Several months ago I picked up a tired-on-the-outside but shiny-on-the-inside '95 Winchester in .38-72, not one of the hottest selling cartridges in the world, but having some experience with its smokeless near twin the British .375 2-1/2" Flanged it seemed an intriguing prospect.
I expected some of the usual neck/throat/groove diameter issues that many American BP lever guns have and I certainly wasn't disappointed. Groove diameter is right at .379" with the throat being a smidge larger, while the chamber neck even with very thin brass doesn't leave enough room for a bullet that diameter. American BP .38 rifle cartridges are typically this groove diameter or a bit larger. Same kind of thing has happened to me with several original .40-60 1881 Marlins and even a couple of older British BPEs using modern thick-walled brass.
I do like correctly headstamped brass when it's available, so I started with a score of Bertram cases. These were fairly uniform with neck walls averaging .011", thin enough, I hoped, to allow chambering of the .377" soft-jacketed Hawk bullets I figured to try first. Along with all this stuff I had ordered a set of RCBS .38-72 dies knowing that they would almost certainly be set up with .375" bullets in mind. The good news is that it is now possible to get 'cowboy' neck expander dies in .376" and .379" diameters; I got those too while I was at it.
The bad news was that the seating die wouldn't accept a .377" bullet. This was a surprise - usually the 'throat' of a typical seating die isn't as closely cut as that, at least with the bunches of dies for other calibers that I've used over the years to cobble together some fairly odd ammo. One way or another, though, I managed to get a few .377" bullets seated into those cases over a mild charge of 5744 for a first trial.
The rounds chambered without excess force but I was troubled after firing the first when the freshly fired case would not accept a .377" bullet easily. This told me that even with .011" neck walls there was not enough clearance in the chamber on firing to allow the bullet to release well. Not good. Often in this situation I might think about neck reaming to thin the walls more, but at .011" they're already pretty thin and getting a reamer or outside turner for that odd diameter wasn't going to be simple.
I usually don't figure that modern jacketed bullets over smokeless powders will "bump up" to fill oversize grooves like lead bullets over BP will do, but the folks at Hawk were confident that in this situation it just might work. Seemed simple enough to try.
Well, to wind up this long-winded post, I got some of their 235 grain .375" diameter bullets which fit both the dies and chamber well and on my first try at a load proceeded to make an inch group at 50 yards using old eyes and older iron sights. Not bad. .375" bullets slid in and out of the fired cases just fine. Heck, the rounds even fed nicely out of the '95 box magazine which the usual .38 rifle flat point won't do so well. It may not be a lion load like Teddy's .405 '95 but it ought to be a peach to keep the pot filled on any continent...