How does changing brass effect load?

[m1ajunkie]

I have a .308 load that I am pleased with so far.

43gr varget
winchester primer
175gr smk at 2.8" avg
WCC nato marked brass

This load gets about 2560fps out of my 20" lmt mws. Shows no pressure signs at all.

The problem is I only have fifty pieces of the WCC brass to load.

What kind of change can I expect if I swap this load over to ppu brass from their 175gr match load? I have lots of that brass and I'm always shooting it to get more. Would it be safe just to load an exact duplicate of my good load but just put in the ppu brass, starting off at 43grs?

I know I can't tell until I shoot it, but would it be reasonable to expect similar results?

 

[ChadTRG42]

Re: How does changing brass effect load?

You will have to compare the case volume between the 2 to know. If the PPU has more case volume, then you will need a little more powder to compensate for the additional case volume, and vise versa. You can measure this by filling up the case with water and weighing each one. The only true way to know is to start low and work up, and shoot it and see.

 

[MitchAlsup]

Re: How does changing brass effect load?

Changing brass changes the load the same way that changine compression in an automobile engine changes the power output.

 

[m1ajunkie]

Re: How does changing brass effect load?

Thanks for the help. I was hoping it would be an even trade.

 

[Greg Langelius *]

Re: How does changing brass effect load?

I would fill one case with the intended powder, weigh it; fill the other, weigh it, and compare. If the new brass holds less, you should drop the charge and work up; and vice-versa.

 

[crackerbacks]

Re: How does changing brass effect load?

Is it possible to maintain the same velocity if you have less case capacity? I shoot alot of FGGM brass but I recently got a bunch of LC brass and I know it has less capacity, therefore I should lower my powder charge. is it still possible to obtain the same velocity with less powder or does more case capacity equal more powder and more velocity?

 

[Greg Langelius *]

Re: How does changing brass effect load?

As long as you're not working with extreme pressures, you should be able to.

Just understand that the same velocity may or may not involve the same bore transit time, and could affect POI and grouping.

I do not load to a velocity, I load to a group size.

Incidentally, 50 cases is my usual loading batch size. My matches require 40 rounds for score. Ten more is plenty for fouling/sighting. It's my preference to load and shoot my handloads within a couple of weeks at most. I don't like having handloaded ammo on the shelf for long periods.

Note that MilSpec require about an additional 5gr of brass in the cartridge base area. Such brass should be marked 7.62. Commercial brass should be marked .308.

I stay away from brass that requires any kind of special ordering, etc. Each year I buy a batch or 100 or 200 commercial cases <span style="font-style: italic">off the shelf</span> from the same lot, and keep them segregated in handloading batches.

Greg

 

[samnev]

Re: How does changing brass effect load?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ChadTRG42</div><div class="ubbcode-body">You will have to compare the case volume between the 2 to know. If the PPU has more case volume, then you will need a little more powder to compensate for the additional case volume, and vise versa. You can measure this by filling up the case with water and weighing each one. The only true way to know is to start low and work up, and shoot it and see. </div></div>
+1 I would do the volume mesurement with water rather than powder since with powder you can never be sure you filling the voids between powder grains the same way in both cases. The water method avoids the problem. Assuming you don't spill any along the way..

 

[Grump]

Re: How does changing brass effect load?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: crackerbacks</div><div class="ubbcode-body">[snip] I shoot alot of FGGM brass but I recently got a bunch of LC brass and I know it has less capacity, therefore I should lower my powder charge. [snip]</div></div>

Do you REALLY know?

Unless you have truly measured both, you don't.

I have a few hundred FC cases that are virtually identical in internal case capacity to three separate lots of USGI brass. It's more recent production. 55.2 to 55.9 grains water, fired from the chamber.

The USGI stuff runs 56.0 average, more than one sample done.

Lapua was 56.5

Older FC was 57.7

Hornady Match was 58.7

Using Varget and a 175 SMK, QuickLoad puts a difference of 22 fps and +2,653 PSI (piezo) when modeling a 1.0 grain difference in water capacity. Velocity for a 22-inch barrel. With that powder charge (45.0, not corrected for real-world results from MY rifle), the smaller case capacity predicts overpressure by 1,083 PSI. Predicted M1A port pressure is 15,653 Piezo, which from my cross-referencing about equals 17,500 CUP (which is what the military used for its measurements/specs), fully 3,000 PSI over max spec.

The very mild overload velocity is predicted at 2659 fps.

Hope that puts it into perspective.

 

[crackerbacks]

Re: How does changing brass effect load?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Grump</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: crackerbacks</div><div class="ubbcode-body">[snip] I shoot alot of FGGM brass but I recently got a bunch of LC brass and I know it has less capacity, therefore I should lower my powder charge. [snip]</div></div>

Do you REALLY know?

Unless you have truly measured both, you don't.

I have a few hundred FC cases that are virtually identical in internal case capacity to three separate lots of USGI brass. It's more recent production. 55.2 to 55.9 grains water, fired from the chamber.

The USGI stuff runs 56.0 average, more than one sample done.

Lapua was 56.5

Older FC was 57.7

Hornady Match was 58.7

Using Varget and a 175 SMK, QuickLoad puts a difference of 22 fps and +2,653 PSI (piezo) when modeling a 1.0 grain difference in water capacity. Velocity for a 22-inch barrel. With that powder charge (45.0, not corrected for real-world results from MY rifle), the smaller case capacity predicts overpressure by 1,083 PSI. Predicted M1A port pressure is 15,653 Piezo, which from my cross-referencing about equals 17,500 CUP (which is what the military used for its measurements/specs), fully 3,000 PSI over max spec.

