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Thread: A little about reloading .308 Win and a little about my view of reloading...

  1. #1

    A little about reloading .308 Win and a little about my view of reloading...

    On another forum, we got to talking about various light-for-caliber (hereafter LfC) .308 Win loads and I mentioned that I was working up some loads for resized pistol bullets weighing from 60 to 90 grains. Egged on by the ever-willing-to-egg-on there at The Hunter's Life, I said I'd start a thread on the matter once I had more to offer than just how to 'squeeze' bullets. Since Stalking Directory is a "mature" forum, and since it is difficult for someone new to get "known", I thought I'd post this thread I started at The Hunter's Life, not so much for the reloading interest, but rather as a vehicle to "introduce" myself a little more thoroughly. Even though this is an 'introduction' of sorts, the "Introduction" site here at SD didn't seem the appropriate place for it. If a moderator thinks it should go there, by all means put it there and rest assured that I will have no 'heartburn' about that.

    Since the .308 Winchester is one of my 'most favoritest' cartridges, I thought I might as well wring every last drop of precision I could from this exercise. Knowing that I'd be facing the slings and arrows of nay-sayers and skeptics, I decided to exercise all the quantitative 'muscle' I could muster. I want the results to be as unambiguous as I can get them. Meaning... I want to minimize the comments about various sources of variability by 1) eliminating as much of the variability as I can and 2) quantifying the rest as much as I can. Strap in, 'cause it's gonna be 'dense'.

    My desire with the LfC bullets is to get 'good energy' (lethal doses - 1000 ft-lbs minimum for white-tailed deer) down range with as little recoil as possible. I spend much of my time with women, young, and inexperienced shooters, and recoil is a very real issue for them. Almost as significant as recoil is "lethality". My opinion on the matter of lethality is that the less experienced the hunter, the more psychologically important it is for the animal hunted to be killed with one clean shot and not have to be chased. The 'experience' is not as "good" if the animal is found dead even shortly after having been chased rather than if it had been killed "in its tracks". Hence the search for a .308 Win load that is "lethal" on the pointy end, yet "friendly" on the blunt end.

    When reloading, one often starts with bullet selection. In this case the suite of bullets from which I was going to choose was:

    .312", 60, Hornady HP/XTP 32010 swaged to .3085"
    .312", 60, Speer HP GD 3986 swaged to .3085"
    .312, 71, Sierra FMJ FN TM RN 8010 swaged to .3085"
    .312, 85, Hornady HP/XTP 32050 swaged to .3085"
    .312, 90, Sierra JHC 8030 swaged to .3085
    .308, 110, Speer SpirePoint 1855
    .308, 130, Speer HP 2005

    A comprehensive search for the best load for 7 different bullets would be very labor-intensive. (Turns out, it’s pretty darn labor-intensive for “only” 3 of them.) To narrow the list a bit, I started off with QuickLoad (hereafter QL) to see how fast I could get each of them going, and what their trajectories and delivered energies were like. Without boring you with the details of that exercise, the results were; that of the resized pistol bullets, only the 85-grain Hornady HP had the ballistic coefficient to 'reach out' to 150-plus yards. I will be going back to fiddle with the others, but for now, the 85-grainer is "it" for the resized pistol bullets.

    At my recommendation, sakorick has been experimenting with the Speer 130 in his custom .308 Win. Turned out, the 125-grain spitzer shot straighter than the 130 HP for him, so he was going to use the 125 for hunting. I know what that 130 Speer HP can do terminally, so I want to develop a load that is laser-accurate. (I have a ‘good enough’ load, but I haven’t really fine tuned it.) So the 130 is in this mix for this exercise.

    The third bullet is the 110 Speer Spire Point. It's a good step up from the 85-grainer, and down from the 130-grainer. While not an HP, it will be 'screaming along', and based on my experience with other similar bullets, I expect its terminal performance to be excellent.

    These loads are being worked up for use in my Ruger M77 with a 22" barrel so I needed to get precise information about the chamber if I was to get the best estimates from QL. The first order of business was fire-forming a case and getting the exact dimensions for creating a "new" cartridge in QL - the ".308 Win My Ruger". (QL allows the user to create new cartridges based on the exact dimensions of a specific case.)

