The rifle only did it with the hornady 400 gr factory ammo.
My reloads using the same cases and 340gr woodleighs never gave a problem.Over five shots they are chronagraphed at an average of 2570FPS

Once I cleaned the barrel I never had any other problems Nor did I have problems with the 340gr loads.

Tophet
The 416 Ruger is only made by one manufacturer. All the factory loads had the same batch number on the packets.

Like 450 Ackley said I think it has to do with the length of the lead in the throat combined with a build up of copper at the start of the rifling. I suspect the projectile may still be in the case neck when it arrives at the lead and this is causing the extra pressure.

I will do a measurement and check this out.

Thanks for your replies.