Originally Posted by spaniel

Bingo. Problem solved. It was darn hot out and you probably could not have held your hand on that barrel late in the session. Hard enough on accuracy in centerfires, a real killer in MLs. I've shot more of those bullets than about anyone, and that is the only thing I've ever seen cause that particular symptom.

Let the gun cool at least 5min between shots when it is that hot. I know it is boring and slow, but there is no substitute for patience when it is hot. Years ago before I figured this out I wasted a lot of money on powder and bullets frustrated that my guns would not group, only to see the same loads shoot fine in the fall when it was 40-50 degrees out.

Just take another gun with you and alternate. Sometimes I'll rotate up to 3 guns so I am always loading or shooting.