My experiences as well Phil, although my guiding experience pales in comparison to yours.
As to bench shooting, some clients cannot even do that well at 100yards, let alone further out.
3 exceptions stand out. 2 lads from England, David Chapman, a meat dealer and Dave Stutley, of Stutley Bros. gun shop. The 3rd was a chap from Hungary, who put his rifle together out of the case, clamped on the scope on then to check his zeros, proceeded to put 3 Sako factory 9.3x62's into a 1" group at 100yards exactly 3" over the bull. That was how he sighted it in, in Hungary. His offhand shot on a grizzly at 20 yards was quite properly done as well, bear just dropped.
The 2 lads from the Uk, helped me with a 2 1/2" 6 shot offhand group with an open sighted .75 calibre muzzeloading rifle (built by my Bro) using open sights, of course. The range was only 50yards, but - it WAS 50 yards & our 2 shots each landed into a 2 1/2" group on centers, 2" over the bull. Spot-on for a 100yard shot. 150gr. of powder + a 600gr. ball is not a heavy load, though.
Some clients can shoot.