Something to do with using a .303 bullet case to repair a busted stem. I think the cartridge was cut in half with the top half going over the broken stem. The rear end of the cartridge was cut off at the primer plate and fitted to the strummel so that the end that had the bullet head in it fitted into the case. It sort of makes sense but how true it is I have no idea.
I suppose you might need a different caliber case for the strummel maybe like a .455 revolver cartridge case or something. Something for you gun fellows to ponder the next time you sit on your Dunhill by mistake and need to make a running repair!
The way I heard this legend was that this military mount enabled the stem to be removed or put back under stress of having to repeal an attack, during WWI, when the standard rule was you didn't remove a stem from a warm pipe until it cooled completely down. Some still observe that rule, and others overlook it with aplomb, but that is the story I read somewhere. Nearly everything in piping passes through an oral history phase; little is recorded first thing at the time. I think guys (and some gals) sit around and think these narratives up while smoking their largest pipes for extended sessions. Just to embellish the tale, in Navy bootcamp in San Diego in 1968, we were drilled with those old Springfield rifles for marching and doing close order exercises. They were heavy and we spent a lot of time trying to make the wood and metal shine, which mostly it didn't. The weight made them pretty good weight bearing exercise after an hour or two of slinging them around. For actual rifle range target practice, we used WWII M-1's. Later aboard ship, they give us firing practice with the standard .45 caliber semiautomatic pistol, and that was genuinely fun, except we didn't have a target, so it was more just firing into the wake of the ship off the fantail. The Pacific Ocean was greatly intimidated.
Nothing about pipes but the 303 was insanely accurate for a rifle of its time. I think it would be really cool to see one of the great makers here to actually make a pipe using a rifle casing. Come on’, a bullet bulldog? That would be pretty cool. A whole pipe line, each named after its respective caliber. Bent bulldog 45 or 50 BMG for example.