Hello there, I just built my first AR15. 18” 223 Wylde Stainless steel, rifle length gas system. I took it to the range for the first time and shot Hornady Frontier 55gr FMJ. No problems what so ever. Winchester, same thing, no issues. But if I were to shoot Federal American Eagle, Wolf or PMC (all 55gr FMJ) it would either shoot one or two bullets and then it won’t cycle the next. It ejects the casings fine but the BCG runs past the next round in the clip and won’t cycle. Any ideas?

Gas block seems to be lined up properly (it’s not adjustable) and buffer spring isn’t any longer than my stock M&P15. Only thing that I have noticed is that the BCG doesn’t appear to be lubricated and I haven’t even shot 150 rounds in it yet. Like I said before hornady and Winchester it shoots perfectly fine but I would like to use cheaper ammo as well.

Maybe the magazine?  Specifically the spring.
Definitely lube the bolt.
Does last round bolt hold open work?  Try one round at a time in the magazine, see if the bolt catches normally.  
White Lithium spray in the buffer tube will quiet it out nicely.  Just a quick blast.
You can verify the gas block alignment by removing the upper, take out the bolt and charging handle.  Blow down the barrel (cup the muzzle device with your hand) and with the other hand block the throat and feel airflow through the gas tube, like you're playing the trombone.

I will try the magazine next time I’m out. My M&P came with a magpul mag. So maybe that’s the answer. I bought these el cheapo mags thinking a mag is a mag. I’m ignorant. And yes when I did one round at a time the bolt will lock back the way it should. I failed to mention that once or twice with a mag and 6 rounds in the clip (cause that’s PA law??) that after the last round the bolt will be engaged in battery and not locked back. I noticed that it did that with the Wolf and PMC ammo. I will also try that method you mentioned to see if my gas block is aligned. Thank you!

You may be sitting on the edge of operation and the two brands of ammo could be slightly too weak to make for a sustained cycle.

You built it and didn't lube it? My fav bolt lube is Superlube grease. Pull the bolt out of the carrier and lube it lightly lube around the rings and down in the cylinder. Lube the whole front half of the carrier lightly. Lube the rails inside the upper receiver.

Also don't skimp on cheapo mags. Magpul, Lancer, Okay,  Metalform.

If it's overruning the next round either it's over gassed, maintenance issue, or it's a bad magazine.

Narrow down the magazine possibilities and maintenance...ie cleaning lubing first

Overgassing can be address in a couple different ways

Well I bought the upper built from PSA. So as ignorant as I am I assumed it came lube. Could be wrong. I couldn’t imagine it being lined and 100 rounds later it’s dry. Is that even possible! But I will lube it up even clean the whole rifle before I take it out again this weekend! Thanks you for your response!

Well I bought the upper built from PSA. So as ignorant as I am I assumed it came lube. Could be wrong. I couldn’t imagine it being lined and 100 rounds later it’s dry. Is that even possible! But I will lube it up even clean the whole rifle before I take it out again this weekend! Thanks you for your response!

I would like to add that I tested the alignment of the gas block by blowing air through it like mentioned from the first post! And it is working properly. My thoughts could the BCG be leaking? How do I test that if I want to find out?

New parts never come with enough lubrication to be ready to run. Always field strip a new upper or lower, clean it, and give it a good coat of appropriate lube. The BCG should look wet... not necessarily dripping but clearly wet. Any lube it had when shipped was definitely inadequate.

That said, clean, lube, test with quality mags and ammo.

You must be speaking of the public ranges located in the state game lands? What part of PA are you in? If your issue isnt magazine induced than it is a gas issue. Just because the gas port is aligned enough for you to blow air through it doesnt mean that it is fully aligned. You may have to pull the gas block after making witness marks to see the carbon print around the port on the barrel. I've had cyclical issues before from a gas block that wasn't properly set off the shoulder of the barrel.

If it's cycling properly with 5.56mm pressure and not with .223 pressure, and not locking back on empty, you have insufficient gas for that configuration of parts.

You can try another BCG to see if your particular BCG isn't gas efficient (or leaking).

The rifle may break in a little bit more still and end up being ok, but it sounds like you either have a loose BCG (need to replace to fix that), a smaller than required gas port (should be .0995 for an 18" rifle gas build), a misalignment of the gas block (pulling the block off and looking at the carbon marks, or using an illuminated bore scope are two ways to check), or something seriously causing some friction that's robbing the BCG of needed velocity to cycle.