Wolfgang,
I'm shooting .375s, since that's all I can find around here. They are none too tight in the chambers. The cylinder mics to 0.374" to 0.3745" depending on which chamber. I haven't slugged the barrel due to some roughness in the throat, which is hopefully shooting out. Actually, I tried to slug it back a couple months ago, but the effort wasn't too successful. The muzzle mics, groove edge to groove edge, 0.371", which is probably a little tighter than the true groove diameter. With 5 groove rifling, it's not easy to mic. I'd like to find some .380 balls.
By the way, I shot 16 gr. FFF, 1cc of Cream of Wheat, a pasteboard card wad, ball, and dollop of Bore Butter to top it off. Caps were RWS 1075s.