I have come across quite a few barrels where the threads are starting to feel weak. This is mainly because the original bolts, and some of the replacement bolts available, do not use the full extent of the thread. Nowadays, if I'm building an engine where originality is not critical, I use 3/8 BSF high tensile allen bolts. I bought a stack of these a while ago and they have proved to be invaluable as they have suitably small heads, plain shanks, and can be cut to the desired length.