Pressure Signs

[zedsdead]

I have a Model 112 with a factory 223 barrel that was rechambered to 223 AI. It's a solid .5 MOA barrel shooting 69gr SMK. The load I worked up for it is 27.2gr of H4895 at 3250fps. At 27.5gr the groups were a hair smaller but I started getting pressure signs.

When I say pressure, it was a little different than what I'm used to seeing. Bolt lift was not any harder than the lower loads, but extraction was hard. I actually had to tap on the bolt handle to get the bolt to come to the rear. Is this a normal sign of excessive pressure? I'm happy with the load I have, just wondering if anyone else has seen this. The action is not timed or trued and the barrel is 26" so I assume the reamer was just ran in far enough to 'clean up' the standard chamber.

Jason

[dcloco]

Post a picture of a fired case....that will help.

Could also be the chamber just needs to be polished as well.

[GaCop]

What do the primers look like? Have you miked the cases at the head? As dcloco stated, could be just a simple chamber polish to eliminate the problem. Pictures of a fired case would really help.

[Blue Avenger]

possible, the .223AI can go overpressure very fast with a case or primer change, seating the bullet a little deeper, ect...

.223 ackley improved primer test
By Greg Idso


Two cases were selected for the test, Federal Gold Medal match and BHA match. All cases were twice fired in the improved chamber to insure a good fit. Pockets and flash holes had been uniformed.15 cases from each brand were sorted out by weight and then sorted again by measuring just ahead of the case head with a calipers. The case was then set in three groups of five for each brand. The primers selected for the test were CCI 450, CCI BR4, and Remington 7 ½. These three were selected for their thick cup to resist high pressures. Five BHA and five Federal cases were primed with each of the primers. The next step was to charge all cases with 27.5gr of Hodgdon Bench mark and top with a Nosler Ballistic Tip bullet seated for an OAL of 2.324.
Range day was not bad for the country I am in. Sunny, 50 degrees with a 7mph wind. Target at 100’ and the rifle on sandbags. The rifle use was a savage 10 action with a Adams & Bennett 24” barrel, Rifle Basix sav 2 trigger, Bell & Carlson stock, Burris 2 piece base, signature rings with +40 inserts and an 8-32x44 Burris Signature scope.
The BHA match brass went first.
CCI 450 primer- 3502 velocity fps with a .823 group
CCI BR4 primer- 3512 velocity fps with a .625 group
Rem 7 ½ primer- 3518 velocity fps with a 1.04 group

Next the Federal Gold Medal brass
CCI450 primer- 3530 velocity fps with a 1.26 group
CCI BR4 primer- 3528 velocity fps with a 1.39 group
Rem 7 ½ primer- 3504 velocity fps with a 1.16 group

3515 fps avg. velocity for all loads tested

The first thing you see is the primers all showed different velocities. Having no way to check the flame from the primes one can only speculate at this point what physically happened during ignition. The second thing is the different case also showed a change in the velocities. Case volume? The BHA case weighed 98.7gr and held 32.9gr of water. The Federal case weighed 99.8gr and held 32.5gr of water. So what we find out is with the same powder charge the BHA case has 0.40 more empty space in it. The lack of space caused a raise in velocity in the Federal cases.
Physics being what they are and all things being related there was a pressure rise with the increase velocity. This showed itself in a couple ways. When the primers were inspected the crater left by the firing pin was gone. In its place was a ring showing the metal was blown back into the firing pin hole On all loads. OVER PRESSUR! The load manuals showed a speed of approximately 3250fps. My avg. for any load tested was3515fps. OVER PRESSURE! Next I mentioned I measured the cases when I selected them. After firing the case were remeasured with the digital calipers. .0002 Is about all the growth you want to see. This is the area in the case with the most brass.

BHA case before firing .3730

CCI 450 after firing .3755 + .025 3502 fps
CCI BR4 after firing .3765 + .030 3512 fps
Rem 7 ½ after firing .3765 + .030 3518 fps

Federal cases before firing .3725

CCI 450 after firing .3760 + .035 3530 fps
CCI BR4 after firing .3790 + .065 3528 fps
Rem 7 ½ after firing .3795 + .070 3504 fps

On all loads you can see that as the velocity went up the pressure caused the case to swell. The Remington 7 ½ primer in the in the Federal case shows an exception, why did it have the most case growth with such low velocity? My guess would be the venting when 4 out of 5 primes blew out. OVER PRESSURE?

[Nor Cal Mikie]

Were you full length sizing the brass or neck sizing? Are the bullets set at a "jump" or "jam"?
One thing that I've run into on my BR rounds is "If the bullets are jumped to the lands, I will see high pressure signs before there actually is high pressure. (stiff bolt lift and cratered primers). That same load with nothing else changed but the bullet jamed about .010 into the lands won't show any pressure signd and you can even increase the powder charge with no ill effects. Other folks havn't see this happen but I have so, YMMV.

[kelbro]

I would polish the chamber a little. The 223 AI brass is a little more 'square' resulting in more sheer force required to remove the fired brass.

FWIW, I used CHE/PRE measurement for my 223AIs and it correlated with the readings on an Oehler M43 system. Some folks discount the method but I now add it to my toolbox when working up loads.

[zedsdead]

Thanks for all the replies.

The brass I was using once fired and that was fire forming. I neck sized them after fire forming. I took some measurements just above the webbing after firing and the ones that were sticky measured about .0005 bigger than the ones that extracted fine. I can't remember the exact number. The bullets were jumped .020-.030. Sorry I don't have any pictures but you guys talking about polishing the chamber reminded me that the fired cases look a little dull, like maybe the chamber is a little rough.

How does an average Joe go about polishing a chamber? Or is it something better left to a smith?

Thanks again for all the help.

[Nor Cal Mikie]

Think more about "full length" resizing before you get after polishing the chamber.
And if you're neck sizing only, you might need to bump the shoulder back a bit.
You can do it with the full length die. Just don't run it down to a hard cam over and see what you get. Anything is worth a try.