Either make sure that the front of the carrier is touching the back of the barrel extension before you pull the trigger.  

If it not, cycle the round out and lock the bolt back, remove the gas piston, then cycle a new round it check to see if the carrier is touching the barrel extension and you can fire the round.


Hence bank that you did not get the barrel/gas block reinstalled correctly and the piston rod is binding up to prevent the carrier from locking the bolt fully closed correctly.

Just because I already had it handy on a copy to note pad currently, just for you and pay attention on the part where you have to make sure that the piston rod is not binding!!!!!  Also, take a look at the center between the feed ramps and the top lug on the barrel extension, and make sure that they are straight up and down against the lower take down lug and piston rod bushing opening; hence the barrel did not slip in the upper receiver barrel socket to cant the barrel when you where tightening the barrel nut for the new float system,and why the piston rod is now binding in the rod guide now to block the bolt from fully closing on load.

Check for a gas leak between the block and barrel that you may have caused when you removed the gas block, and causing problems now.

Also, make sure that the piston rod is not binding on the the barrel nut through channel/ upper receiver gas tube channel.

Lastly, shotgun the upper open and make sure that the barrel extension did not rotate in the upper receiver barrel socket when you tightened the barrel nut.