will turning off gas raise chamber pressure?
- Thread starter Winny94
- Start date
No. Peak chamber pressure occurs long before gas hits the gas port and is not affected by gas port settings. So the PEAK pressure will not change.
If you are seeing pressure signs with gas on, it's due to: load too hot, gas port settings too high (overgassed and trying to extract while too much pressure still in chamber), possibly long headspace condition, or a combination of all three.
If you size your brass too much for the chamber, it creates possibility of having case slam forward too far and leave a small gap between bolt and case. After firing, primer is unsupported and can flatten a bit. Measure fired vs sized brass to see how much change you are getting in base to shoulder and if it's more than .005", back your die out a touch until sized brass is .002-.003" shorter than fired brass and see if problem goes away.
It's factory new brass. I just measured an unfired case vs my fired cases and growth at the shoulder was .0125". So that number is a bit shocking, now I'm considering just running a very low charge to fireform it all before testing charge weight.
Yep, the pressure sign is likely the growth combined with short brass letting primers back out a bit. I'd just shoot a slightly lower charge that's accurate for fireforming them work up new load with fired brass sized properly.
0.0125" growth ?
If you sized your Factory new brass that short, it's life is gone.
If that's factory sized brass your headspace is wrong or the brass is crap to begin with.
If you sized your Factory new brass that short, it's life is gone.
If that's factory sized brass your headspace is wrong or the brass is crap to begin with.
Something doesn't seem right then.
Did you make a typo with the 0.0125" ?
Might be correct but with cheapo brass I find 0.004" short, and I have a barrel than is 0.0015" long.
Once fired (with spring back) is 0.005" longer than virgin brass.
Bump it back about 0.002" giving me close to 0.003".
I fire form, then use once fired (up to 3rd fired) for matches.
Maybe someone that uses Lapua could comment.
One thing I might add, turning off the gas will increase muzzle velocity, skewing your load development.
Nope, no typo - hence my concern. Barrel maker came highly reccoemnded and obviously brass does too. I'm going to reach out to the mfg and see what is said.
AR15: Ranier Arms 16" SDM SS barrel with RA adj. gas block, shooting IMI 5.56 77gr SMK, SiCo Saker 556k.
I only shot 3 rounds with gas shut off while I was adjusting my gas block, average was 2812. Once my gas was adjusted my average was 2748 over 50 rounds. If I remember correctly each click was about 10 fps in MV loss.
Making 6mm predator form 6.5 G brass is supposed to be a Neck size only operation.
Your necked down new Lapua brass grows 0.0125" after firing.
Correct?
Is your neck sizing pushing the neck down?
Your necked down new Lapua brass grows 0.0125" after firing.
Correct?
Is your neck sizing pushing the neck down?
I experienced the same thing with some Lapua 6.5 Grendel brass. The shoulders move forward .010" in my minimum spec chamber on the first firing. I know the barrel is correct because I chambered it myself, and also the base to datum dimension of fired brass in that chamber matches my 243 LBC as well.
I didn't bother to contact Lapua since I'd already loaded most of the brass before realizing it (I hadn't initially thought to measure the new brass at that time), but I should have. About 1 in 5 experienced cracked necks after firing as well, indicating they weren't annealed properly either. That's pretty disappointing for Lapua brass IMO.
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