Couple weeks ago replaced handguard on one of my ARs. took it with me last week and inserted a magazine and racked a round in the chamber. Flipped the safety off and pulled the trigger and nothing happened. I then noticed that the bolt was not fully closed. Pushed the forward assist and could not get it to fully close the bolt. No problem I will just help the forward assist close the bolt. Once the bolt was closed the gun functioned normal. Problem solved, I had cleaned the bolt when I put the new handguard on so I thought I did not get something in time. Always a first. Shot the mag out and went to put another mag in and racked a round in only to find the same thing, the bolt would not fully close. So I helped the forward assist again but as an after thought I decided to look at things alittle closer and removed the the mag and tried to pull the bolt back. Nothing doing. had to help the charging handle but finally got the bolt to free up. So today I have been playing with it and thought I screwed up the bolt when I took it apart but everything looks ok. Took a spare bolt assembly and put it in the upper and function tested it and it worked fine. Put a dummy round in and it does the same thing will not fully close. Without a round in the chamber everything on both bolts works correctly. My question is when I changed the hand guard could I have changed the headspace? I did not remove the barrel or turn it.

The head space is built in to the barrel. You can remove the barrel and the head space is still the same even if your holding the barrel in your hand. Replacing or changing the forearm should not have any effect on chambering a round. Was this reloaded cartridges or factory stuff?

Did you do anything to the barrel nut?

Lube...

curious what brand carbine is it?

Factory ammo and yes I replaced the barrel nut. This set has run flawlessly until now. Bushmaster upper with a super ultra match 24" bull barrel.

even replacing the barrel nut wiht an extension, would be hard to do anything wrong.

Fired cases don't look wildly weird?

I"d take a brush and scrub the bolt locking luck recesses really good. I'd almost think that doing the change, somehow some crud got in there, and it could even be as small as a kernel or three of extruded powder if you ever had a round come apart in the chamber and had to push a bullet back out and had a powder spill in there or such, while fighting this issue.

Beyond that, if the barrel never moved, and you just swapped nuts and then the barrel was checked to still be seated in the reciever and would not budge, once the replacement nut was in place..

The other thought, when replacing nut, you still have to line up the gas tube... is the gas tube perfectly in line? If not it'll fight the part of the BCG that has to engage it... it will bend over, with pressure, and that might also be whats happening.

Trying to think back too, over many years and probably over a million rounds in ARs, I THINK I had a charging handle crack at the end... and bend up a hair and create some issues...I think...

Sounds like it is binding on the Gas tube...I would bet on it.

This makes everything better. I would check and make sure everything is well oiled first, then start looking for issues.

Gas tube dragging shouldn't make it lock up so tight that you can't eject the round. I am going with dirty chamber. Maybe some carbon or something flaked off during the forearm change out and got in the chamber.

This.

Is it possible you under torqued your new barrel nut?

Thanks for all the advice. I torqued the barrel nut to specs but will check all your suggestion. Elk hunting right now but will get back to that project next week.