old colonl,
You are right to be hesitant to use a bullet too large to fit into a fired case (especially a jacketed bullet). It sounds like your rifle has an odd number of grooves;if so it is difficult to measure a "slug",because grooves are not across from each other."Rolling" the slug between the jaws of a dial or digital calipers gives a pretty good approximation of the diameter,but can be a couple thousanths off.The most accurate way is with a special micrometer, having a "V" anvil, or a common micrometer and special "V"block (included angle depends on number of grooves). You can easially measure the bore diameter with a micrometer and an expandable ball guage though.
I wouldn't worry about using any bullet that will slip into the case (which seems to be .427" or less).To prevent your loose bullet problem,the expander should be .002-.003" less than that.You can reduce the diameter by grinding with a toolpost grinder;or if you go to a 44 spc/mag expander,you can reduce it enough by spinning it in a lathe/drill press/drill motor against abrasive cloth backed by a file.
I only go through the "drill" I described with the 2 targets, etc. when I'm trying to work up the best load.After this is done, I don't worry about cooling the barrels either. Always shoot it, holding with both hands, however. All this is just my opinion,I wouldn't say anybody else is wrong.I hope it helps.
Mike