    That done, I needed to precisely measure the chamber in order to select a seating depth/overall length for each different bullet. For long bullets, I prefer 1 caliber seating depth - for short bullets, I use 67% of 1 caliber. In the case of the .308 caliber, 67% is 0.206", and since these are definitely short bullets, all of them were going to be seated 0.206" deep as long as the chamber would allow it. Using the Hornady (originally Stony Point) Chamber All, I measured the chamber length from the bolt face to the lands. In this rifle it is 2.288". Clearly, all the bullets could be seated 0.206" deep.

    The special case (I made this one, but they can be purchased in common cartridges, or you send them one of your wildcat cases) next to the Overall Length Gauge.

    The gauge with the case screwed on.

    The case showing the rod that extends inside the case and positions the bullet against the lands when the whole contraption is inserted into the chamber.

    After seating the bullet against the lands, the gauge is removed from the chamber and the length is measured from the head of the cartridge to the where the ogive of the bullet hits the lands. In this case, 2.288". One should measure this distance for each bullet tyoe you use, OR measure the ogival length of each of your bullet types.

    Armed with that information, I could get pretty good ‘numbers’ from QL for each bullet. However…

    As those of you that have been reloading for some time probably know, “serious” reloaders weigh each batch of cases they get and sort them by weight. While it is rarely, if ever, explained WHY they do this, the implication is that those cases that have similar weights will have similar case capacities. Previously, when I have casually looked at this, I have not found the relationship between case weight and case capacity to be predictable. 'Til now, I haven’t performed a comprehensive examination. Now I have, and the clear conclusion is that weighing cases in hopes of getting cases of similar capacity is an exercise in futility. (Here is one place I expect those ‘slings and arrows’ of which I spoke above to start flying.)

    I used a sample size of 54 cases. There were cases from three “lots”. Two were military surplus, (White City Cartridge 1984 and Lake City 1993), and one was “civilian” (Federal Cartridge .308 Win HE).

    I have a scale capable of weighing to 0.001 grams (0.01534 grains). To give you some idea of this level of precision, one granule of I3031 weighs approximately 0.003 grams. So I can weigh with a precision greater than one granule of powder. (That's granule not grain.)

    To do this “right”, each case had to be prepared identically. First, each case was full-length resized in a Lee resizing die. Then each case was trimmed to 2.010” exactly. Because I would be filling each case with water, each case was “primed” with a spent primer. The primer was inserted ‘backwards’ so that what ever variability between the spent primers was, it was not an issue when inverted. The inverted primer sealed the case uniformly regardless of what type of primer was used.

    The procedure was:
    Weigh the empty case and write the weight on the case.
    Tare the scale.
    Fill the case with water using a pipette.
    Wick off the meniscus so that the water was exactly even with the mouth.
    Empty the case of water.
    Write the weight of water (case capacity in cubic centimeters (cc)) on the case.

    Here's a picture of what the meniscus looks like.

    Here it is after wicking off hte meniscus. This may look concave, but that's an illusion. I assure you the level of water is absolutely flat across the mouth.

    Once the weight and capacity in cc was recorded for every case, I entered all of the data in Excel and performed a simple linear regression. The first regression was with all the cases pooled. Then I regressed case weight on case capacity for each separate case group – WCC, LC, and HE. The results are unambiguous – there is NO relationship between case weight and case capacity within a given cartridge. Of course there will be at least two rebuttals to the above statement.

    First, someone will argue that the sample size is too small. Wrong. Based on internationally accepted statistical standards, a sample size of 54 is sufficient to determine the magnitude of the variability of a sample that demonstrates the variance of this sample.

    Second, someone will argue that “good” (read “expensive”) cases from “good” manufacturers would render results that show a relationship between case weight and case volume. While I cannot say with certainty that this is not true, until I see THE DATA that PROVES it, I’m sticking with the results from this random selection of case manufacturers. There are two reasons to do so. First, the process of ‘drawing’ a case requires that a mandrel be used for the inside of the case. Therefore, the most uniform dimension of ANY case should be the volume. As a manufacturer, it would be silly to spend money on weighing cases to the 0.001 grams, when the volume will be determined by a mandrel? Second, one of the many “givens” of reloading, is that military brass is “thicker” and therefore weighs more, and therefore has a smaller case capacity, and therefore causes higher pressures in non-military firearms. This data says exactly the opposite. The WCC cases are lighter - not heavier than civilian cases - and have higher - not lower - volumes. Exactly the opposite of what common “wisdom” asserts. Therefore, I am completely unconvinced that “good” cases will have a high correlation between case weight and case capacity.

