When you measure them, you should find that the average size is around .173 or just under that. I measured a bunch a few years ago - it helps to have a bench stand for your mike! BB's are designed to have windage in a .177 bore. A .177 pellet gun that also shoots BBs has to have either a plastic clip or a magnet on the bolt to keep the BB's from rolling out the barrel.
.177 * 40/39 = .17744", so at first glance it looks like a #16 bit (.1770) is a bit too small. However, unless you have a super precise setup, you're probably drilling slightly oversize anyway (maybe .178), so the #16 bit is a simple solution.
Of course, drill bits don't really make round holes. What I did for my .177 hand gonne was to get a #16 drill blank and make a simple D-bit hand reamer out of it. I drilled with a #17 bit (.173) and reamed to .177, so now I have a round hole that is a bit smaller than I would wish, but close enough if I don't get any oversize BBs. To deal with that, I made a ball gauge that would only pass BB's less than .173.
I posted a picture of my ball gauge earlier:
http://www.gboreloaded.com/forums/index.php/topic,237021.msg1099341613.html#msg1099341613Oh yeah, I forgot to mention that I was using Daisy BB's in my initial tests. later ran some of my son's Crosman BB's through my ball gauge and found that there were more oversize BB's.
andy
