Run brass ammo, and see if the same problem.

Hence some new chambers are on the rough reamed side, the poly case ammo will grab more than brass case ammo, and can cause problem.

As for rig, get a few hundred rounds through it and cleanings, and the live fire/cleanings may polish up the chamber conditions that the poly case ammo is no longer a problem.


Also to point out, new rifles come with protective coating in the bore and chamber, so need to do a detail cleaning of the rig with CLP and bore/chamber brushes, to remove that before the rig in put into action.  Also, do not use Hoppes to clean the barrel.  It contains a solvent that stays behind to protect the bores, and can cause cycling and fouling problem on the quick side.


If rig does have problems with brass case factory ammo, then start looking for leaks or blockages in the gas system.  Hence gas key leaking to top of carrier, gas tube not indexed with key, gas tube leaks at FSB and so on.

As Dano already covered it, start with a thorough cleaning, making sure you get the chamber clean.  I like to make sure no solvent or lube is left behind in the chamber by swabbing it out with denatured alcohol, as this ensures there is nothing that would hinder the extraction of the empty casing.  Then make sure you soak the 7 bolt lugs with a good lubricant, add a full drop to each lug as this will ensure to reduce the friction during the unlocking cycle.

I would also suggest try inserting just the bolt carrier (No bolt, cam pin, or firing pin) into the upper receiver, and check to see that it freely moves in/out of the upper receiver and isn't binding on anything.  Specifically checking that the gas key is mating with the gas tube and not binding at all, as if it is then you may have a misaligned barrel nut or gas block that is off center to the upper receiver gas tube hole, or possibly a badly machined upper receiver gas tube hole.  Also the bolt carrier shouldn't bind inside the upper receiver, if it is you could have a badly machined upper receiver, an upper receiver that was pinched when put into a vise, or badly machined our out of alignment gas key.  If you find something binding, try and figure out what it is.

Check to make sure the gas key is tightly screwed down to the bolt carrier, as loose gas key screws happen caused lack of or improper gas key staking.

You may consider trying some quality American made 5.56 ammo from a reputable manufacturer, just for process of elimination of it being under gassed.


CY6
Greg Sullivan "Sully"
SLR15 Rifles
DefensiveEdge.com
(763) 712-0123