Range Report Pressure Signs at *expected* or lower velocities?

[Grump]

Well, I'm really sick of load development now, but maybe that has a little to do with trying to do OCW groups with fuzzy mirage today.

I'm getting pretty low in the velocities from a 22-inch M1A with three powders now, and if it weren't for the primers I'd just say fooey, Federal brass is just too soft.

Anyway, here are the results:

AA 2520
FC brass
WLR primers
1680gr Noslers
About 70 degrees

42.5
2604 fps at about 7 feet
Group okay
Flat primers
I thought a lot of people drove these up to 2650 fps out of 22 or 24 inch barrels...

43.0
2667 fps
Group okay
Flat primers
one slight ejector mark

43.5
2711 fps
Lousy group
Flat primers, slightly more so than the previous two loads.
None of the three caseheads showed any marks. Huh???

44.0
2784 fps
Lousy group
Flatter primers, ejector marks on two of three

44.5
2799 fps
Lousy group
Very flat, ejector marks on two, extractor line on one, primer fell out of one of those two, and one case shows no signs other than the flat primer.

Did not shoot the 45 and 45.5 loads in the batch.

These speeds are above what was expected for the charge weights out of that barrel, but the ejector wipes came much sooner than I ever expected.

Now the 155-gr loads with Varget.

Did with FC and LC86 brass:

LC86
44.5
Velocity must be more than 1115 or 1973! (stupid chrono)
Group okay
Flat primers, one case has both extractor and ejector marks
Group hits 2.8 inches lower and 1 inch to the left of the 168-gr loads.

45.0
Velocity unknown
Group okay, vertical is superb
Velocity unknown
Flat primers, two cases have ejector marks, one of them has the flat-line extractor mark.

45.5
Velocity probably 2697
Group okay
Ejector marks on only one. Curious.

Switch to LC86 cases
still WLR primers
still 155-gr bullets

43.5 Varget
Velocity 2772
Group okay
NO marks on case other than extractor snapover on the extractor rim

44.0 Varget
Velocity 2831
Group okay
Primers look flat-max load, but not as "mushroomed" as with the Federal cases. Does not appear to be traceable to the "radius" from the casehead to primer pocket is sharper than on the Federals.

44.5 Varget
Velocity unknown
Group okay
Again, primers flat, NO traces of brass flow into the bolt's nooks and crannies.

Even tried some Sierra 175s for old time's sake remembering the not quite so good 173-gr M72/M118 military ones that held okay to 1,000 yards.

42.2 Varget
Sierra 175s
FC Brass
2622 fps
Group lousy
F210 primers
Primers flatter than I like.
Some faint ejector marks, I would normally call this overpressure.

Same as above, use Federal 210s instead
2574 fps. Same general pressure signs as with the WLR primers.

So, what's up? Shouldn't I be getting more speed before getting these pressur signs?

Thanks.

 

[chadg]

Re: Pressure Signs at *expected* or lower velocities?

tag

 

[MitchAlsup]

Re: Pressure Signs at *expected* or lower velocities?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Grump</div><div class="ubbcode-body">I thought a lot of people drove these up to 2650 fps out of 22 or 24 inch barrels...

Now the 155-gr loads with Varget.

2574 fps. Same general pressure signs as with the WLR primers.
</div></div>

The fast velocities are almost invariably bolt guns that can take pressures right at the limitations of the brass.
Gas guns need to be operated at pressures that are right when the bullet passes the gas port in the barel. Thus, different powder==different pressure curve might be necessary to achieve both good velocity and the correct pressure as the bullet passes the gas port.

 

[Grump]

Re: Pressure Signs at *expected* or lower velocities?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: MitchAlsup</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Grump</div><div class="ubbcode-body">I thought a lot of people drove these up to 2650 fps out of 22 or 24 inch barrels...

Now the 155-gr loads with Varget.

2574 fps. Same general pressure signs as with the WLR primers.
</div></div>

The fast velocities are almost invariably bolt guns that can take pressures right at the limitations of the brass.
Gas guns need to be operated at pressures that are right when the bullet passes the gas port in the barel. Thus, different powder==different pressure curve might be necessary to achieve both good velocity and the correct pressure as the bullet passes the gas port. </div></div>

Pardon me for being so dense, but I thought chamber pressures determined whether pressure signs like ejector wipe or extractor marks appeared.

I also thought that the newer reloading manuals with sections for loads in M1As using military brass listed only powder/bullet combinations which gave correct port pressures.

How is a port pressure imbalance going to affect pressure signs on the brass? I'd really like to know that one.

 

[MitchAlsup]

Re: Pressure Signs at *expected* or lower velocities?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Grump</div><div class="ubbcode-body">Pardon me for being so dense, but I thought chamber pressures determined whether pressure signs like ejector wipe or extractor marks appeared.</div></div>

Correct, chamber pressure is recorded on the brass when excessive.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">I also thought that the newer reloading manuals with sections for loads in M1As using military brass listed only powder/bullet combinations which gave correct port pressures.</div></div>

Also correct

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">How is a port pressure imbalance going to affect pressure signs on the brass? I'd really like to know that one. </div></div>

I was commenting on one reloaders inability to get bolt gun muzzle velocities from a gas gun. Now it may be that one can get those MVs and those port pressures, but the selection of powder is much more constrained in the simultaneous optimization process.

 

[Falar]

Re: Pressure Signs at *expected* or lower velocities?

I'm getting the same results with my AR10 and up to 46gr of Varget and 155gr Scenars through my 21" Stainless 1/10" Polygonal barrel. The ejector marks start to become pretty distinct beyond 46gr though the primers aren't really flat. Velocity is only in the mid 2700s and I was hoping for at least 2800. I thought gas guns would be at a 100-150fps disadvantage but now I know it is even more dramatic.