Alright I just recently got my Stag 16" M4.
Bought the upper complete and built the rest myself. Fired 90 WWB 55gr FMJ through it perfectly. Cleaned and lubed afterwards. I go down to the indoor range that just recently opened where I have to buy their stuff.
So they give me 60 rounds of Winchester USA 55gr FMJ 5.56mm. The Q3131.
First couple rounds fire fine, and then after that, after every round, I have to pull the charging handle. It just refuses to cycle another round into the chamber. Jammed ALOT too. Took it to the gunsmith there, he stripped it, oiled it, said it looked perfectly fine.
He tried one last thing, where he put a little oil on the 4 rounds left and they shot fine...
My gun is chambered for 5.56mm. So I go back to the desk and ask for different ammo. Luckily, he found some Federal American Eagle .223 55gt FMJ. Shot flawlessly.
YET, the next guy after me came in with a RRA Carbine, using that Q3131, and it shot fine, yet I bought 3 boxes and had nothing but trouble.
Why? My gun HATES that stuff.
Here is a pic. To recap, Q3131 shot the worst I have ever seen. Switched over to the American Eagle making no changes to the rifle, and shot 40 rounds flawlessly.
dubisteinspundloch.com/DSCF4898.JPGETA - Update on page 2. Still doing it, but American Eagle shot fine, yet didn't lock the bolt back after the last shot. It locked back fine after I pulled the charging handle. So it's not the mag. The bolt just didn't go back far enough.