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Thread: Effect of OAL on Pressure

  1. #1

    Effect of OAL on Pressure

    Hello chaps.

    Quick question for the reloading crowd. I took a couple of ladders to a BDS range day today.

    I've been trying H335 under a 110gr SP in .308 and Reloder 15 under a 150gr SP.

    I couldn't get close to maximum loads at all. I was expecting that there might be a point before maximum where pressure signs started appearing, but I was barely above the starting loads when I started seeing pressure signs (brass flowing into the extractor recess the most obvious, some stiffness in the bolt also). In spite of the pressure signs, the way they were shooting made them feel like they were light, mild loads - certainly not "maximum pressure" loads.

    Both lots were loaded within the limits of the data, above the minimum OAL (I don't have a comparator / measure-to-ogive gauge yet). The 110s at 2.580" and the 150's at 2.725". For both, I could seat the bullet further out of the case, though probably not to max OAL for the cartridge in the case of the 110s.

    What I'd like to know is whether a difference of a tenth of an inch or two seating depth is likely to make much of a difference to pressure? I.e. if I've got another 3-5gr before I get to maximum load, should I seat further out of the case and try another ladder to see if I can get further towards maximum or is it not likely to make much difference as to how far I can go?

    I should mention that one load of the 150's looked very promising (one ragged hole at 100m) but that the rest of them were mediocre with respect to accuracy. None of the 110's performed particularly well, so I either need a new approach or to try a new powder.

    Thanks for any thoughts / pointers you can give.

    Adam.

  2. #2
    Hi. Have you modelled the loads in a simulation programme? If not, if you want to send me full details of loads then I am happy to run them thru the Quickload software programme. Will give you an idea on pressure etc etc. Might help gain some insight. Not perfect as cannot replicate your specific gun but might help.

  3. #3
    Quote Originally Posted by cambsman View Post
    Hi. Have you modelled the loads in a simulation programme? If not, if you want to send me full details of loads then I am happy to run them thru the Quickload software programme. Will give you an idea on pressure etc etc. Might help gain some insight. Not perfect as cannot replicate your specific gun but might help.
    It's a kind offer but what would that ascertain? Not much I think, because quick load is a generic ballistic guide, much as reloading data is a guide. The data contained in load information is what the provider got on that day, with that LOT of powder, from their barrel, cut with their chamber. It will never transfer straight across to your rifle and your loads. My 7mm-08 topped out with Varget at 42.5 grains -which is starting load on Hodgdon's site, but a mid range load in Hornady's book. This is why there are constant reminders to "START AT MINIMUM LOAD AND WORK UP" because, "minimum" sometimes ain't, and maximum is sometimes never approached. It just works that way. When you start getting pressure signs you're pretty much done but that's OK. Unless it's being caused by something amiss in your loading technique, you are probably still getting the performance listed but using less powder to get there.

    As to relieving the pressure signs by seating out a bit. Probably won't help a measurable amount. Even if it did, what would you do then? Continue to add powder to try to match your loading data? You'd soon be right back in the same boat.~Muir

    PS: I had once a 17 MachIV that blew primer out of the case with starting loads. My best load was 15% below starting load.

  4. #4
    As a rule of thumb deeper seating with a given load will increase pressure while seating a bullet further out will reduce it.
    However, while I have always adhered to reloading manual data as a general rule, I have a situation with my current loads that allows me to exceed by .2gn the max charge shown with a load that provides constant good accuracy without any visible external pressure indications on the fired cases.

    Indeed these cases have now been loaded and annealed to provide further longevity with what are still very good useable cartridge cases. You may anyway find that bullet manufacturers data and propellant manufacturers data are different..... - But if you exceed max data listed, go into this territory with great care. ATB
    Blaser K95 Luxus Kipplaufbüchse .25-06Rem. Zeiss 8x56, 110gn Nosler Accubond = Game Over!

  5. #5
    Thanks all for your responses - much appreciated.

    To cambsman: Thank you for your offer to put them through quickload - out of curiosity if for no other reason, I'll send you a PM later today with the load details.

    To Muir: I take your points and if seating depth won't make that much of a difference I probably need a new powder. The range between max and min was (I think) about 6-7 grains but I was getting pressure signs after 1.5gr above starting. You asked me if I'd try to match loading data - actually I would. I only had four loads, 0.5gr between them, to test with a 1.5gr range and none of them were good. It would be good to try and generate some more possibilities though, with a whole tub of powder sitting on the shelf with no purpose to it at this point. If I could get back within safe pressure, I can get two more loads to try for every grain extra I can get in the case. I could cross my fingers that there would be an accurate load in those and try to develop it. I'm not sure I'm explaining myself very well, but hopefully you get the idea.

    To deeangeo: Thanks for your reply. I think I'm too much of a newbie at this to be anything other than conservative / cautious, so I won't be straying into above-maximum territory any time soon. I guess I am curious though - I wonder if because the loads showed pressure signs whether they are actually "out-performing" the data. I.e. I'm getting the pressure they expect with 50gr of powder with a load of 45gr - does that mean I also get the velocity associated with 50gr? I don't have access to a chronograph, but I'm curious.

    Thanks again guys.

    Adam.

  6. #6
    Quote Originally Posted by neutron619 View Post
    Thanks all for your responses - much appreciated.

    To cambsman: Thank you for your offer to put them through quickload - out of curiosity if for no other reason, I'll send you a PM later today with the load details.