The very mild overload velocity is predicted at 2659 fps.

Hope that puts it into perspective.

</div></div>


Wow, I think my brain hurts!!! I appreciate the info, I will have to do my own testing. Is the overpressure of 1083psi considered extremely dangerous or will maost guns handle it?

 

[Grump]

Re: How does changing brass effect load?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: crackerbacks</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Grump</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: crackerbacks</div><div class="ubbcode-body">[snip] I shoot alot of FGGM brass but I recently got a bunch of LC brass and I know it has less capacity, therefore I should lower my powder charge. [snip]</div></div>

Do you REALLY know?

Unless you have truly measured both, you don't.

I have a few hundred FC cases that are virtually identical in internal case capacity to three separate lots of USGI brass. It's more recent production. 55.2 to 55.9 grains water, fired from the chamber.

The USGI stuff runs 56.0 average, more than one sample done.

Lapua was 56.5

Older FC was 57.7

Hornady Match was 58.7

Using Varget and a 175 SMK, QuickLoad puts a difference of 22 fps and +2,653 PSI (piezo) when modeling a 1.0 grain difference in water capacity. Velocity for a 22-inch barrel. With that powder charge (45.0, not corrected for real-world results from MY rifle), the smaller case capacity predicts overpressure by 1,083 PSI. Predicted M1A port pressure is 15,653 Piezo, which from my cross-referencing about equals 17,500 CUP (which is what the military used for its measurements/specs), fully 3,000 PSI over max spec.

The very mild overload velocity is predicted at 2659 fps.

Hope that puts it into perspective.

</div></div>


Wow, I think my brain hurts!!! I appreciate the info, I will have to do my own testing. Is the overpressure of 1083psi considered extremely dangerous or will maost guns handle it? </div></div>

I'm not saying that anything which is over spec is safe. Personally, I prefer to not go there.

But since I had two sources 10+ years ago say/hint that normal rifle caliber brass caseheads start to "flow" but not fail at somwhere like 73KPSI, and we have lots of people backing off their "max" loads only .3 or .5 grains of powder charge (.308 at least) once they see "ejector marks", I believe that the rifle is not the limiting factor.

I don't trust the brass. It's getting re-used.

And most of the legendary Glock kB! events reported here on the InterWeb are really just case wall blowouts (split barrels have indeed happened, but far less often and at least one was a poorly-executed aftermarket barrel anyway), two reasonable conclusions are:

1. The brass case really is the limiting factor; and

2. The firearm need not violently disassemble itself to give a shooter a very, very bad day. Gas release from case failures is to be avoided if you don't like bruises, bulged magazines and possible blindness. All of those have happened without the rifle being the reason--and the damage to many firearms has been caused by the gas release, rather than being the cause of the gas release.

 

[Greg Langelius *]

Re: How does changing brass effect load?

I overfill with a drop tube for case volume measurement and average ten drops. Consistent enough for my purposes.

Greg

 

[crackerbacks]

Re: How does changing brass effect load?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Grump</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: crackerbacks</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Grump</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: crackerbacks</div><div class="ubbcode-body">[snip] I shoot alot of FGGM brass but I recently got a bunch of LC brass and I know it has less capacity, therefore I should lower my powder charge. [snip]</div></div>

Do you REALLY know?

Unless you have truly measured both, you don't.

I have a few hundred FC cases that are virtually identical in internal case capacity to three separate lots of USGI brass. It's more recent production. 55.2 to 55.9 grains water, fired from the chamber.

The USGI stuff runs 56.0 average, more than one sample done.

Lapua was 56.5

Older FC was 57.7

Hornady Match was 58.7

Using Varget and a 175 SMK, QuickLoad puts a difference of 22 fps and +2,653 PSI (piezo) when modeling a 1.0 grain difference in water capacity. Velocity for a 22-inch barrel. With that powder charge (45.0, not corrected for real-world results from MY rifle), the smaller case capacity predicts overpressure by 1,083 PSI. Predicted M1A port pressure is 15,653 Piezo, which from my cross-referencing about equals 17,500 CUP (which is what the military used for its measurements/specs), fully 3,000 PSI over max spec.

The very mild overload velocity is predicted at 2659 fps.

Hope that puts it into perspective.

</div></div>


Wow, I think my brain hurts!!! I appreciate the info, I will have to do my own testing. Is the overpressure of 1083psi considered extremely dangerous or will maost guns handle it? </div></div>

I'm not saying that anything which is over spec is safe. Personally, I prefer to not go there.

But since I had two sources 10+ years ago say/hint that normal rifle caliber brass caseheads start to "flow" but not fail at somwhere like 73KPSI, and we have lots of people backing off their "max" loads only .3 or .5 grains of powder charge (.308 at least) once they see "ejector marks", I believe that the rifle is not the limiting factor.

I don't trust the brass. It's getting re-used.

And most of the legendary Glock kB! events reported here on the InterWeb are really just case wall blowouts (split barrels have indeed happened, but far less often and at least one was a poorly-executed aftermarket barrel anyway), two reasonable conclusions are:

1. The brass case really is the limiting factor; and

2. The firearm need not violently disassemble itself to give a shooter a very, very bad day. Gas release from case failures is to be avoided if you don't like bruises, bulged magazines and possible blindness. All of those have happened without the rifle being the reason--and the damage to many firearms has been caused by the gas release, rather than being the cause of the gas release. </div></div>

Sounds like trying so squeeze out an extra 50fps isn't worth the risk.
Thanks for the info!