Well I can tell you 100% I have seen a lot of old brass that I would describe as 'glass like;' you can easily take a piece between your fingers and snap it or, if thin enough, actually crumble it into pieces. Seen it, done it, worked with I can't even count how many guns carrying it and sure not going to shoot a 250 year old barrel made out of it. Nor do I think that proofing old barrels in any way indicates safety because the proofer, unless he is xraying or magnafluxing it or something like that, has no way of knowing what stresses, cracks or fissures he may have just aggravated.