Ed, if twist steel barrels pass the Proof test for either black powder of nitro then they are proof tested to the same standard and safety as any modern types.
For export it has to be of the same Proof standard as those for domestic U.K. sale. The onus is on the seller to ensure that it conforms and if not it is an offence to even just offer it for sale. The Police can take the prosecution but usually it would be one or other of the Proof Houses that take the case to court as they would be contacted in any case to provide expert evidence. Hope that clarifies a little. Lagopus.....
