Leman cranked out probably thousands of rifles in both full-stock and half-stock varieties, so no caliber would be much of a surprise (TOTW has a 33 cal currently listed for sale)- these were "factory made guns".
They also made many percussion locks which were sold "everywhere" and used by various builders.
Sometimes the rifle gets attributed to Leman simply because the lock and characteristic drum was used on an unsigned rifle.
And yes, 38 cal was somewhat common and the good thing is you can use the .375 ball used for cap/ball revolvers - so easy to find ammo.
Sorry, no idea what twist they may have used with a 38 cal..
You generally won't encounter many .38 caliber guns when you're out and about. Replica Arms once carried a "Plainsman" which was a .38 and used a .375" patched ball. Many years a ago I had a Hacker Martin rifle in .38 and it shot well. Falling between the .36 and .40, the were never hugely popular but would take most critters just short of Bambi size.
The smith who built my Leman "squirrel" rifle has owned a fair number of originals over the years including several 38's. Since I was interested in a smaller "lean" half stock we spent a lot of time talking about them and going over his pics, even if he didn't currently own one. We couldn't find a 38 cal barrel for my build and didn't want the loooong wait to have one built. And he wasn't all that thrilled with the 40 cal barrels in current stock at that moment in time. So we went with a 45 cal while trying to keep the "spirit" of the 38 cals he'd owned and others he'd handled. He created the impression in me that the smaller calibers were pretty common for Leman's eastern markets, even if larger cals went west.
Since we were playing with fantasy, I decided to go with a high grade stock after he told me he had rubbings taken from engravings on the originals he'd owned. Nope, my Leman isn't anything that would pretend to go west and wear buckskin. But I'm pretty confident a discerning "gentleman" in St. Louis and points south and east would have been proud to own it.
I have an original rifle that is "around" .38", the .375-380" balls seem to be a good fit. I would think that small caliber rifles that were carried and used far from civilization would be a good choice for an all around, use it for anything rifle, that would be easy on lead and powder consumption. That was how the rifle I have was used, and was involved in some of the "Indian wars" in Oregon after 1846. (it also traveled with the Donner party) Not really "war", but militias chasing around small bands of Natives with the occasional skirmish or shoot-out. Anyhow, I think .38 and .40" were popular at one time, for the reason of getting the most shots out of a pound of powder and lead.
Oh yeah, nice rifle. I'm not sure why a man would not take it out west, into uncharted territory, when his life would depend on it. I think Buckskin Bob would not hesitate to take such a rifle. As long as I could afford it, I'd choose it over a more plain-jane rifle. I have a very nice Jeager, but I sure don't hesitate to hunt, hike, and explore hard with it. I'd take it out West, if there still was a West.
I suspect the "craze" for larger calibers out west had all to do with Lewis and Clark's accounts of multiple shots and treeing incidents with grizzlies while using 54 calibers. I'm guessing humans were humans back then, and decisions were often made based on style and fashion more than experience and common sense.
It’s easy to determine twist. Start a snug jag/patch combo. Mark the ramrod with a pencil at the muzzle. Wrap the other end of the ramrod with a piece of tape with a tag sticking up. Slide the ramrod in until the ramrod has done a half turn. Mark the ramrod at the muzzle with a pencil. Now measure the distance between your pencil marks and multiply by 2. Odds are 1 in 48”.