Tried to attach.
It's a rcbs. Why marked 38 / 9mm

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Most dies today are dimensioned towards the minimum side of the spectrum. Sizer dies (in my opinion) size brass down more than needed. Expander dies tend to favor the smallest diameter bullets for a given range for any particular caliber. Larger bullets are accomodated by simply using (again, in my opinion) an excessive amount of belling.
If you mic the shaft of the expander plug, I wouldn't be surprised to hear that it's about .354".
The priority of die makers is to ensure that you end up with minimum dimensioned ammo purely for the sake of functioning.
They don't care if the results are compromised accuracy and shortened case life. For better or worse, they realize that most reloaders are just looking for cheap ammo that goes "bang".

My personal experience has been that special attention needs to be given to die dimensions. If you want accuracy, this is just one more area that needs to be considered. I bought a set of 44 Special dies from Redding. They were ridiculously undersized. A sized case looked like a 44-40. The inside neck dimensions were .424". The expander plug wasn't much larger. Loaded ammo was just about worthless. My carefully cast lead bullets (.432" diameter to match cylinder throats) were squashed in diameter, and the bases were visibly distorted. I called Redding and they told me the dies were within spec. They didn't care less about my concerns.
So, I made my own brass laps and used lapping compound to open up the sizer die. Eventually, I got it so sized cases had a .429-.430" I.D. I used an old Lyman M die with a stepped expander so I could press the bullets partially into a case. The resulting ammo looked better and certainly shot better. (For someone who just loads .429" jacketed bullets, my modified dies would be a problem.)
The same thing has happened for me with every revolver cartridge I've tried from 38 spl to 455 Eley. (Lee makes one single 45 sizer die that they sneakingly package for 45 ACP, 45 Colt, and 455 Webley.)

It's too bad the die makers don't pay closer attention to this. They should be specifying die dimensions in writing so customers can make informed purchases.
Conversely, most gun makers cut their chambers on the larger end of the spectrum. Again, the idea is to accomodate the "I just want it to go bang!" crowd.

Per SAAMI, 9x19, 38 Super, 38 Spl, and 357 mag all have groove diameters rhe same--nominally 0.355". If shooting lead bullet in any of those cartridges, you might need a slightly larger expander.

As to the OP's ?

Because it works for both calibers.

Cheers!

P.S. Just like you can shoot 38 Special & 9mm from the same barrel in a Ruger Convertible with two cylinders...

There is a recent post titled "9mm, the caliber for every man", or something very similar. As I recall there was included in the post a link to many cartridges, handgun and rifle, which fall within the dimensions of being 9mm. These include .38 Spl. and .357 Magnum. If you find the post it should be quite clear why the expander is so marked. Refer to STORMINORMAN's post and his comments, he is absolutely correct.

__________________
Gunsmithing since 1961

We are discussing multiple cartridges using bullets within a very small range of diameters. 9X19 is usually loaded with jacketed bullets of 0.355", lead bullets of 0.356". .357/.38 Special is usually loaded with jacketed bullets of 0.357", lead bullets of 0.358". The greatest variation in diameters is 0.003" (3/1000ths).

A neck expander with tapered "button" allowing expansion of case necks is a pretty good idea, allowing the reloader to fine-tune the work for the bullets and cartridge cases in use (not all are exactly the same).

My old Browning Hi Power 9mm pistol will readily accept jacketed bullets of 0.357" diameter and cast bullets sized to 0.358", no problems at all.

My S&W 9mm pistol will not accept cases loaded with the larger diameter bullets. This indicates a somewhat smaller chamber dimension.

Manufacturing tolerances are a fact of life. When dealing with all the variables (cases, bullets, chambers, bore/groove dimensions, magazines) the tolerances can "stack up". What works readily with one set of variables will completely fail with another set of variables.

Having the ability to produce finished ammunition using components from different manufacturers, different production lots, etc, allows us a lot more flexibility.

__________________
Life of the party until 8:00PM

I didn't use an "expander" until I had reloaded a few thousand 9mms (and an unknown number of thousands of other handgun calibers). I merely flared the case mouths. Out of curiosity, and seeing it referenced on forums, I bought a Lyman M die in 9mm, seems to work, but any better? I can't tell. My cast didn't get swaged, all my bullets got seated straight and case necks didn't split any sooner with a case mouth flare. Jes thinkin' out loud...

