Some of the guns by that maker seem to have barrels made from seamless tubing, which is nowhere near strong enough for a firearms barrel, even with black powder. There were also allegations that some guns were never submitted for proof or proof certificates were copied and included with guns that had not been proved. Illegal, of course, but often proof laws are not enforced on guns to be exported.
FWIW, in most countries, gun parts are NOT required to be proved since they are not assembled for sale. So kit guns and the like will not be proved.
Jim