    In conclusion on the case weight/case capacity relationship: Don’t waste your time weighing cases. Within a cartridge, cases with similar weights will not yield cases with similar capacities.

    Below you can see the graphs in which are plotted the case weights on the horizontal (“x”) axis, and the case volumes on the vertical (“y”) axis.

    Here is a graph of all 54 of the cases. As you can see from the legend, the WCC mil-surp cases (green) are the lightest and have the highest volumes. Just the opposite of what gunwriters (ptooey) will tell you.

    Note also that there is no way to predict capacity based on case weight.

    Here are the Federal HE cases alone:

    Here are the Lake City cases alone:

    Here are the White City Cartridges cases alone:

    It should be clear that there is no relationship between case weight and case capacity.

    Once the case capacity for all 54 cases was measured and recorded, I could use QL and find appropriate charges for each bullet. Theoretically, if I adjusted the case capacity every time I calculated a charge that produced a specific chamber pressure for a given bullet, the resulting muzzle velocities and exit timings would be identical. In fact, that was indeed the case.

    I chose 56,565 PSI for the upper limit. This figure is the CIP (European) standard for the max chamber pressure for Mauser 98 actions – an action I use frequently to build rifles with. This figure is well below, (about -6%), the SAAMI max of 60,191 PSI for the .308 Win cartridge. At the low end, I chose 52,667 PSI. It is the default value used by QL for the .308 Win cartridge as the “safe” ceiling for the .308 Win cartridge. The third value – 54,616 PSI - was simply half way between 56,565 and 52,667.

    Having chosen the pressure boundaries, I could input each case’s capacity and calculate the charge that gave the exact (within about 20 PSI) chamber pressure I was looking for. It was tedious, but but productive. Namely, by altering the charge appropriate to the case capacity, I could get each case to ‘generate’ the same chamber pressure and therefore MV and bullet exit timing. This means that small changes in case capacity compensated for by charge adjustment, yields uniform velocities and exit timings.

    From that point it was simply a matter of setting the charge specific to the case capacity to give either 56,565 PSI chamber pressure, 54,616 PSI, or 52,667 PSI. Each case should then theoretically produce the same MV and exit time. Once that was done ‘on paper’ (in QL), I had all the charges I needed to load 6 cartridges each of three charges for three bullets, or a total of 54 cartridges. (6 x 3 x 3 = 54) In the end, this all took about 16 hours to accomplish for the 54 cartridges. And this is why I didn’t get to the range today. J

    In summary:
    I loaded 18 cartridges with the 130-grain HP, and a charge of I3031 to yield chamber pressures of 56,565 PSI (6 cartridges); 54,616 PSI (6 carttidges), and 52,667 PSI (6 cartridges).

    I loaded 18 cartridges with the 85-grain HP, and a charge of I4227 to yield chamber pressures of 56,565 PSI (6 cartridges); 54,616 PSI (6 carttidges), and 52,667 PSI (6 cartridges).

    I would have loaded 28 cartridges with the 110-grain spitzer, and a charge of I4198 to yield chamber pressures of 56,565 PSI, 54,616 PSI, and 52,667 PSI, but I found out after I did all the paper-whipping that I only had enough I4198 for 2 cartridges. L I’ll have to recalculate for another powder I have on hand.

    Here is part of the table I created that has the proper charge for every case to produce the specific chamber pressure.

    I used my .308 Win precision bullet seating die for seating every bullet to the exact seating depth of 0.216.

    Here are pictures of the RCBS Precision Bullet Seating Die.

    [COLOR=white]Here is a picture of the loaded cartridges:

    While I have several rifles fitted with pressure sensors, I don’t have a .308 Win-chambered rifle fitted with one or I would use it and get chamber pressure data to test if the theoretical estimates (of chamber pressure) made by QL are accurate. I will be measuring muzzle velocity though, and the MVs can be used to estimate chamber pressures. In other words, the MVs that QL estimates are based on the theoretical chamber pressures. Therefore, if the spread of the MVs is as small, it would indicate that the chamber pressures are uniform.


