Determining COL

caorach

Well-Known Member
As per my recent thread on the Nosler Partition I am approaching the stage where I will be thinking about fine tuning a load and almost without fail everyone has recommended working on COL so that is what I'm going to do.

I've loaded some partitions for testing to the 308W max of 2.800 inches and looking at my magazine on the Blaser I'm not sure I will be able to go significantly longer than this.

I've never messed with COL before and have no idea how far the lands are from my bolt face or how long I need to make my OAL using the partitions to set the end of the ogive a given distance from the lands. I am aware there are various "make shift" methods for doing this and also a range of things that can be bought to do the job (the precision mic for example).

So, what methods are recommended for measuring the distance to the lands and converting this to a correct COL taking the ogive into consideration etc? Is something like the precision mic worth the money as it looks like it will give repeatable results while many of the make shift methods appear, to be, to be prone to some vagueness?

As ever all views, advice and opinions are gratefully received.
 
I'll tell you how I did it, but obviously there are many ways to skin a cat so standard disclaimers apply! :D

I made myself a 'gauge' using a case I had resized and trimmed. I polished out the inside of the neck slightly with a dremel so that a bullet would seat slightly loose, but not so loose it could easily slide in and out just by handling it.

Then I experimented by seating that bullet in further and further until it got to the point that I could comfortably chamber it whilst feeling a tiny bit of resistance whilst closing the bolt. At that point the action of closing the bolt was pushing the bullet into the case against the lands. You can put a bit of engineers blue or pencil on the bullet to confirm that it is indeed pushing against the lands if you like.

I then repeated the process several times, closing the bolt and seating the bullet into the case, then very carefully extracting and measuring using calipers and a suitable comparator.

ReloadingArticle035.jpg


That gives you your baseline. Then every thou smaller (using the same calipers/comparator) is a thou off the lands.

Alex
 
The depth of detail of the answers depends on how much detail YOU want.

The BEST way to get the dimensions of your chamber is to cast it. You can use a sulphur (yech! stinks and is a mess, but is cheap), or you can use CerroSafe. It is expensive, but is simple, safe, and reusable.
Next best, and a very close second to casting, is the use of the likes of Hornady's Chamber-All. I understand there are some UK competitors. From what I read, "six of one, half a dozen of the other".
Next are the "make shift" methods. They are cheap, but less precise and generally more 'complicated' operationally. If done properly, they are precise enough.

The "mic" isn't necessary, bit it makes the process 1) repeatable, and 2) "easier". That's usually what you get when you "pay for it". Unbeknownst to many, you can buy one mic body, and only have to buy new inserts as you exapnd the number of calibers (CALIBERS, NOT cartridges) you load.

If you want detailed info from me, send a PM, but I'm sure there are plenty here that can help you.

Regards,
Paul
 
Instead of polishing out the neck you might split the neck using a fine tooth blade such as those on the Junior hack saw. Then debur the cut. The splits will allow the neck to give slightly yet hold it tight enough to get a reading :D. I have cases adapted like this for most of those that I reload I do not have a comparitor liek the Stoney point.

Now to my mind the comparitor is only of much use in measuring the bullet jump IF the comparitor is the same diameter as the bore and leade of YOUR rifle and as rifles will differ in this dimension it makes me laugh when someone posts they seat their bullets say 0.005" off the lands. For one thing the postion of the ogive can and will often vary more than that is a box of bullets. Makers use more than one bullet forming die in production so the chances of gettign bulelts formed in different dies in one box is more than likely ;). Then add to this that they are most likely using a commercially made comparitor and the chances of it being the same diameter as their barrel is very slim.

Tin hat doned and securely tucked under the manlet :norty:.
 
Now to my mind the comparitor is only of much use in measuring the bullet jump IF the comparitor is the same diameter as the bore and leade of YOUR rifle and as rifles will differ in this dimension it makes me laugh when someone posts they seat their bullets say 0.005" off the lands.

I'm not sure I agree with this. So long as the comparator is measuring to the same point on the bullet each time then it doesn't matter whether that point is exactly the same point that the lands touch the bullet because the readings you are taking only have to be relative.

When you take the modified case/bullet out you know it is touching the lands. When you seat that bullet X thou further into the case it has to be X thou back from the point of contact in the rifle chamber. All the comparator does is try to give a consistent point of reference on the bullet.

