These are just my thoughts on the issue, most have already been covered.

Ruger & Hornady where trying to get 416 Rigby, 24 or 26 inch barrel performance out of a 20 inch tube. The pressure, even with the special developed propellants has to be right on max. If this is the case any warming of the propellant- unless temp stabilised like the AR powders- is going to really be pushing the safety envelope.

The Hornady projectiles- DGS- in my 416 Rigby have a bit more push in them than my handloads with Bertram and Woodleigh pills but produce over 100fps less at the muzzle. These Hornady pills may have harder lead or jacket then the others.

The lead in the CZ550 Rigby is certainly long enough and it seems that in the Ruger it is short. This in itself is not good as far as reducing pressure.

Combine any or all of the above and problems are likely especially if the copper has really fouled an already tight barrel.

As some have not had any pressure problems with their 416 Rugers it is also possible that some barrels could be tighter or throats shorter. As con said it may pay to get the factory barrel slugged.

Another thought comes to mind. Did any of the first shots from magazine have the problem or only no2 onwards. Just thinking the projies might not of been crimped properly and have been forced back into the case and compressed the powder????

Cheers
Greg