  2. #2
    By the way...

    Since the charges are listed in grams, you probably didn't notice that the difference in charge between the lower chamber pressure figure - 52,667 PSI - and the upper one - 56,565 PSI - is about 1 grain. That means that the chamber pressure in the middle is about 0.5 grains from either the high or low. So... of the 18 cartridges per bullet, I have 6 each at half-grain intervals.

    I thought it was interesting that charges that produced the QL max (52,667 PSI), were almost (but not quite), exactly one grain less than the charges needed to produce my max pressure of 56,565 PSI. I am hoping therefore, that one of the three loads (lo - medium - high) will be near a timing "sweetspot". That's why you see the "node=" note on the charge table.


  3. #3
    Since this exercise is about 'recoil, among other things', here are some recoil numbers.

    First, recoil for what I call an American "standard". A 180-grain bullet doing 2800 f/s in a rifle weighing in total (scope + rifle) 9.5 lbs.

    Here are the same calculations for the 60-grain HP:

    Here for the 85-grainer:

    Here for the 110-grainer:

    Here for the 130-grainer:

    There's a lot of "stuff" on this page. I look primarily at the figure for "Energy of Recoiling Mass".

    Here's a table comparing the "Energy of Recoiling Mass" for each of the above bullets.

    180 - 19.77 ft-lbs
    130 - 11.63 ft-lbs
    110 - 8.99 ft-lbs
    85 - 7.23 ft-lbs
    60 - 6.04 ft-lbs

    To give another 'ruler' with which you might judge these values, here's a .223 Remington shooting a 55-grain bullet at in a 8.5-lb rifle.

    The "Energy of Recoiling Mass" is 3.29 ft-lbs.

    More later.


  4. #4
    OK - I actually did get to the range today.

    I decided to keep it simple, and only took four rifles:

    1) Ruger M77 in .308 Win,
    2) My hand-made .375x.284 Win,
    3) My .416x.348 Win Cape gun, and
    4) My new-to-me BSA Martini in .22 LR.

    I'll report the results for the .308 here.

    Just to remind everyone:

    I had 3 loads (6 cartridges each) for 130-grain Speer HPs, and 6 loads (also 6 cartridges each) for the 85-grain HP/XTP swaged down from .312" to .3085", and 1 load of two cartridges for the 110-grain Speer Spirepoint.

    This is the target that has all the shots:

    The 110-grainer was to get I4198, but when I "went to the cupboard, the cupboard was bare" of I4198. I only had enough for two loads. They are at the center bottom above.

    Over-all I was quite pleased with these groups. Most of the fliers were called. I was looking for a really good load for the 130 HP, and I think I've got one. Of course I was also checking out the swaged 85-grainer. It performed better than I thought it would. I'd hunt with it.

    Here is the image of the first group I shot of the 130s.

    I had 6 cartridges of each load, so I shot 3 shots, let the rifle cool completely then shot the second three. So shots 1 and 4 are from cold barrels. Each shot is numbered because each case had a different capacity and therefore a different charge, and I wanted to be able to correlate charge, shot placement, and muzzle velocity. I used I3031 for the 130.

    Here are the stats for the first group:

    Shot# . . Case Capacity . Charge . . MV

    1 . . . . . . . . . 53.62 . . . . . 44.41 . . 2973
    2 . . . . . . . . . 53.62 . . . . . 44.41 . . 2977
    3 . . . . . . . . . 53.62 . . . . . 44.41 . . 2985
    4 . . . . . . . . . 53.65 . . . . . 44.42 . . 2971
    5 . . . . . . . . . 53.65 . . . . . 44.42 . . 2979
    6 . . . . . . . . . 53.70 . . . . . 44.45 . . 2982

    MV Max Spread was 12 fps. Pretty darn good. Average is 2978.
    Group size (not counting #4 which was a called flier) is 0.948" windage by 0.682" elevation.

