Hector

Those rifle barrels are massive, and would almost certainly handle the pressure of full nitro loads, but it is the action strength that would give me most concern if you were considering shooting full nitro loads.

Of course, you could do so without getting it reproved for full nitro? Remember, the British proof procedure is "semi-destructive" in nature, and IMO may permanently damage a gun, even if it passes - meaning not destroyed'!

The shape of action balls are suggestive of Webley Screw Grip shotgun action, but this is an early example, so that could be misleading.

What is the all-up weight with the rifle barrels fitted? If it is much under 11 pounds, it would be a brute to shoot, even if it did pass re-proof for full nitro loads without cracking the action.

In this context, I understand that the majority of shotguns that fail re-proof do so, because the action cracks, not because of any weaknesses in the barrel, and that is at shotgun proof pressures! So, if the action is originally for a shotgun, you would be taking a huge risk subjecting it to full .500 NE proof pressures. Remember that this is an early gun, and the steel technology is not as good as even that used on guns built after WW1 that utilsed a lot of the advances in steel technology made during that war.

IMO it may be best if you stick to "Nitro-for-Black" type loads, which BTW, are nothing to sneeze at.