Undersize Sizing Dies

[AlaskaMike]

Anyone who has experience with the Lee (or other manufacturer) undersize sizing dies, do you notice whether they increase neck tension on the bullet?

I have a couple of different semi auto pistol calibers where I'd like a little more neck tension, and I notice Lee makes these undersize dies, and I'm wondering if it might be worth trying them out.

Thanks,
Mike

[Harter66]

The only thing I've measured of late has been a heard of 45 Colts dies .

Any die that I have counted on being small was nominally correct .
Every die I expected to be nominally correct has been under sized .
Only and 8×57 I needed to do a .316 neck delivered for me for a fat 7.7 that needs a .318 . Hornady neck die .
I ended up with custom bushing neck dies for a 6.5 Japanese . I needed it to be .270/1 .
308 does a nice job for a 30-30 neck die . Lee currently at hand .
I'm doing an adaptor plug to make a 45 ACP FCD a 45 Colts sizer die .
I have neck dies for a hand full of cartridges and cheat on a few more to get what I want. Like using a 223 die for a 222 neck die and the 30-30 and 308 , 7.7 and 8mm , I have a 6.5-257AI die I use for a neck die for 257 Roberts and 25-06' .

As far as under sized goes Small Base or match dies should get you there .

Where cal /cartridges are close you might even be able to use the next one down for the desired affect IE 270/7mm with just .007 , there's .02 from 6.5 to 280 but only about .01 to 270/6.8 and only .07 from 25 to 6.5 .

[Sam Sackett]

I might be a little confused here…..

If you are talking about a sizing die, that die squeezes down the casing externally. Most size down below what is required for neck tension. These dies are to make the case fit back into the chamber.

I think to get more neck tension you need to focus on the expander die. This is the die that gives final neck diameter and will govern neck tension. If you want more neck tension, you will need a smaller expander.

Or am I missing something?

Sam Sackett

[Carrier]

I use a Lee under sizing die for 9 mm. Had issues with bullet set back no matter what I tried and use Hornady, RCBS and Lyman dies. It solved the issue with bullet set back. Now some can argue that I’m doing something wrong with crimping as I tried everything I could think of and it is only with 9mm that I had the issue with.
Only With jacketed or plated bullets that I use it. With cast I don’t need to.

[jetinteriorguy]

I can’t say I’ve noticed increased bullet tension in my 9mm with the undersized die, but definitely helped with reliability in all my pistols. I have a couple with tighter chambers that have benefited greatly from this. The only way I’m aware of to increase bullet tension in pistol ammo is proper adjustment of the crimp die.

[AlaskaMike]

The reason I asked this was because I've had a couple of instances of bullet set back in .45 Auto. Apologies that I didn't mention the caliber in my initial post. Tight chambers haven't been an issue for me, just neck/bullet tension.

Thanks to Carrier for your experience with this die in 9mm with respect to bullet set back.

With the .454 Casull, some sources recommend a dedicated .454 Casull sizing die which is a little tighter than a typical .45 Colt die, the idea being that sizing the brass a couple thousandths smaller will increase neck tension. So it only makes sense that the same concept would apply elsewhere. Yes, the .454 is a roll crimp cartridge, but we all know that neck tension trumps a heavy roll crimp.

I don't want to modify my expander die because I only flare my brass just barely enough to accept the bullet. If I reduce that anymore, then it won't be possible to seat the bullet.

Sounds like I need to just buy the undersized sizing die and try it out.

Thanks for the responses.

[jetinteriorguy]

The reason I asked this was because I've had a couple of instances of bullet set back in .45 Auto. Apologies that I didn't mention the caliber in my initial post. Tight chambers haven't been an issue for me, just neck/bullet tension.

Thanks to Carrier for your experience with this die in 9mm with respect to bullet set back.

With the .454 Casull, some sources recommend a dedicated .454 Casull sizing die which is a little tighter than a typical .45 Colt die, the idea being that sizing the brass a couple thousandths smaller will increase neck tension. So it only makes sense that the same concept would apply elsewhere. Yes, the .454 is a roll crimp cartridge, but we all know that neck tension trumps a heavy roll crimp.

I don't want to modify my expander die because I only flare my brass just barely enough to accept the bullet. If I reduce that anymore, then it won't be possible to seat the bullet.

Sounds like I need to just buy the undersized sizing die and try it out.

Thanks for the responses.

Perhaps you’re over crimping it causing the case to bulge a little which would definitely mess up neck tension. This is not an unusual or uncommon thing to happen. Especially if you’ve been increasing your crimp trying to increase bullet hold while it’s actually having the opposite effect.

[David2011]

I have some undersized Lee dies for auto loader cartridges so I understand what you have. Sam Sackett’s analysis is spot on. It doesn’t matter how small the case mouth is sized (within reason) once the expander enters the case. The expander determines neck tension. Maybe a Lyman M die or similar custom made stepped expander die would achieve your goal.

[jmorris]

What is the ID of the 45 ACP die you have?

Is your low neck tension brass random from same MFG or all one in particular?

[Willbird]

IMHO neck tension can only be around .0016" on average no matter what we do. That number came from Varmint Al several decades ago and I have seen it in print elsewhere too.

The theory is if we fire a case inside a .250 neck in a chamber it will swell to fit the .250 and then when pressure drops spring back .0016" smaller, ending up .2484" outside. So if we turned our brass so our loaded round measured .249" we would never need to size necks, they would spring right back.

It also says that if we use a .250 M die and expand a case neck, that is will spring back smaller and end up .2484" inside.

I have tested this theory quite a bit over the years and it has proven to be true, now it might be .0018 or some very close number to .0016 but the basic number is pretty darn good. Otherwise you just end up using the bullet to expand the neck (Dillon IMHO loves to do this) or you size the bullet down, or some of both. No doubt the case neck anneal has some impact perhaps...annealing necks does change how much shoulder bump you get in my experience.

Now one thing we MIGHT achieve if we sized a neck smaller, say on 44 magnum, then expanded it full depth is a smaller area below the bullet that stops the bullet from being pushed deeper into the case.

And of course a smaller die might let us use mixed brass where some cases are way thinner, I used to have a bunch of RP 45 acp brass that my Star sizer die would not size enough to properly hold a bullet.

Bill

[GregLaROCHE]

I would focus on sizing the case neck, not the boolits.

[Sig]

I haven't needed a U die for 45 but needed it badly for 9mm. Using .355" J words & either Federal or Blazer brass, failed the push test every time. First I thought it was the PTX over expanding the brass. I turned down the PTX so that it was only flaring the case mouth. Still no joy. Then I swapped out my hornady sizing die for a RCBS. Still no better. The Lee U die without a doubt gave me the increased neck tension I needed. The standard sizers work fine with cast.