    Here's the second group of 6:

    Shot# . . Case Capacity . Charge . . MV

    1 . . . . . . . . . 53.70 . . . . . 44.93 . . 3001
    2 . . . . . . . . . 53.70 . . . . . 44.93 . . 3005
    3 . . . . . . . . . 53.70 . . . . . 44.93 . . 3009
    4 . . . . . . . . . 53.70 . . . . . 44.93 . . 3003
    5 . . . . . . . . . 53.70 . . . . . 44.93 . . 3008
    6 . . . . . . . . . 53.71 . . . . . 44.93 . . 3009

    Max Spread was 8 fps!!! :COOLdude:Average is 3006.
    Group size (not counting #1 because I don't want to ) is 0.967" windage by 1.077" elevation. I like it!

    Here's the third group:

    Shot# . . Case Capacity . Charge . . MV

    1 . . . . . . . . . 53.70 . . . . . 45.43 . . 3034
    2 . . . . . . . . . 53.70 . . . . . 45.43 . . 3038
    3 . . . . . . . . . 53.70 . . . . . 45.43 . . 3054
    4 . . . . . . . . . 53.70 . . . . . 45.43 . . 3043
    5 . . . . . . . . . 53.70 . . . . . 45.43 . . 3039
    6 . . . . . . . . . 53.71 . . . . . 44.99 . . 3041

    Max Spread was 20 f/s. Average is 3042.
    Group size counting all 6 is windage = 0.985" and elevation = 1.539" Group size not counting #3 is, windage = 0.985" and elevation = 0.620.

    Now here's the stuff you've been waiting to see - the 85-grain data. I used I4227 for the 85s.

    Here are the stats for the first group:

    Shot# . . Case Capacity . Charge . . MV

    1 . . . . . . . . . 53.82 . . . . . 38.74 . . 3455
    2 . . . . . . . . . 53.83 . . . . . 38.74 . . 3461
    3 . . . . . . . . . 53.85 . . . . . 38.76 . . 3451
    4 . . . . . . . . . 53.85 . . . . . 38.76 . . 3452
    5 . . . . . . . . . 53.85 . . . . . 38.76 . . 3467
    6 . . . . . . . . . 53.85 . . . . . 38.76 . . 3467

    MV Max Spread is 16 f/s. Average is 3458.
    I aimed 1" to the right of the Point of Aim for the first 5 shots when I shot #6, so I am going to move it 1" to the left. Group size is 0.330" windage by 1.386" elevation. Without #3, elevation is 0.539".

    Here's the second group of 6:

    Shot# . . Case Capacity . Charge . . MV

    1 . . . . . . . . . 53.85 . . . . . 39.24 . . 3499
    2 . . . . . . . . . 53.85 . . . . . 39.24 . . 3491
    3 . . . . . . . . . 53.85 . . . . . 39.24 . . 3505
    4 . . . . . . . . . 53.86 . . . . . 39.25 . . 3489
    5 . . . . . . . . . 53.86 . . . . . 39.25 . . 3497
    6 . . . . . . . . . 53.86 . . . . . 39.25 . . 3501

    Max Spread is again 16 f/s. Average is 3497.
    Group size is 0.682" windage by 1.673" elevation.

    Here's the third group:

    Shot# . . Case Capacity . Charge . . MV

    1 . . . . . . . . . 53.93 . . . . . 39.76 . . 3537
    2 . . . . . . . . . 53.97 . . . . . 39.79 . . 3547
    3 . . . . . . . . . 54.00 . . . . . 39.81 . . 3541
    4 . . . . . . . . . 54.00 . . . . . 39.81 . . 3551
    5 . . . . . . . . . 54.08 . . . . . 39.84 . . 3561
    6 . . . . . . . . . 54.11 . . . . . 39.86 . . 3565

    Max Spread is 28 f/s. Average is 3550.
    Group size is 1.600" windage by 1.105" elevation. I didn't let the barrel cool between the first three and the second three shots for this group, and that may account for 5 and 6 moving right. They were not called fliers, and I was holding true. Notice that they are the two highest velocities also.

    So therere you have it. I'm quite pleased with both the 130s and the 85s. As I said above, I'd use the 185s for hunting and not think twice about it.

