The government contracted with Penn gunsmiths to build rifles in 1797. These had 44 inch barrels, I believe, and were about 50 cal. Lewis had 15 of these rifles rebuilt to take on the expedition in 1803. They were cut shorter(the records don't say how short), and apparently bored larger. The lock designed for the 1803 Springfield was fitted. A rebuilt 1797 contract rifle with a 36 inch barrel in .54 cal., straight taper, would be a very well balanced and handy rifle. I don't ride a horse but I'll bet if there were any advantage in a short rifle the mountain men would have carried a 30 inch barrel instead of the longer ones found on most hawken rifles. I'm building a fullstock flint hawken with a 1" to 15/16" tapered barrel 36" long. It seems to hold and point naturally, and if it had 6" less barrel it would be very muzzle light, to my taste, not to mention ugly. But to each his own.