I recently completed my first AR build, and began experiencing some cycling problems within 75 or so rounds.  I am new to the piston world, and am looking for any suggestions of what the problem may be other than what I suspect.  The two main problems I had were these: 1. The BCG would eject the casing, but not fully cycle to the rear, and would stick. 2. The BCG would over ride the next round after ejecting the previously fired casing- I believe it wasn't fully traveling to the rear before beginning its forward travel.  I believe that these issues may be cause by three possibilities: 1. Ammo- I was using American Eagle 55 grain 5.56 FMJs, but may need something that produces more gas. 2. The gas block was set for normal fire, but may need to be put on a different setting. 3. The ST T-2 (4 oz.) Buffer- May be too heavy.  These are my theories, but I am open to any suggestions as to anything else it may be.  I was unable to spend more time at the range trying to figure it out due to optics problems that were plaguing me as well and it was getting near to closing time.

Here is what I'm running:
ST Lower with a PSA LPK and ST T-2 Buffer
DSA Upper with the AA Carbine Length Piston and SMI Bolt
Radical Firearms 16" Carbine Length 1:7 Barrel with a YHM Phantom Compensator
All Magpul Furniture

If the bolt isn't going all the way back, here's what you need to do:

1. Make sure nothing is binding. With the weapon cleared, the bolt carrier should travel all the way to the rear and back forward without sticking or binding. With the bolt locked back, you should be able to use a small flat screwdriver to move the op rod rearward, and it should smoothly spring forward without any sticking or binding. Once you have everything moving smoothly, do one or more of these:

2. Switch to a lighter buffer. Heavy buffers will slow down cycling rate and reduce the total distance the bolt carrier travels.

3. If you have an extra-power action (buffer) spring, replace it with a standard-power spring.

4. Drill the gas port in the barrel larger, using a numbered drill set so you can open the hole a few thousandths of an inch at a time. Try everything else first, because you can damage the barrel if you do it wrong, as well as go from too little gas to too much. You'll also have to remove and replace the gas block every time you drill, which is a hassle.

Alright... I figure the most complex of my problem would be the buffer itself, but apparently not.  I'll see what I can do as far as a standard weight buffer goes and will post the results once I'm able to get it out to the range again.

Thanks!!!

I had similar issues, but  in my case the gas port was undersized--.063" which is the standard size for a 16" carbine barrel, but too small for a 10.5" carbine barrel. The bolt only went back halfway even with a standard carbine buffer. I've drilled out the gas port, but haven't had a chance to get out to the range to verify proper cycling yet.

Before you do anything else, I would try some different ammo.  Also, be sure to try some actual 5.56 versus .223.  For example, I frequently get a failure to hold the bolt back on the last round when using PMC Bronze -- but, PMC Bronze is notoriously underpowered.  And I'm running a standard carbine buffer weight and spring rate.

I had a very similar issue it turns out my barrel should was cut I  the wrong spot so my piston rod ended up being too long for the system causing alot of cycle issues I sent it I to adams arms and they shortened the rod now it works great. Sorry for the typos on my phone

I'm currently shooting American Eagle standard 5.56 55 grain FMJs. I got my buffer and new optics this week, and am debating on braving the cold to try again tomorrow.

I got mine working; I ended up drilling out the gas port to .101" It now cycles reliably with everything I've put through it so far.

You have a lot going on.  Start with stock parts.  Make sure nothing is binding, as others have said, and giver her a go.
A quick test for alignment is to flip her upside down and see if the gas block is aligned with the upper receiver as there should be no wobble.

Customer service at AA is GREAT!
I too was having similar problems with my AA piston upper, but Cody at AA payed to have it shipped back to AA, and repairs were made. Now she's eats any brand of ammo and runs flawless!!!
AA sets a new standard in customer service!!!

Thanks all for the tips. I swapped my 4.0 oz buffer for a standard 3.0 oz, and my troubles pretty much vanished. I was able to zero at 100 yards with American Eagle 5.56 55 grain FMJs. I changed over to Monarch .223 55 grain soft points, and they went everywhere. Needless to say, I won't be spending any more money on those. Due to having the 1:7 twist barrel, I will be experimenting with heavier rounds.

Glad to hear it was the buffer. Btw, is this a midlength system?

Edit, isn't the 1:7 intended for 55gr?

IIRC, the 1:7 was implemented to help stabilize the heavier M855 (62 gr) round.

I'm running a carbine-length system.

The 1:7 twist was meant to stabilize the heavier rounds.

1:7 is more appropriate for pistol-length barrels because velocities are lower from short barrels. At lower velocities, a faster twist rate is needed to stabilize a bullet.