Curious what this is needed for.
Undersized bullets?
Tight chambered gun?

 

I'm guessing this is something like small base?

I would not use anything undersize loading lead, I'd rather go the other way. My 9mm bores slug .3555-.3565 so I'm loading .358 bullets. With a tight bore and/or real tight chamber I could see a purpose, neither of those are common conditions for 9mm.

Hell I use a 38S&W expander in my 9mm dies to avoid swaging my lead bullets.

 

I have never had a need for any undersize die. The regular ones have done a good job for me for a long time.

 

Undersizing works the brass more, possibly reducing the number of times you can use it. I don't see why you would have a problem shaving bullets, since you need to expand the 9mm case mouth before you seat the bullet. What problem are you having with the standard die that makes you want to try an undersized die?

 

I use a small base on my ar10 dpms. If I use a standard die they won't chamber.

Didn't figure this out until I had Mark Hunt take a look at it.

 

I am seeking to improve the case gauging of my rounds. I currently use a Lee Carbide FS die but I have a few rounds that still have a bulge near the base and I was wondering if an an undersize die would improve that but at the same time I don't to fix one problem and create another problem elsewhere in the loading process.

Thanks everyone for your comments and feedback!

 

I have a few rounds that still have a bulge near the base

Do the ones with the bulge fit into your chamber? If they do, you do not have a problem that needs to be fixed.

 

Lee factory crimp die resizes again during the crimp, all the way to the base. It isn't undersize but it certainly helped in one of my 9mm 1911s that had a slightly tight chamber. The seating or crimping could bulge the case slightly.

You can rent chamber finish reamers if you need to get a tight chamber up to normal size.

I also use the RCBS small base AR sizing dies for 5.56 and 300BLK.

Maybe they do work the brass more, but I have tossed maybe a few dozen 9 or 556 cases for splits or cracks or other issues in many 10Ks of rounds, so I am not worried about it.

 

Probably not a fix to your problem. I dont think an undersized die will size any farther down than a std one.

 

I thought "undersized" dies were just small base dies, which means they specifically size further down than a standard die.

 

lee bulge buster is what you need

if you already have a lee FCD in 9mm, you can rig up one with a punch that will fit into the die

 

This.... the bulge buster was specifically designed to fix this issue. And the issue is typically caused by unsupported chambers... Aka: The Glock Bulge

 

Undersize means a few thousandths smaller full length.