ATTENTION
This is exactly why people need to DO THEIR RESEARCH before they start monkeying around with their stuff. A knowledgeable gunsmith/armorer would not have just cut down the barrel without also opening the gas port to the correct dimensions. What baffles me even more is why you cut down a 16" to begin with and then had someone weld a large flash hider on there making it longer.
I am almost certain that if you were to replace the barrel (which I highly recommend) with a good 16" barrel it would start to function properly.
I would also look at getting some other types of ammo not weak stuff and seeing what happens as well.

