As you progress into the wonderful world of handloading, you might notice that bullets are sold as being of a particular diameter. But different diameters are available even for the same cartridge. Which diameter to choose?
For autopistols, the rule of thumb is that jacketed bullets should be the same diameter as the bore (hole through the center of the barrel) while bare lead bullets will be .001” over bore size. For instance, the bore diameter of most 9mm handguns is .355”; with jacketed bullets you’d go .355”; with lead bullets it’s .356”. Simple.
Of course, in reality there are exceptions to every rule. There was a time 9mm autopistols were notorious for having extremely variable bore diameters, which could lead to accuracy issues when fired with ammunition where bullet diameter didn’t interact well with bore diameter. Over the decades, 9mm bore consistency has tightened up immensely, so today, unless firing an old gun, it’s a non-issue.
If it IS an issue, the solution is to “slug the bore,” i.e. drive a soft lead bullet all the way through the barrel, and measure with a caliper the diameter of the resulting, swaged-down bullet. Then choose bullets of that diameter if handloading jacketed bullets, .001” over if handloading bare lead. Bullets even .001” UNDERsized for the bore down which they’re being fired can lead to lousy accuracy, at the extreme the bullet may even tumble as it travels through the air, because it wasn’t stabilized by good contact with the rifling as it traveled down the bore.
It gets a bit more complicated when handloading for revolvers, because there are TWO relevant measurements to take into account, not just one: (1) bore diameter, (2) the diameter of the chamber mouths at the front of the cylinder. According to Old West gun authority Mike Venturino, who has done an immense amount of experimentation with 19th Century firearms, both original and reproductions, ideally we want cylinder chamber mouths to be no more than .001” larger than the bore diameter, The sad truth is, frequently they’re much larger than that.
For instance, on every Second or Third Generation Colt Single Action Army or foreign-made clone in .45 Colt that Mike has ever examined, the barrel is a tight .451” (originally .45 Colt bullets measured .454” but they’ve been .451” for decades now, so they can use the huge number of bullets available for the .45 ACP) but the chamber mouths are a sloppy .456” or .457”.
Going the typical “choose lead bullets .001” over bore diameter” route (which translates to .452” in this case) gives horrible accuracy in guns with .456”-.457” cylinder chamber mouths since that allows the bullet to enter the barrel’s forcing cone (the flared portion of the barrel, located on the end facing the shooter as the gun is fired) cocked off its axis.
On the other hand, matching bullets to the cylinder chamber mouths, i.e. going .457” or .458” with lead bullets, seem a bit much from a standpoint of increased pressures, resulting from having to swage down a bullet a full .006” or .007” over bore size. Mike splits the difference and uses bullets of .454” in SAAs with mismatched bores/cylinder chamber mouths. The increase in accuracy is notable; any increase in pressures is not.
Mike’s rule of thumb: Remove the cylinder from the revolver, and drop a bullet through every chamber. If it just squirts right through the chambers and falls on the floor, that bullet is too small and will give horrible accuracy. If it hangs up in the chamber with just the nose protruding from the cylinder front, then most likely that bullet diameter will give good accuracy.
For most of us, slugging barrels and checking bullet-to-cylinder chamber mouth fit, then choosing bullets of a particular diameter based on what that tells us, is not something we’ll ever have to do. Some of us will, and for those folks it’s good to know how to analyze and solve bullet/bore/cylinder chamber mouth incompatibility issues.
