Gun part in need of Viagra! One of our guys had a little comp problem while shooting his AR-15 the other day. Apparently the threads were not straight on either the rifle or the comp (probably the latter). He was not injured and the rifle is probably OK too. The comp will need some minor adjustment.
You can take 3 things from this:
1. Always wear your safety glasses even when shooting long guns.
2. Stuff happens. It could have happened to any of us. We all trust threads.
3. Threads must be straight when putting on a comp. This can be checked by eyeball looking through the bore, or by using a rod. There is probably an even better way.
Using live ammo to check concentricity is not my first choice, but apparently it does show you what you need to know.