I believe the reason the fellow's middle finger was getting wacked by the trigger gaurd is because of his odd and less than firm and secure grip. No way to firmly hold the rifle with your thumb along the top lever. So on recoil the rifle is moving not in concert with the shooter but loosly, all over the place relative to how it would move with a firm and proper grip.

(Got to wonder about the firmness of the forward hand's grip on the barrels too, under the circumstances.)

With the not inconsequential recoil of a 470 and the rifle moving entirely too freely in recoil because of the weak grip, I'm guessing that the rifle opened because the shooters thumb pushed the top lever open.

Now the question becomes, "Was the action properly closed to begin with?" If yes than the reloads and chamber are both in question. If no then the head seperation could possibly, maybe have been caused or exacerbated by the extra length caused by the less than fully closed action. Either way using a set of go no-go gauges will eliminate the possibility of a too long chamber. If no gauges are available, ditch the reloads (would do that in any circumstances since time is too limited to rule the ammo ok or trouble along with all the other possibilities to be ruled out,) shoot some Federals with the rifle tied to a tire as Curl suggests and see what happens. Check the brass from that chamber and see how it fared.

If the rifle behaved for a few rounds and the spent brass checked out the I'd try with a proper grip and the Federal ammo. The fellow will probably find that the rifle doesn't recoil nearly so much with a proper firm grip. I'd also take the care to push the top lever to make sure it is where it should be before firing.

I'll second the recomendation (really a requirement) that all ammo should be checked in both chambers before a trip for game, especially dangerous game. Also, ammo ought to be checked at 90* rotation four repetitions in each chamber, you would be supprised at how much brass is out of round at the base or not square to the base.

Best of luck to the fellow in the short time available to work this through and regain confidence in himself and his rifle.

JPK