I figured it out, but can't explain it. Thoughts?

[democracywins]

After building many AR15 rifles without ever having any issue, I challenged myself to build my first pistol. After all the reviews the common theme was to build around a 10" or above because of the trouble with the gas systems on the 7" uppers. However, being who I am, and liking a challenge a built a 7" upper. It was a challenge getting the gas system running right, and even after I got it dialed in it would run great sometimes but them sometimes have a failure to feed. After many hours of tweeking and testing I realized that for whatever reason this gun will not run 55gr PMC Bronze. Every failure to feed it had would be on a PMC bronze round. I find this strange because every other round I have ran through it runs perfectly fine. Including but not limited to PMC xtac 55gr and 62gr, Wolf Gold 55gr, American Eagle 55gr (black and red box), PPU 55gr, and Colt Zinc. And although I can't explain it I know what the issue is and I just avoid PMC bronze in my pistol. I just thought maybe someone who knows a little bit more than me could explain, bc to be quite honest this is very weird to me.

 

[sloughfoot]

A couple of years ago I ran about a dozen 55 grain factory ammo loads in 10 round batches over a chronograph.

Out of a Colt 20 inch barrel, they were all running 3100 to 3200 fps until the PMC Bronze was fired. It ran about 2600 fps. I fired a another 10 thinking that the chronograph gremlin had appeared, unnoticed. The second ten was also about 2600 FPS.

I concluded that PMC Bronze 55 grain FMJBT is downloaded severely. At least that lot was. If this is indeed true, it would be even worse in a AR pistol. Not enough gas to cycle would probably be the result.

BTW, Winchester white box Q1313 is by far the hottest 55 grain FMJBT out there. Including all the Mil-Surplus stuff from all over the world that I chronographed that day. Almost 3300 FPS.

My data posted here is from memory. I have the days' results saved somewhere. If you like, I could try to find it and post it. I did post it here a couple of years ago if you want to search my history.

 

[wsenefeld]

https://www.indianagunowners.com/fo.../167415-223-55-grain-loads-chronographed.html

I'd contact PMC and see what they have to say about that ammo. They list the Bronze 55gr at 3,200 fps.

democracywins, what buffer, bcg, and muzzle device are you running? I'll be assembling my 7.5" upper later this afternoon.

 

[democracywins]

Fairly certain it is a phase 5 buffer, may be omega...honestly can't remember. Bolt is a standard from JSE, here is link to one that I have JSE 7" 1x9 SS 5.56 Micro Gas Block UTG 4 Rail Pistol Float and still have standard cage on the end. Been doing a lot of research to see if I can find a cage that may help give my gun the added boost in pressure it needs to cycle pmc bronze. Because it will almost do it, but just comes up a little short...

 

[throttletony]

underpowered ammo,
also, have you looked at a pigtail gas tube?

 

[democracywins]

I have never messed with one of those. Think that may be worth it? Any recommendations on a muzzle brake that will "turn up the juice" if you will?

 

[throttletony]

I have never messed with one of those. Think that may be worth it? Any recommendations on a muzzle brake that will "turn up the juice" if you will?

Not sure if a pigtail would help or not, but I know some people swear by the (especially in carbine and pistol lengths). I was just throwing that out there as an option to consider.

Now for brakes and comps, those usually do not turn up the juice in my understanding, if anything, they suck the juice out. (I'm speaking from more pistol experience where running underpowered ammo through a comp will be detrimental to correct cycling -- with a comp you want more gas volume, whereas a brake is just to tame down recoil regardless) a muzzle brake on a rifle is different than a comp on a pistol, but they both try to get similar end results. Feel free to correct me if I'm wrong.

 

[throttletony]

Other things to consider in order of likeliness-to-screw-with-you:
Gas block properly installed?
Gas port undersized or obstructed?
No other obstructions impeding bolt cycling? (machining chatter marks that are now dirty, poor finish, etc.)

(assuming it runs other ammo well... I'd look to the ammo first of all in your case)

 

[Dwight D]

The problem with the PMC bronze is due to insufficient "time under the curve". A readers digest explanation is that the you can have 2 loads with same bullet weight and velocity but using a different propellant can cause one load to take a few micro seconds longer to reach peak pressure. The time the load stays below peak pressure is called the "time under the curve". Depending on application, this "time under the curve" can work for you or against you. For example most saboted loads perform better with loads displaying longer curves because it puts less stress on the sabot. Another example would be a Desert Eagle in 44 mag. It is very similar in design to an AR pistol. Rotary bolt, and gas operated. These are very fussy guns and are regulated from the factory for 240 gr loads as the 180's peak out too fast. I worked with one extensively and the lightest bullet I could get it to run with was a hot hand load and a 210 gr bullet. Never would run right with 180's. However if I had kept on using different propellants I might have been able to get it too run. Bottom line, nothing wrong with your weapon, it is just a combo from PMC with the lighter 55 gr bullet and a propellant that peaks out a little quick for your short barreled platform. Just 2 cents worth from an old Marine shooting instructor.

 

[wsenefeld]

I have never messed with one of those. Think that may be worth it? Any recommendations on a muzzle brake that will "turn up the juice" if you will?

The Noveske kx3 and kx5 along with the Spike's Tactical Barking Spider increase the back pressure and help alleviate cycling problems. The pigtail gas tube causes more problems than it fixes. The tube itself is longer but the dwell time remains the same. A shorter gas tube on a pistol length system will give you higher pressures. The further the gasses have to travel, the lower the pressure.