    Truth is, these are the best groups I've ever gotten for the 130. But there's something here that is a bit subtle, yet a very big deal. Look and the differences in loads, yet all the group sizes are roughly the same. That speaks volumes to me regarding matching the case volume to the charge as precisely as one can. Among these 50-odd cases, there is a full grain difference in case capacity. That is a HUGE difference when it comes to chamber pressure and therefore muzzle velocity, and by extension, precision (accuracy). Truly, discounting the fliers, I could "live" with any of these groups.

    Relative to the Optimal Barrel Timing Theory, the best group for the 130 - group #1 - was right on the node. "Calling it" for the 85-grainer is a bit more difficult. The node is at the timing of the third group. I think that group might have scored better if I hadn't rushed the last three shots. Still, group 2 or groups three are OK by me.


    I'm not sure I'm ready to go through all of what I went through to get these results. It was a LOT of work. However, it turned out better than it ever has for the 130, and better than I expected for the 85. I'm gonna hafta think about it for a while.

    Last edited by gitano; 27-09-2010 at 20:24. Reason: So shots 1 and 4 (not 3) are from cold barrels.

  5. #5
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    Bonnie Scotland
    Hi Paul. What a read, fascinating. Maybe a bit much for me to respond to in detail though. I wonder...What are your thoughts on HfC loadings? esp 6.5mm re: medium game, at close range and middle distance (i.e. out to 300yds) using bullets with goodish BC's.

  6. #6
    Oh dear.......................... now you have asked. He has a monster that finnbear would love a .50 Alaskan if I remember correctly for which he was making bullets from cases. 500+ grains of bullet I believe that a heavy. Oh as you might have guessed Gitano is a scientist and to be honest he makes my head spin.

  7. #7
    Quote Originally Posted by Tamus View Post
    Hi Paul. What a read, fascinating. Maybe a bit much for me to respond to in detail though. I wonder...What are your thoughts on HfC loadings? esp 6.5mm re: medium game, at close range and middle distance (i.e. out to 300yds) using bullets with goodish BC's.
    Thanks for taking the time to read that 'thing'. It's a bit tedious I'll grant.

    As to the 6.5 with HfC bullets:

    My personal generalization about HfCs and LfCs is that HfCs are "best" in calibers above something like .358" and LfCs are "best" for calibers like .284" and below. With calibers from .30 to .35 using "middle of the road" bullet weights. Of course as soon as one makes such a generalization, especially with "boundaries" one runs smack into someone that disagrees, and has the "proof" to back them up. No arguments from me on "appropriate" bullets, only on what I like and why. So...

    For "medium game" - is that the likes of fallow and sika? - I think the 6.5 is an excellent caliber but I would be using 129-grainers instead of 140s or 160s. I don't see them (never having shot one though) as requiring one to break some of the locomotor skeleton to "anchor" them before they stomp mud-holes in your butt like you would with a Cape buffalo or coastal brown bear.

    I have a little calculation I perform when I'm looking at a new-to-me cartridge (not caliber). I call it the energy "sweet spot". Now before I get taken to task as an "energy (KE) lover", let me quickly add that I am NOT. I like my cake and want to eat it too. Therefore I want to deliver the most energy to the critter, AND I want to penetrate as far as possible. I consider momentum a more appropriate measure of a bullet's penetration potential than KE, because... It is momentum that must be overcome by the animals flesh, to stop a bullet.

    So what I do is take all the bullets available that I am interested in comparing within a given caliber and 'paper-whip' some loads that theoretically stay well below the SAAMI or CIP max chamber pressures. Using those MVs, I calculate the delivered energy and momentum at 100, 200, and 300 yds for each bullet and plot that data in a graph of bullet weight vs delivered energy/momentum. This almost always yields a hump-shaped graph. Sometimes the "hump" is toward the lower-weight end of the bullet weight spectrum, and sometimes it is toward the heavy end.

    In the case of the 6.5x55 cartridge, it peaks at the 129-grain weight. As the case capacity goes up - say to a .264 Win Mag - the weight "sweet-spot" of a given caliber often moves a little to the right. But not always. This sweet-spot graph is sort of a comparison of BCs because the better the BC the more energy/momentum the bullet will carry down-range. However, some lighter bullets squeeze so much more MV out of a given case that increasing the case's capacity doesn't really do anything significant energy-wise.

