lee undersized sizing die 9mm

I have good luck with using the Lee Factory Crimp Die after seating the bullet. It's the only thing that has worked well for my revolvers which have very tight tolerances

Do the plunk test with a magic marker to determine where/why they don't fit your chamber.

https://www.shootingtimes.com/editorial/reloading-tips-the-plunk-test/99389

9mm is a tapered round, lowering the die will make everything smaller.

What are you using now, die/shell holder wise?

You supposedly can use a Lee Bulge Buster with a 9mm Makarov Lee FCD to clean up the very bottom of a 9mm Para case. I have not done this myself, yet.
But I have MAK to load so buying the FCD and Bulge Buster makes some sense.

https://www.egwguns.com/smithing-tooling/undersize-reloading-dies/

I think I had a set from these guys, with carbide inserts.

Are you reloading FMJ or cast/plated/lead?

IMO, you need to find out where your problem is first. Can you easily chamber and remove a sized and unloaded casing? If so, then take a look at your bullets as Maxxhavoc indicated. If oversized, then the FCD may help, but with its own set of considerations/controversy. If not, maybe the bullets are going in crooked. The FCD may help here also, but it is better to figure out why and fix the problem. As fxvr5 indicated, using a marker is often helpful. Excess crimp can create bulging of the case below the crimp and reduce neck tension.

For a more permanent fix, you could hone your chamber to open it up a little.

As said above, use the magic marker trick to see where the round is binding. be sure to 'paint' the bullet with marker also to see if the bullet is hitting the lands.

Other ways to diagnose if the case is sized small enough:
- size some empty brass and see if it fits the chamber.
- flare and then taper crimp the empty brass and see if it fits the chamber.

I also use a 9mmMAK factory crimp die and push the empty brass through to uniform my 9mm cases before sizing. This helps resize the area where the sizing die cannot reach.

If the empty brass fits, then you can seat a bullet and test. Did you check if the bullet is hitting the lands?

Hockey7711 said:

It's difficult to do with this gun as the barrel is not removable, and only accessible through the ejection port. I'll give it a try though.

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I'm curious - what gun?

Good question, a scorpion?

Some others:

What makes you think it has a tight chamber?

Does it run on factory ammunition?

What malfunctions are you having with your reloads?

Do you own a case gauge?

Hockey7711 said:

One of my guns has a tight chamber. I heard about the Lee undersized sizing die, but can't seem to find any available.
Does anyone know of a similar die from a different manufacturer that would also work?

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► Due to the construction of the standard Lee 9mm Sizing Die, I believe all cases come out under-sized. Lee makes a good die, but they also prevail in keeping their costs low. The 9mm case represents a unique design problem. Unlike 45ACP, 40 S&W, 38Spcl, 357Mag or other common straight-walled cases, the 9x19 Luger case has a taper. If Lee were to use a tapered carbide their cost would go up considerably. So I believe they use a non-tapered carbide ring, which works terrifically well, but leaves the 9mm case slightly undersized. SAAMI defines maximum case dimensions, and so being undersized is simply not an issue.

► If you are having issues chambering your finished rounds, due to the tapered 9mm chamber I suggest you also check your: 1) Taper Crimp and 2) your finished cartridge OAL with respect to your barrel's maximum allowable OAL. Many barrels with "tight chambers" also reduce the freebore, thus making the reloader use OALs shorter than those listed in some reloading manuals. The OAL shown in your reloading manual is NOT a recommended cartridge OAL. It is merely a report of what the test lab used. You can only safely use their OAL with that specific barrel and that specific bullet. Every other conceivable bullet-barrel combination has to be checked on an individual basis.

You may be OK if your barrel looks like the one from the test lab....



Hope this helps.
.

Hockey7711 said:

Just checked a factory round, it was .36 at the mouth and .37 after

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This is one of the issues which needs correcting. You cannot safely reload ammunition working to 2 decimal places. In reloading, dimensions as small as a human hair make a BIG difference. You'll need to gain measuring instruments that can measure to at least 3 decimal places to be successful in this hobby. You need to measure increments 10X finer than where you are currently.

When you report ".36" we can only assume you mean "0.360", which far too tight for 9mm taper crimp. In fact 0.370" is also far too tight. The best diameters for 9mm taper crimp are in the 0.379" to 0.376" range. No more, no less.

Hope this helps.

Hockey7711 said:

This is a GSG MP-40
It's not just my loads, it has also done it with factory ammo.
Could it be the chamber has very lo tolerances that even as you fire 50 rounds or so, it gets dirty enough not to chamber a new round?

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Hence, my post #16. If you are not comfortable doing this yourself, a gunsmith can easily do it.