Yesterday a retired gun dealer friend of mine was given a S&W AR15 by an old customer of his who asked him to sight in a newly attached 'scope for him.
The customer also supplied 3 or 4 boxes of .223Rem for the task.
My friend arrived at the range and proceeded to attempt the job, but the rounds would not chamber fully. After many attempts and chamber cleanings....still no luck. He took the rifle to a nearby gunsmith and asked him to look for a problem.
After a short time, the solution came to light - if you'd like to try to work the answer out for yourself, I'll post it in the next 'reply' to this topic.
MP