shooting match today and all stages went well until 600 slow fire.  bolt failed to lock open for single loading w/ mag in place.  i used both South African and Q3131A ammo but same results.  changed mags and same results - still had to lock it open using charging handle.  after the match, i shot one round of black hills ammo with the mag in and it locked back properly.  this has not happened with this ammo before. any ideas?  possibly underpowered ammo?  gun was naturally dirty (my son and i were using it together in relays) but well lubed.  anyone else seenthis before?  

Normally at 600 yards we are using 75 to 80 grain bullets and that tends to drive the rifle harder than the 55 grain bullets it was designed around.

Failure to lock open is a common problem, with lots of threads already in existence on this forum.

Common issues are: Rough or dirty chamber causing sticky extraction; High pressure rounds causing stickyy extraction; Sticky bolt stop not coming up quickly enough; Sticky single loading mag not raising the bolt stop quickly enough; Worn gas rings; Loose or leaky carrier key; worn gas tube.

Good Luck!

my gas key was loose.  this happened on another AR and i cannot believe i forgot to look at it on this one![:I]   i cleaned the chamber and carrier, etc. real well before the shoot so i don't figure it was too dirty.  i'll lube the bolt stop up as well.  thanks for the help.