Recognizing pressure signs in a gas gun

[SABuzzard]

I'm sure this has already been covered, but since Google hasn't re-indexed the site I can't find anything.

I'm comfortable with my ability to identify signs of pressure in a bolt gun, but not so much with a gas gun. More specifically an AR 15. I'm working up a supersonic load for my 300 blk out SBR (10" barrel). What are the basics to look for? Primers are an obvious one... What else?

 

[insight3b]

I use a shell holder to gauge pressure. I just tuck it inside the ammo box when I go to the range. I check a few cases to make sure they'll still fit in a shell holder. If they still fit I'm still safe.

 

[ksthomas]

SABuzzard,

I'm in the process of building up a rifle with the same basic specs as yours (10" SBR, AR platform in 300 BlkOut) so this one caught my eye. The pressure signs for full charge loads in ARs are essentially the same as they are for any other autolader. Micing the cased head is a good place to start, and primer appearance is always something to keep an eye on (even if they're not always one to be believed). Chronographing is another, since the capabilities of the 300 Blk Out are already pretty well documented. In other words, if you're getting substantially more velocity than pressure tested loads without other pressure signs, then the velocity itself IS a pressure sign. Condition of the primer pockets are another one to pay attention to when reloading those cases. Are the pockets still snug? Does it take a degree of pressure to seat the next primer, or does it drop in with very little noticable resistance? What I'm saying is, absent the standard of bolt lift "feel" common to bolt guns, you need to take a look at a broader array of pressure signs, and take them as a whole.

My bigger concern is with subsonic loads, as this is such a departure from most loading experiences. I suspect I'll be watching my chrono more than anything else with this one, but it should be an adventure! Looking forward to starting this one once all the ATF paperwork clears. Keep us posted on what you find!

 

[Bartdog]

Pressure will also show itself as a "smear" on the bottom of your case next to the primer. If you see any marks there, you are reaching high pressures.

 

[SABuzzard]

Thanks for the replies/info

 

[SABuzzard]

SABuzzard,

I'm in the process of building up a rifle with the same basic specs as yours (10" SBR, AR platform in 300 BlkOut) so this one caught my eye. The pressure signs for full charge loads in ARs are essentially the same as they are for any other autolader. Micing the cased head is a good place to start, and primer appearance is always something to keep an eye on (even if they're not always one to be believed). Chronographing is another, since the capabilities of the 300 Blk Out are already pretty well documented. In other words, if you're getting substantially more velocity than pressure tested loads without other pressure signs, then the velocity itself IS a pressure sign. Condition of the primer pockets are another one to pay attention to when reloading those cases. Are the pockets still snug? Does it take a degree of pressure to seat the next primer, or does it drop in with very little noticable resistance? What I'm saying is, absent the standard of bolt lift "feel" common to bolt guns, you need to take a look at a broader array of pressure signs, and take them as a whole.

My bigger concern is with subsonic loads, as this is such a departure from most loading experiences. I suspect I'll be watching my chrono more than anything else with this one, but it should be an adventure! Looking forward to starting this one once all the ATF paperwork clears. Keep us posted on what you find!

Thanks Kevin. Very helpful.

I'm still below manual max loads and I've been watching velocities with a chrony. Your info, plus that from others, is a good double check for me. I'm also using a 300 blkOut Handi Rifle w/ 16" barrel to test loads at the same time. On average I get an extra 125 FPS with the Handi.

First-time hand loading for an SBR + first time using ratchet mount/QA suppressor + new caliber to me + first-time hand loading for gas gun + having to carefully select alternative components than those specified in manuals due to crazy hoarding = very interesting experience.

I'll report back once I'm able to eliminate a few variables and provide meaningful info.

Good luck with the build!

 

[SABuzzard]

Oh and I've been using "once fired" Lake City brass that was cut and necked up. I think the company I got it from did a good job, but it's not nearly as consistent as new factory 300 BlkOut brass. I can see a variety of different minor marks on the case heads from the different weapons that fired it. No idea how much, if any, impact this brass would have on the consistency of my loads, but it's just one more potential variable in the mix.

Just received some new Hornady Brass. Going to try that and then compare it to the necked p LC.