I just started reloading for a 1911 in 10mm what should the measurement at case mouth be and should i taper crimp.

Figure bullet diameter (.400") plus cases thickness (.020") minus ~.002" for the crimp & it should be about .418-.419" at the taper crimped case mouth.

What type of bullet are you using? Jacketed usually get a firm crimp while lead, coated lead, or plated get just enough to remove any case flaring/bell-mouthing done & will be close to a zero crimp (.420").

.

__________________
Waiting for the break of day

...the cartridge head spaces on the case mouth. Therefore you can over crimp, especially with a soft bullet.

__________________
"He was kinda funny lookin'"

Definitely taper crimp the 10mm.

I load only home cast bullets (Lee 175 grain sized to .401"). I do not measure case mouth diameters of any ammo I load. Just follow the die manufacturer's directions, and make sure the loaded ammo chambers in your pistol the way it should.

Be careful not to over-crimp.

Taper crimp and only crimp enough to remove the belling of the case mouth and get it pressed flush with the side of the bullet. If the belling is not removed, then the edge of the case mouth can get caught on the edge of the chamber or possibly wedge into the chamber, slightly short of being fully seated in the chamber. Too much crimp and the case mouth can dig into the bullet and potentially prevent the case mouth from headspacing correctly in the chamber. Case neck tension is what will hold the bullet in place while the rounds are in the magazine and during the feeding cycle.

__________________
VCDL, GOA, NRA

I use a taper crimp with cast lead bullets. Works for me....

__________________
USMC 69-93 Combat Pistol Inst.

Brian Pearce (IMHO the most knowledgeable writer out there on handloading) wrote in Handloader #253 to taper crimp Starline brass to .4180" at mouth (p.44).