AR pressure signs...
by rob 
, Thursday, July 02, 2015, 14:20 (3436 days ago)
Last week I had a chance to shoot my loads in my AR with CFE223 powder and Barnes 55TSX. 27 grains was very accurate and 27.4 was almost as, so I came home loaded 10 each and today tried them again. Last week I had no issues of any kind but this week 27.4 was double feeding every one. Fed singly and they did fine for the most part but some failed to extract. When they double fed it mangled the cases. I fired 5 rounds 27, then 5 27.4 (double feeds), followed by 5 @27 (no probs) and finishing with 5 single loaded 27.4 and that's where some failed to extract. I can't imagine what this could be if not high pressure. I'd say primers look fine but that's almost a given short of extreme high pressures.
AR pressure signs...
by Cherokee , Medina, Ohio, Thursday, July 02, 2015, 15:45 (3436 days ago) @ rob
My two cents, the 27 is what your gun likes. Double feeds are generally mag related but could be other things too.
AR pressure signs...
by rob , Thursday, July 02, 2015, 16:11 (3436 days ago) @ Cherokee
Same mag throughout and it only acted up with the 27.4 load. I think the pressure is such that the case is giving sticky extraction while at the same time it has higher pressure on the gas key trying to unlock the bolt so it's stripping the bolt off the rim leaving the case in the chamber or partially extracted and trying to feed a new round while the chamber is occupied. That's my best analysis but it's not worth firing any more to figure out...can't be good on the rifle.
Those are some of the signs of over gassing
by Miles 
, CIVITATES AMERICAE, Thursday, July 02, 2015, 17:18 (3436 days ago) @ rob
If your riffle likes the 27gr load, stay with it.
Those are some of the signs of over gassing
by rob , Thursday, July 02, 2015, 18:09 (3436 days ago) @ Miles
That is what I suspected, thanks. Definitely staying with 27 grains...just curious about the double feeding and failures to extract from a learning standpoint.
Do you remember where is was throwing the brass?
On the single loaded ones that ejected of course.
Do you remember where is was throwing the brass?
by rob , Thursday, July 02, 2015, 20:38 (3436 days ago) @ cas
About 2-5 feet and a little to the front. Not too unusual. It's gotta be too much pressure because it only did it on the book max load. Last week I fired four at 27.4 grains and no issues but I was using nickel once fired brass from Winchester Razorback rounds. The primers didn't seat as deep as I like so I switched to Remington brass. I suspect the nickel cases are slicker and a bit more rigid so they ejected easier but not certain of that.
AR pressure signs...
by MDF99 , SW Ohio, Thursday, July 02, 2015, 20:14 (3436 days ago) @ rob
If rifle is a carbine short tube gas system it will eject earlier and more powerfully. It may benefit from more extractor tension in form of extra power spring or an added O-ring.
barrel length?
by bj , Thursday, July 02, 2015, 21:25 (3436 days ago) @ rob
I'm not familiar with that powder but I've noticed that powder burn rate plus bullet weight has to be matched to the position of the gas port in the barrel. With a fast burning powder and original 20" barrel you sometimes don't get enough gas pressure at the port for reliable ejection. Maybe a short barrel and slow powder could go the other way and have too much pressure at the time of ejection.
Also assuming that the chamber is reasonably clean- An AR in this country will run for a long time without problems but eventually the chamber will get dirty enough to offer too much resistance to ejection.
I've noticed a lot of products on the market related to extractors- springs, o-rings, etc. I don't know how necessary they are because a stock AR can be very reliable but maybe it is worth a little time to make sure the extractor system is operating properly.
Weather these days is very hot, which will aggravate pressures. Do you have a chrono? Your cases and/or gun may not show pressure signs but if you are getting velocities that are out of character for your load your pressures might be a bit high.
barrel length?
by rob , Thursday, July 02, 2015, 21:45 (3436 days ago) @ bj
It's a 16" Carbine. The CFE223 is a modern powder designed for AR use. Going by Hodgdons own data for this powder and exact bullet. I'm not surprised to see pressure signs...why I always work up carefully. But the gun has never had an issue except this one time and the second time out with this load. I really think the nickel brass last week compensated for the pressure. The extractor is extremely tight, no issues there. I'm just gonna stick with 27 grains...it was the most accurate of the two loads both weeks and I'm not a velocity chaser at all. I do have a chronograph and I have clocked my other AR load with H335 and IIRC it's somewhere around 2700 FPS but I'd have to double check. I usually work up a safe accurate load, confirm the most accurate powder charge and the chrono it. However, 27.4 grains may be a mild velocity by that standard alone but in my gun with a short gas 16" barrel might just be a bit much whereas an 18" mid length gun would handle it just fine...which, incidentally, is what I'm gonna build to match my lower when I get around to it and put this upper back on its original lower.