I agree that you won't know the true distance from bolt face to the lands, but if all you want to do is seat the bullet X thou back you don't actually need to know.

I agree that bullet inconsistency will affect it though... but what can you actually do about that?

Alex
 
As per my recent thread on the Nosler Partition I am approaching the stage where I will be thinking about fine tuning a load and almost without fail everyone has recommended working on COL so that is what I'm going to do.

I've loaded some partitions for testing to the 308W max of 2.800 inches and looking at my magazine on the Blaser I'm not sure I will be able to go significantly longer than this.

I've never messed with COL before and have no idea how far the lands are from my bolt face or how long I need to make my OAL using the partitions to set the end of the ogive a given distance from the lands. I am aware there are various "make shift" methods for doing this and also a range of things that can be bought to do the job (the precision mic for example).

So, what methods are recommended for measuring the distance to the lands and converting this to a correct COL taking the ogive into consideration etc? Is something like the precision mic worth the money as it looks like it will give repeatable results while many of the make shift methods appear, to be, to be prone to some vagueness?

As ever all views, advice and opinions are gratefully received.

You hit your own answer. Your starting point is the length that allows a cartridge to freely feed from your magazine. Worrying about how much longer you than that is silly unless you want to load single shot. Start at this length and move inward in increments. Seating out is not always the route to the most accurate load. I helped a kid reload 30-06 once using 150 grain bullets. He found that his groups tightened markedly as the OAL decreased. His final groups were half the size of his initial groups.

I also chuckle when i hear .005" off of the lands. Few die/press/ bullet combos will allow that accuracy on a repeatable basis.~Muir
 
I'm not sure I agree with this. So long as the comparator is measuring to the same point on the bullet each time then it doesn't matter whether that point is exactly the same point that the lands touch the bullet because the readings you are taking only have to be relative.

When you take the modified case/bullet out you know it is touching the lands. When you seat that bullet X thou further into the case it has to be X thou back from the point of contact in the rifle chamber. All the comparator does is try to give a consistent point of reference on the bullet.

I agree that you won't know the true distance from bolt face to the lands, but if all you want to do is seat the bullet X thou back you don't actually need to know.

I agree that bullet inconsistency will affect it though... but what can you actually do about that?

Alex

If this is the case then one could use a .22" bullet comparitor for all calibres after all once you measure your test bullet/case with it you can then move it back "X" number of thou...

Somehow I get the feeling that all those extra calibres ones are like most fishing equipement. Designed to catch the angler and his wallet and not more fish ;).I will give this some thought and see if I can knock up some gauges to check the exact diameter of the leade and bore on the .280 then make up a suitable comparitor just out of interest. Although I have never even looked at the Stoney point of Hornady I am sure I can make one up.

Now as to if it's worth the time :confused: :-| hmmm that requires thought.
 
If this is the case then one could use a .22" bullet comparitor for all calibres after all once you measure your test bullet/case with it you can then move it back "X" number of thou...

Absolutely. The only reason for the various comparator sizes as far as I can see is to try to get the datum point roughly in the area of the ogive where the bullet dimensions are supposed to be more consistent, as opposed to towards the tip, or indeed ON the tip!
 
I've loaded some partitions for testing to the 308W max of 2.800 inches and looking at my magazine on the Blaser I'm not sure I will be able to go significantly longer than this

i load my .308 r93 to 71.5mm to the tip (from memory) with 125 b tips - thats as long as the mag allows and is still some way from the lands. i have all the measuring stuff but in the blasers it doesnt seem to make that much difference, compared to a turn bolt.
 
I load my .308's to 2.8" which is just about the max for the magazine although the lands are quite some ways further - don't recall as it seemed academic at the time. As Muir points out, I don't want a single shot...

P.S. I borrow a stoney point OAL gauge and buy the case whenever I get a new rifle...
 
Regarding Brithunter and CSl's back-and-forth:

One is relative, and one is absolute. Relative can "work", but requires more knowledge and skill by the user. Absolute is 'easier' to use, but usually costs more money. It is not difficult to have the 'comparator' on a "Lock'n'Load" type device be EXACTLY the diameter of your specific rifle:

1) Slug the bore,
2) Ream the "comparator" to the dimension of the bore.

