Re: Problem with ar10
Check the brass of the extracted round to see if the primer ignited. If the primer primer was struck, AND there are obvious signs of ignition inside the brass i.e.: fouling inside the brass, then you had a squib round, i.e. - no powder in the brass and primer pushed the bullet an inch down the barrel.
If you pulled the empty shell out, and the powder was all over the inside of the chamber, the the bullet kept going forward when the bolt stopped as you were chambering the round. This is unusual, but eh, it could happen, likely that under that loose of neck tension would have allowed the bullet to fall out a long before you loaded it.
It unlikely that the bullet was seated too long and got stuck in the lands. A bullet that long would not fit in the magazine in the first place. You would have had to chamber load it. AND, there is not enough tension in the lands to hold a bullet while you pull the bolt back. This scenario is not probable for a few other reasons.
Any of these is very unusual with factory rounds especially Federal GMM. You need to examine the brass and bullet very carefully and diagnose what happened.
It is generally not good to send a bullet down the barrel with one already stuck there. Im not totally sure what would happen, but I am certain you would not like the result.