Probably proofed to modern 12 bore shotshell smokeless pressures in plastic or paper cases - both are available in England - I am not aware of any commercial loads today made up in Brass for sale for these guns - it's possible, of course. I know there are specialty shops, but that isn't the same thing at all.

There is no distinction between thick brass and thin - brass is all thin - paper and plastic are thick.

Seems to me the BP proofs run around 8,000LUP for normal loads, while the smokeless proof means loads developing up to 12,000LUP - are suitable. This, for smoothbores, at any rate. Perhaps someone else knows if bore rifles are proofed higher than this.

A 12 bore would need upwards of 7 drams black powder to meet the BP proof load's working load, while the higher, smokeless proofing would allow modern shotshell pressures - these guns are not proofed for normal Nitro-express rifle pressures with smokeless loads - only shotshell/slug loads with smokeless - far as I know - which knowledge is limited in this regards.

All modern shotguns, rifled or smooth, are proofed for modern shotshell loads with shot or slug - and that's 12,00LUP max pressure. All factory shotshells are held to about 11,500LUP - whether it's a trap load or a 3 1/2" magnum. ALL shotshells are held to the same breech pressure. Low brass trap loads develope identical pressure with the high brass 3 1/2" magnums- they are equal in pressure, but not in payload. Most handloaders know this.

If a gun has 2 1/2" chambers and 1/4 turn in 26" equalling 104" twist & is nitro proofed, I'd be using whatever load regulated with either black powder or smokeless or both. Suffice to say, you will not be able to buy ammo that will regulate - or produce max pressures for that matter. Just because an old gun withstood the nitro proof, does not mean it's regulated for that load. It was regulated for the BP load and the new proof just says you can use smokeless in it for increased performance. Hitting isn't in the equation.

I've a friend with a 10 bore 2 7/8" Wesltey Richards, like new condtion, he icked up back in the early 70's. He sent it to H&H to be checked over & a new case made for it. The told him it was the original finish and the tightness of the action would loosen up with some shooting as the gun had hardly been fired. They also asked to nitro-proof the gun, to which he answered positively. They reproofed it to nitro standards, which means he can shoot modern smokeless shotshells in it or handloads duplicating those and is not restricted to using black powder loads - that's all. Smokeless developes only a bit more pressure than black - 3,000LUP or so which isn't much - but - it has a different pressure curve which differenciates it from black powder.