I do it all the time, and I have individual comparators for most of my individual rifles.

I'm always taken aback by reloaders of rifle cartridges that want to do things "fast" or take shortcuts because "that takes too long". Really? What's the hurry? (Don't start in with pistol and shotgun ammo. That's NOT what we're discussing here.)

Regarding Muir and other's magazine length comments:
Magazine length is not in my experience the MOST COMMON limiting factor to cartridge length, chamber length is. Of course there are exceptions; the most common ones are in semi-automatic and automatic rifles. In my experience, and certainly when I cut a chamber myself, the cartridge length limit is ALMOST always the chamber length. Again, there are exceptions, but there is NO reason to start with the assumption that magazine length will be the limiting dimension. Measure the chamber and measure the magazine. The shorter value determines the 'starting point.

With respect to "on the lands" for "accuracy" (precision actually):
The benchrest/competitive shooters have caused this approach to be far too widely accepted as generally "good". After years and years of fiddling with this concept, the reality is that selecting a seating depth that puts a bullet "on the lands" is far more likely to present problems than it is to improve precision. CAN it improve precision? Certainly. WILL it improve precision in a over-the-counter stalking rifle? More likely than not, "no". There are many reasons and I'm not going to go into them here. More important is the potential for problems caused by trying to set seating depth "on the lands".

There are very real problems with trying to get your COLs to be "on the lands". The most significant of which is that SMALL errors, caused by component variation or lack of attention to detail by the reloader, can lead to serious problems, the most significant of which are 1) "jamming" the bullet into the lands causing SERIOUSLY increased chamber pressures, and 2) having a SLIGHTLY elongated neck get 'squeezed' in the leade, resulting in SERIOUSLY increased chamber pressures. The potential for "bad" usually exceeds the TRUE potential for "good" when trying to get the COL to be "on the lands".

With the advent of the "on the lands" concept for jacketed bullets, ballyhooed by ignorant or dishonest gunwriters (ptooey), 'regular Joes' forgot (or were mislead) that MANY rifles and cartridges did "just fine" when the bullets weren't seated to be on the lands. In point of fact, for those that are paying attention, there is a COL "sweet spot" that occurs somewhere between 0.030" and 0.150" OFF the lands. That 0.120" window is pretty large, and leaves LOTS of room to experiment.

All of the above noted, it should also be noted that the "INTO the lands" concept is one promoted by CAST BULLET shooters for more than a century. BUT... cast bullets are NOT jacketed bullets, and the differences are almost 'night and day'. Practices and concepts between the two do not overlap except at the most general levels.

Finally - Here is a general step-by-step sequence for locating your rifle's precision "sweet spot" for a given bullet.

1) Determine the MAX length your rifle can "handle" - be that limited by magazine or chamber.
. . . . .a) If limited by the magazine, determine how far back from the lands the "magazine-limited" length is,
. . . . .b) If chamber-length limited, note the actual chamber length,
2) Starting at about 15 thousandths from the actual chamber length, (or as close as you can get if magazine-limited), load 3 or 4 or 5 rounds at progressively decreasing lengths in increments of 0.030" until you get to 0.150" short of the actual chamber length.
3) I promise you that barring some truly extraordinary circumstances, ONE of those loads will produce a smaller group than the others.
4) Turn your focus to 'fine tuning' THAT load by shortening the COL increments to 0.010" increments (0.005" if you really think your equipment will allow you that level of REPRODUCABLE precision). Once you have found THE precision "sweetspot", you can, if you must, fiddle with charge. I would recommend against doing that unless your best groups are still larger than you are comfortable with in a stalking rifle.

This method has 'worked' for me for decades, and it will 'work' for you if you use it.

This particular element of reloading is, to me, identical to the element of reloading called "sighting your 'scope in". There is a very simple, very straight-forward procedure for sighting in 'scopes that WORKS and works EVERY TIME if followed correctly. Yet, I still see people wasting boxes and boxes of ammunition because they either don't know the procedure, or they are in too much of a hurry to perform it properly. If the procedure outlined above is employed, it is the VERY odd 'scoped hunting rifle that will not shoot 3 or 4 shots into a 4-cm circle at 100m.

Best of luck,
Paul
 
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