    To Muir: I take your points and if seating depth won't make that much of a difference I probably need a new powder. The range between max and min was (I think) about 6-7 grains but I was getting pressure signs after 1.5gr above starting. You asked me if I'd try to match loading data - actually I would. I only had four loads, 0.5gr between them, to test with a 1.5gr range and none of them were good. It would be good to try and generate some more possibilities though, with a whole tub of powder sitting on the shelf with no purpose to it at this point. If I could get back within safe pressure, I can get two more loads to try for every grain extra I can get in the case. I could cross my fingers that there would be an accurate load in those and try to develop it. I'm not sure I'm explaining myself very well, but hopefully you get the idea.

    To deeangeo: Thanks for your reply. I think I'm too much of a newbie at this to be anything other than conservative / cautious, so I won't be straying into above-maximum territory any time soon. I guess I am curious though - I wonder if because the loads showed pressure signs whether they are actually "out-performing" the data. I.e. I'm getting the pressure they expect with 50gr of powder with a load of 45gr - does that mean I also get the velocity associated with 50gr? I don't have access to a chronograph, but I'm curious.

    Thanks again guys.

    Adam.
    Adam,
    I just re read your original post and I was struck by the fact that you re getting pressure signs with two different powders and two different bullets. And you say that it seems to be mild despite the 'signs' you're getting? That indicates that something else might be the problem. Are you using the same LOT of brass for both? FL resized? ~Muir

  7. #7
    Quote Originally Posted by Muir View Post
    It's a kind offer but what would that ascertain? Not much I think, because quick load is a generic ballistic guide, much as reloading data is a guide. The data contained in load information is what the provider got on that day, with that LOT of powder, from their barrel, cut with their chamber. It will never transfer straight across to your rifle and your loads. My 7mm-08 topped out with Varget at 42.5 grains -which is starting load on Hodgdon's site, but a mid range load in Hornady's book. This is why there are constant reminders to "START AT MINIMUM LOAD AND WORK UP" because, "minimum" sometimes ain't, and maximum is sometimes never approached. It just works that way. When you start getting pressure signs you're pretty much done but that's OK. Unless it's being caused by something amiss in your loading technique, you are probably still getting the performance listed but using less powder to get there.

    As to relieving the pressure signs by seating out a bit. Probably won't help a measurable amount. Even if it did, what would you do then? Continue to add powder to try to match your loading data? You'd soon be right back in the same boat.~Muir

    PS: I had once a 17 MachIV that blew primer out of the case with starting loads. My best load was 15% below starting load.
    Muir. It was only meant as a data point. As I said in my post the software won't model his specific rifle etc etc

  8. #8
    Quote Originally Posted by cambsman View Post
    Muir. It was only meant as a data point. As I said in my post the software won't model his specific rifle etc etc
    I understand. I guess what I was trying to say, and said badly, is that the OP needs to deal specifically with what is happening in his rifle because his results to date are not in accordance with any data set. Didn't mean to be curt.~Muir

  9. #9

  10. #10
    Quote Originally Posted by neutron619 View Post
    I wonder if because the loads showed pressure signs whether they are actually "out-performing" the data. I.e. I'm getting the pressure they expect with 50gr of powder with a load of 45gr
    That doesn't sound correct. To be getting pressure signs at 10% below max load published in a reputable manual is unusual.

    First thing I'd do is check your scales. If you are using a beam scale, make absolutely sure you are setting it up correctly. There are some models where it is easy to confuse the settings.

    If you are using a digital scale, did you check and if necessary re-calibrate it with a test weight ? And did you check the tare of the pan hadn't drifted during the loading process ?

    If you don't have a test weight (you absolutely need one if using digital scales), then try weighing a bullet. They are normally quite close to the stated weight. E.g. a 50ish grain .224 bullet, if your scales won't cope with your .308 bullets.

    Pull one of your loads and re-weigh the powder, to be certain.

    I am very fussy, and use two digital scales. I weigh powder using one that resolves to .02 grains, then check again on a coarser scale with 0.2 grain resolution. It only adds a few seconds.

    I also weigh the completed cartridges as a final check. You need digital scales to do these checks. A beam scale is great for weighing defined powder charges, but pretty much useless for weighing anything else.

    And every time I weigh another charge I check that the empty pan still reads zero on the scale. They can drift during a session.

    Even if I used a beam scale, I would also check every load with a digital one.

    Next question, are you using .308 brass or 7.62 ? If the latter, it may have a significantly smaller internal volume due to the thicker brass.

    As for seating depth, unless you are pretty much filling the case with powder it is unlikely to have a strong effect, unless you are seating your bullets extremely deeply.

    According to Lee manual the useful capacity of a .308 case is 3.43cc. Vmd of H335 is 0.06450 cc per grain. Reloder15 is 0.07060.

    A quick look at my manuals suggests 48.0 grains of H335 under a 110gr bullet would be about max. 46.3 gr of Reloder15 for the 150 grainer. Do these numbers tally with your load data ?

    I calculate that these loads would fill the case to about 90% (H335) or 95% (Reloder15), which is pretty much ideal. My gut feel is that these loads should be pretty tolerant of varying seating depth.

    One other thought: are you cleaning the lube off the cases thoroughly? If the cases are greasy, or you have left the chamber oily, you might well see pressure signs at otherwise safe loads.

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