About powder measures - I've found, in test-weighing thrown charges, the smaller the measure's inside diameter, the more consistant the charge thrown. With 3/8" inside diameter, even 100gr. charges for large bores can be thrown or stricken that vary no more than 1/2gr.
The little calibres also add the problem of a small hole in the bore to pour the powder into and the more consistant the charge thrown, the more consistantly they hold accuracy - in a graduated degree, that is, as mentioned on the other small bore thread.
The smaller the bore, the more exact everything must be to maintain accuracy. Perfection in the ball cast and consistancy in the material used for patching. What this means, is a .50 or .54 might allow powder charges to vary 1 to 3gr. & very small defects in the ball yet still cut small groups, whereas a smaller calibre gun, like a .30 or .32 will not reward you with such slopiness.
For my .32 powder measure, I use 5/16" tubing, or trimmed down (lathe) .223 cases along with a reduced diameter pouring spout on the horn. The small hole makes pouring the powder charge into the small measure more exacting along with hitting the small bore with the charge an easier task. Due to my rifle preferring 35gr., the turned .223 case is too small, ie; too low in capacity, so I use brass tubing with a wooden plug in one end for a measure. They are very easy to make and very consistant in thrown charges.