Hi guys, I am using my ELISCO M16 this morning.
It has a 13.5 in barrel(converted from the standard 20 in that the military uses) I was firing about 100 rounds ion full and semi auto with no problem. But my last 40 rounds, i had some jams due to short stroking and bolt not locking open after last shot. Ammo was assorted, I noticed that the "blast" became less loud than before( almost like a squib), so I assumed its the ammo thats underpowered or defective. Or did the dirty rifle(100 rounds fired caused it). thanks.

Sounds (no pun intended) like it could be the ammo.
Was the rifle clean and properly lubed prior to your range session?
Jams/FTL could also be caused by a bad magazine.
I don't believe it was due to the weapon being dirty, I have fired 500+ rounds out of mine without cleaning, and it showed no signs of problems.