There is a variety of asbestos(about 3 main types). The grey one which was used on roofs is cariogenic, like all asbestos. I know electricians who cut it daily for decades who 40/50 years later have not gotten asbestosis as the grey one is not that harmful. But if they were cutting blue asbestos, they would be long gone.
I know people who removed it themselves and had it collected by the specialist company. Despite the grey asbestos not being the worst, you need to be careful removing it. In the UK some councils will tell you that you can remove it yourself or any builder can.
Asbesto last about 50-60 years minimum. Grey asbestos roofing is really cement with about 10-15% asbestos. It breaks down gradually, so you might not need to remove it anytime soon. Although the less broken down it is, the better.
Gun Barrel will have to be replaced eventually, as it rots unlike copper.