Size to throat for autos?

[BenW]

Hey all,

I did a cerrosafe cast of my S&W M&P in 40 cal, and I want to nail down what diameter I should size to before getting a lubrisizer die. My throat diameter just in front of the chamber is 0.4027 (measured with mics). The groove diameter at the beggining of the rifling is 0.4020", and the muzzle diameter is 0.4005". If I sized to 0.403", as I am inclined to, it'd be larger than the throat diameter, and I'm already having picky chambering with 0.402 boolits.

Should I size to throat diameter, or can i go larger than throat wihout too much of an issue?

Thanks,
-Ben

[35remington]

The gun has to run. If you get picky chambering with anything over .402" why would you want to go larger?

Your pistol is not enough of a precision instrument to be worried about it to this degree. Size to .402" so the gun runs and you're groove diameter or over and be done with it.

Modification of chamber may be needed otherwise if .402" is picky about feeding as well.

[Bigslug]

Methinks it's going to depend on the boolit profile. If you've got an LFN/WFN or TC style slug that begins to narrow shortly after exiting the case mouth, you're probably OK. If you have too much of a SWC shoulder exposed, grief will probably ensue.

[243winxb]

.401 Diameter

[rsrocket1]

My M&P 40 barrel is very tight at the throat and I do have to size my boolits to 0.401" otherwise they will either have to be seated to where the cone meets the vertical part of the boolit or they won't pass the plunk test and will sometimes cause the slide to stop a fraction of an inch before going fully into battery.

My Lee 401-175-TC's drop at 0.4025" - 0.403" and I did have to size them to 0.401. With powder coating, they were close to 0.405" so sizing was a big help because if I didn't, I was having to seat deep and use the FCD which was swaging the bullets down. With the slickness of the powder coating, I was noticing that some of the bullets could be pulled out with my fingers. They would not go in any deeper, but being able to pull them that easily was disturbing to me. I now size and use the separate Lee taper crimp die without the sizing ring (https://fsreloading.com/lee-precision-taper-crimp-die-.40-smith--wesson10mm-90782.html) and the bullets stay snug.


127723

[BenW]

Thank you for the replies.

I thought about it, and figured that if I can't get that ideal .001" over groove size, I needed to go soft so obturation could help. I loaded up some rather soft rounds (should be around 1-1-98 Sb-Sn-Pb) and sized to 0.4015". I used the lee 175 TC mold, dropping around 180 grains before lubing. I seated a little deeper than I used to, at 1.135" with a 0.423" crimp (heavy for cast, but required to feed in firearm). Powder was IMR 700x at about max load.

I got no leading and accuracy was great even after 100 rounds. I was going to order a custom 0.403 die, but I think I'll stick with what I have.

Thank you for the help,
-Ben

[MtGun44]

Groove diam sizing, then set LOA to throats or even select a different boolit,
so that you can chamber. Be sure to taper crimp as a separate operation, this is
critical to getting rounds to chamber properly, in addition to LOA.

Bill