Possible but sounds like a trigger problem since it doubled, probably a bad disconnector.
Load one round in a magazine and fire it, it should lock the bolt back, repeat 10 times. This will tell if it's a gas/ammo problem or not.
I've not used H335 yet so I don't know about the load but it sounds reasonable since I'm looking at a 28 gr load with 50gr bullets to try myself.
It's funny, playing with different buffers, I've had the bolt beat the magazine follower and skip a round, then I've had too heavy a buffer and wouldn't lock back and was sluggish. Always seems to be the same combo it was designed around that works...
