Nominal bore is .323 and .314 bullets ain't gonna work.
A single 00 buck over 5 grains of Unique will make it go bang and oughta kill beer cans up to 25 yards. Don't resize your cases for this. Just deprime and reprime by any convenient method, charge, and thumb press the buckshot into the neck of a fired case. I usually seat the ball flush and add a smear of bullet lube when loading single buckshot.
Cast bullets intended for the 8x57 will work fine. Starting loads for the 8x57 with cast bullets bullets should work, too. With a 170 grain .324 cast bullet, either 13.0 grains of Red Dot or 15.0 grains of Unique is a safe charge.
I've never had a Lebel, but I've shot a whole pile of other military bangers like this. There are likely better loads for any given rifle but these work in almost anything.