My AR15 300 Whisper won't always chamber CTXT

[shaun1921]

I just built up a AR15 300 whisper with the following:
1. Colt Model 6721 (flattop)
2. Model 1 sales 300/221 16" barrel threaded by ADCO for Thunder Trap
3. Daniels defense free floated 12" forearm
4. JP industries low profile adjustable gas block

I took it out to the range today and used factory Corbon 220 gr ammo. When I closed the bolt it would not go fully into battery. It stoped about 1/4" - 1/2" short. I was not able to push the forward assist hard enough to get it closed. I tried to extract the round the bolt would not move back. After 10 minutes of cussing I was able to get the round out. I then hand loaded each round and about half of the time I had trouble closing the bolt. I have no idea what is happening other than I could have a chamber that is too tight. Since I am using factory ammo I thought it may be something else. Please post below or email me if you have any ideas. The only good news was the gun is very quite with the AWC thunder trap and it shot tight groups.

[GREYGHOSTt]

did the round you extracted have any rifling marks on the bullet?
what does the outside dia of a unfired round measure at the neck?

You may have a short throat. or you might have a tight chamber.

the outside neck of the loaded round should measure about .320-.322 dia.. if it is a true whisper configuration.