I agree with 4seventy.

You also must consider that if the gun was properly bolted shut at the moment of recoil there would be some pressure on the bolts. This pressure would resist whatever slight pressure the shooter could have exerted on the top lever during recoil.

Also consider the time delay. The recoil is relatively slow compared to the flight of the bullet in the barrel. Think of the recoil of the slide and barrel of a 1911 .45acp - the action that unlocks that mechanism and results in cycling the gun. By the time the rifle has recoiled and enough pressure has been exerted against the toplever to open the action the bullet is long gone and pressure is down to nothing.

To me the head separation, in conjunction with the other occurrences, indicates the gun wasn't fully in battery. How it could fire in that position is a mystery to me, but I am unfamiliar with the Chapuis lock design.

I also have no faith in reloaded ammo from somebody else, no matter how "experienced". I think this gent should get some factory ammo and do the routine of tying the rifle to a spare tire (or tyre if in Oz) and pulling the triggers with strings. It will do just fine, I predict.

That's my take on it, FWIW.

Curl