how do i know what cal this bullet is?

[Tim Helms]

 

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[villagenut]

measure it

 

[BosnMate]

You need digital calipers to measure in the thousand's of an inch. Example, .38 pistol measures .357 inch, while a 45 ACP measures .452, but the lead bullet for my 45-70 measures .457. Muzzle loading bullets are smaller than bore size so they can be loaded from the muzzle, like a .45 rifle ball can be .440 or .445, while a cap and ball revolver bullet/ball is over bore size, .451. It's confusing, but you can see why to know the exact caliber needs measurements in thousands.

 

[fyrffytr1]

You can buy an inexpensive digital caliper from any local auto parts store or home improvement store. My first one came from Lowe's but only lasted about six months before it started going haywire when I tried to measure an item. My last one came from O'Reilly Auto Parts and has been a good one.
Performance Tool W80152 - Caliper | O'Reilly Auto Parts