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Discussion Starter · #1 ·
Just a question out of curiosity. 9mm fmj bullets have a diameter of .355. 9mm cast lead bullets have a listed diameter of .356. Same for 40 smith. The fmj is .400 but the cast is .401. Why is that?

1/1000 is not much and I wonder why they even bother to list such a small difference.
 

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from what I understand (I asked my gun buddy this) is the cast ones have more "give" and need it to get a good seal in the rifling.
 

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cast size is larger because of manufacturing tolerences of the rifleing from manufacturer to manufacturer,,so that you can size down to your bore. and in some cases older guns have more variation,, .001 to .003 can shoot as is in lead but not jacketed,, and it gives a better seal if it is larger,, if too large can cause over presure,, takes more presure to get a larger bullet started,, i have several guns the same caliber but different size bore,, i cast larger and run them thru sizing die and label the boxes accordingly for the gun,,, i tend to be a bit anal about sizing .001 larger then bore,, for me i seem to get better accuracy,(kind of a hangup from competion shooting in previous yrs),,i shoot cast in almost everything i have,, is cheaper and never have to go look for the kind of loads i want,, just reach in the cabinet and pull out what i want..stay with hard cast,, and use gas-checks for magnum load and rifles,, also have to drop the speed a bit to prevent leading and stripping the rifleing as lead is to soft to maintain a grip if u push it tooo fast..for rifle stay under 2400fps,, pistol stay under 1400fps, at least in my guns and my lead recipe is what i can get by with.
 

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Discussion Starter · #5 ·
Can cast bullets really maintain an increase of 1/1000 over FMJ? Can either one maintain a tolerance of +/- 1/1000? That seems pretty tight for bullet making operations.
 

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As stated above, cast lead are usually oversize to make a better seal surface to the rifling.
The more gas blowby you have on a cast lead bullet, the more it'll lead your bore.

A tighter fitted lead slug will also usually be more accurate.

When you're casting the bullets factors like lead temp, mold temp, alloy, etc can make for pretty wide variations in the final diameter of an as-cast bullet.
But...once they're cast and cooled, you run them through a luber/sizer to swage/shave them to the final finished dimension and apply the lube so yes the diameter will be fairly consistent. Probably just as consistent as any jacketed bullet...some are pretty good, some a sloppy.
 

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Wow, some stuff I ain't heard in more than 30 years of reloading and 18 years of casting my own! Lead bullets are oversize to seal the hot gasses behind the bullet in the barrel. Hot gasses escaping around the base of the lead bullet will melt the sides of the bullet depositing lead in the bore. Jacketed bullets (and gas checked lead bullets) are not subject to melting because the copper alloy jackets (and checks) have a much higher melting temp than lead. A lead bullet of any hardness will lead the bore of the gun if it is not slightly lager (.002"+) than the barrel's groove diameter (the largest part of the barrel.

My as-cast bullets can vary by .005" but when they are lubed and sized they vary less than .001"...
 
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