Recognizing chamber pressures signs in your AR

[BORGY76]

I was watching some videos on what to watch for when getting near the upper limits of your powder charge, but this was is a bolt action. Brass should show signs right. Anymore things to check, BCG? Assuming you are not on or buried in the lands. I would say I'm planning to start .010" with the brass.

 

[GLShooter]

Swipes and flattened primers are my methods. Hardness if bras and the amount of my gas can skew your evaluation. Timing is a lot more important in the AR then many realize. I've worked with lots of wildcats and a Pressure Trace has verified a great deal of data we did by the seat of our pants. It was gratifying to see we were exactly where we wanted to be.

 

[BORGY76]

When you get to higher pressures, does the round stick in chamber . Or have you gone too far at that point if the cartridge doesn't eject?

 

[Jeff Porter]

You should know before that. Primers get flat and flow around the firing pin to make a little volcano. Swipes in the brass on the head from the ejector pin during extraction when the bolt is unlocking will give you a stop sign or at least the stop ahead sign before you blow up the brass enough to stick a case. Cycling issues and ejection angle will give you a clue as to where you are on the pressure curve too.

 

[Scott-ak]

When firing a new load from scratch I shut the gas off. When Im satisfied there are no pressure issus when static I turn the gas on and tune the rest of the system

 

[243winxb]

High pressure signs.

 

[jepp2]

@243winxb ALWAYS has great pics to share.

I have recovered a lot of once fired mil brass, and fired a lot of XM193. The most common sign I see is brass flow into the ejector opening of the bolt and loosened primer pockets. But those are not the starting signs of high pressure, those are indications of pressure being way too high.

I shoot a lot of 20 Practical in a gas gun. I don't normally load that hot, but sometimes I get velocities are over 4,000 fps for the 32 grain bullets. That is too hot (above the 55K PSI limit) and those show no signs of pressure.

I use Quickload and shoot everything across my chrono, and it gives me a very good indication of pressure. I can get to excessive pressure (more than SAAMI) and have no pressure signs. So just start low and watch for any indication of pressure and back off.

 

[jelenko]

Now, that's cratered!

 

[Twdmar]

You should know before that. Primers get flat and flow around the firing pin to make a little volcano. Swipes in the brass on the head from the ejector pin during extraction when the bolt is unlocking will give you a stop sign or at least the stop ahead sign before you blow up the brass enough to stick a case. Cycling issues and ejection angle will give you a clue as to where you are on the pressure curve too.

I would agree with all of this except for the ejection angle part. Variables in the gas system make that a much more muddled sign.
Flattened primers can vary based on the primer used and you should be looking for hard primers. I load hot for the 600 yard line and routinely get flat primers and ejector swipes but I know I am approaching the edge already and plan for it. I rarely get cratered primers though and that pushing a 80smk over 2800 in a 20". The crater may be as much a matter of a large pin hole as pressure.
Damaged case rims can be a result if high pressure but also a poor speed gas system. If the bolt is being forced back too soon because of something like an oversized gas port rim damage can occur.
The AR bolt is pretty sturdy. The m855a1 is loaded very hot and will break bolts. Your load data should be well within those limits. Follow the book data and you will be fine. If you need more speed, do a 20 cal. If you start driving it really hard, you just loosen primer pockets. I also wouldn't jam bullets and might even seat them deeper than .010" because the bolt closing the can push the bullet out of the case. You may get more pressure if you have one that winds up a little jammed versus another round that doesn't touch the rifling.

 

[Jareds]

I bought an el cheapo “Mas Defense” upper in 300 blk to use with a suppressor, went out shooting after assembling the rifle, everything went and looked fine until I started to resize my brass and noticed the first case took way way too much force to resize so I looked closer at them and every case was bulged pretty badly in the last 1/4 of the case. Not really sure what would cause that maybe a reamer wobbling around? Not sure but after seeing that I don’t trust it to shoot with the suppressor on it. If they didn’t chamber it correctly maybe the barrel threads aren’t concentric to the bore and whatnot. Now it sits collecting dust at least it was cheap.

 

[Twdmar]

I bought an el cheapo “Mas Defense” upper in 300 blk to use with a suppressor, went out shooting after assembling the rifle, everything went and looked fine until I started to resize my brass and noticed the first case took way way too much force to resize so I looked closer at them and every case was bulged pretty badly in the last 1/4 of the case. Not really sure what would cause that maybe a reamer wobbling around? Not sure but after seeing that I don’t trust it to shoot with the suppressor on it. If they didn’t chamber it correctly maybe the barrel threads aren’t concentric to the bore and whatnot. Now it sits collecting dust at least it was cheap.

Loose chamber probably. Sorta like once fired brass from am m249 that needs squeezing down to rim. Did you sharpie it to figure out where its sticking?

 

[Jareds]

I didn’t. The only thing I did was take the barrel off and put a piece of resized brass in the chamber and wobbled it around and there was a lot of wiggle room! Then put a piece of the bulged brass in and it fit just fine even with the bulge. After that I lost interest in it. I probably should’ve spent more money on a better barrel initially but lesson learned.

 

[Jareds]

I didn’t. The only thing I did was take the barrel off and put a piece of resized brass in the chamber and wobbled it around and there was a lot of wiggle room! Then put a piece of the bulged brass in and it fit just fine even with the bulge. After that I lost interest in it. I probably should’ve spent more money on a better barrel initially but lesson learned.

Instead of buying a different ar barrel I bought savage axis turn bolt with a 16 inch barrel chambered in 300 blk for the suppressor and couldn’t be happier with it. I replaced the bolt handle with an oversized knob, bought a 10 round magazine a guy makes on eBay with a 3d printer and part of factory savage magazines and a Boyd’s stock. Very quiet with subsonic ammo and the banish 30 suppressor. That’s the rifle I take out when I want to show people who don’t have any experience with suppressors how quiet a center fire can be.

 

[cem]

There are no pressure signs that show on the case at the margins.

Velocities with identical loads in similar rifles should be similar.

The classic example from bolt guns is that early writings on the 7mm STW when it was a wildcat were all by well known gun writers and all reported over pressure loads as showing no signs of excess pressure.

By the time pressure signs show on the normal case in a SAAMI chamber pressure is already too high.

There are however many exceptions with cases and chambers that look like over pressure signs but have other causes. For instance over reaming a crimped primer pocket can leave a new primer with inadequate support and looking like an over pressure sign. Extra space in the bolt face around the firing pin will leave a cratered looking primer at normal pressures - hence some bush the bolt face and so it goes.

I have a couple of cases around here someplace that were slower powder bolt gun loads in a gas gun and the rim is well and truly bent out of shape. The chamber pressure was reasonable but the port pressure was way too high. Such things can distort readings as pressure can mean chamber or port or even muzzle.

Bob Hagel and Ken Waters had differing views on pressure signs and an ongoing debate in the pages of Handloader and Rifle. In hindsight Hagel was clearly loading above SAAMI pressures while Ken Waters was doing the best he could with knowledge and facilities available to him at the time. I have a nice jig to measure case head expansion to tenths and today find the measurements interesting but too noisy to be considered a reliable pressure sign.