    Here's a couple of "sweet-spot" graphs for a wildcat I made - a .375" bullet on the .284 Win case.

    I'll get the ones I did for 6.5mm and post them, but at the moment, the external drive with those images on it isn't 'cooperating'

    One of the interesting "things" about the 6.5mm sweet-spot graph is that it is fairly "flat". Meaning that it's pretty difficult to choose an ineffective bullet weight. Put another way, any of the "regular" bullets available in 6.5mm will "work" pretty much as well as any other. At least in terms of delivered energy and momentum. The only caliber I have seen with flatter sweet-spot curves than the 6.5mm is the .338.


  8. #8
    I went to another computer and pulled up some old 6.5mm graphs. They're not exactly what I was looking for, but they'll be more to your interest than the .375. Also, clearly my recollection of the 6.5s "flatness" was wrong. These are not particularly "flat" sweet spot curves. In fact, if I was pressed, I would have to say that the 6.5 caliber is one that seems to "prefer" HfC bullets. This of course would be consistent with the history of use of this caliber.

    This is the curve for the .260 Win.

    As you can see, the 140s as a group do the best in delivered KE. Three of them are better than the best 129 and all of them are at least as good as the 129 as a group. The 155 does "just fine" too.

    Here's the same curve but for the 6.5-06 cartridge. As you can see, the shape of the curve doesn't change, but the magnitude does. The theoretical sweet spot is still about 135 grains, and the 140s still "dominate". Look at the 155 though. Its relative improvement is considerable, and would be the "best" bullet - delivered energy-wise - in that cartridge.

    Here are the momentum graphs for the 6.5-06.

    This one is for 300 yds. This graph illustrates the "weakness" of KE as a predictor of penetration. Kinetic energy is of course a function of the square of velocity. Therefore small increases in velocity produce large increases in KE. However, it isn't KE that the animal's flesh has to "resist". It's momentum. When one considers the "penetrating power" of a set of bullets, the heavier ones 'prevail' when the correct characteristic is examined. ONE of the questions for selecting a bullet then becomes, "Do I need the maximum penetration for the game I am persuing?" As far as I'm concerned, the answer to that question is NOT a matter of public policy, but a very personal decision.

    Here are the theoretical lines at 100 , 200 , and 300 yds. The 100 yd line is on top and the 300 yd line is on the bottom. You can see that the momentum sweet spot moves left as the range increases. This is due to the relatively poor BCs of the heavy bullets. In my opinion, bullet manufacturers tend to make ballistic "pigs" of HfC bullets. Especially in a "traditional" caliber like the 6.5mm, the HfC bullets are all round noses except the 155. If the manufacturers would make some spitzer boat-tails, they would deliver considerably more "energy" to greater distances than the ligher-weight bullets do with their higher MV and BCs.

    Of course there are mitigating circumstances that essentially require HfC bullets to be RNs. First is "standard" magazine lengths. If those HfCs were SPBTs, they very well might not fit in a "standard" magazine. Second, if you have a "long nose" and have to be seated deeper to accomodate the magazine, the boat-tail is going to take up valuable case capacity. Finally, if you make a HfC SPBT, it's greater length is going to require a faster than "normal" twist rate.

    So MY point is HfCs in "small" calibers are generally a "bad" idea. If you need greater penetration, you might want to go up in caliber instead of selecting a HfC bullet. Except in the 6.5mm -


  9. #9
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    Quote Originally Posted by gitano View Post

    So MY point is HfCs in "small" calibers are generally a "bad" idea. If you need greater penetration, you might want to go up in caliber instead of selecting a HfC bullet. Except in the 6.5mm -

    Now you're learning... ~Tom

  10. #10
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    Further, by using higher pressure cases than the venerable Swede (which can of course itself be tweaked by using some of our clever European powders ) it becomes possible to reap advantages of good launch velocities for heavy 6.5mm projectiles with good BC's. The gain is, flatter shooting and better terminal ballistics at a wider selection of impact ranges. IMHO, of course

    ps. So long as you don't then wipe all the gain away be hacking 8" of yer bbl, obviously.
    Last edited by Tamus; 24-09-2010 at 19:59. Reason: ps.

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