Thanks guys.
I found it in a drawer. I looked to see a full set i use to use. Kinda stumped me. Cant figure out why i ever bought it .
I'll go read STORMINORMAN's stuff. Thank you

I really can't take credit for the post Alk8944 mentioned, but I kinda' remember reading it...

It may be in the Ammo forum, I'm not sure...

Cheers!

P.S. I have used the Lyman M-style expander for 9mm, 380acp, 357 SIG, 38 Special & 357 Magnum upon occasion. I guess I could also use it when I reload for a friend with. 9x18 Makarov as well: he needs to get back to me with some brass 1st!

Well, I found it: it is by Forte Smitten Wesson in the Concealed Carry Forum...

Cheers!

9mm Makarov uses bullet dia of .366".
It's actually a 9.3mm caliber.
Those tiny 95gr JHP Makarov bullets make great plinkers in my 9.3x74R rifle.

Contact Us | RCBS I would ask RCBS. Photo is of my newer RCBS 9mm Luger expander. It will not double as a 38 spec or 357 mag expander.

What does your 38/9mm expander measure? The diameter measured is everything.

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I'm wondering why you feel that M-type expander would not work in 38 Special or 357 Magnum cases?

What diameter is off?

Cheers!

It is typically used to expand cases after sizing in the 38 special or 9mm luger die set before seating the bullet.    It could have been sold separately at that time. 

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So, as i suspected i bought this back when i started 10+ yrs ago, not knowing the decaper/ sizer i would also buy would bell the case itself.
A noob move on my part... Shows the process of learning.

It'll stay new & stay in a drawer until someone goes through all my chit and desposes of it.

.3528" is to small for a 38/357 mag bullet. After spring back, even smaller.

But if you wish, a .3575" or .358" bullet can act as an expander. Just keep in mind, the internet said , a soft lead bullet may be sized smaller by the case? I have not tested it.

The "M " step is only to help bullet start & alignment for 9mm, .355" to .356" bullets.

As always, free to do as one likes. Your choice.

I use .358 and .359 cast bullets in .38 Special/.357 Mag. A flare of .3577 isn’t going to work out for me.

I use Lyman M dies, RCBS cowboy expanders or NOE expander plugs for pretty much every cartridge I load.

For simplicity I have been using .357, 135 grain LRNFP in my 38 Spl and 9mm for years. The same for "Extreme" plated bullets for general shooting and games inside 25 yards! Accuracy in both cartridges is great in purpose built 1911's.

The .357 LRNFP in 9mm solved a "keyholing" issue I was having in a Kart barrel.

I have no idea what size (dia) either Dillon expander die is!

Smiles,

That .3577 measurement is at the step: the flare portion of the expander starts above that and goes way higher than .359...

Jus' sayin'...?

Cheers!

P.S. When using the M-type expander I can feel the step through the lever and the slight additional resistance above that (the flare portion). Works exceptionally well with coated lead bullets.

I prefer to be able to set the bullet in the case a bit so it can get a straight start into the case. Just relying on a flare makes it more likely the bullet will end up cockeyed. Just because a die can work doesn’t mean it’s the best option.

I shoot thousands of 9mm and .38 Special rounds every year. I’m OK with spending $10 for an expander plug specifically designed for my cast bullets. Others might feel like it’s a waste of money or unnecessary because they only shoot jacketed.

That step will always be below the flare...

When the crimp (taper or FCD) is applied the step will, most probably still be there: my flared 9mm brass will still drop into an ammo checker or pass a plunk! test...

I do prefer the Hornady bullet seating dies because they actively help to corectly orient the bullets straight into the cases.

Cheers!

P.S. I just flared about 425 9mm cases this morning.

The M step just lightly goes past the case mouth. The flare part of the expander is not needed, if using normal 9mm diameter bullets, up to a .3575" diameter bullet.

The taper crimp die removes the expanded part of the M step.

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	9MM Luger.jpg (43.4 KB, 5 views)
	RCBS9MM2021 M TYPE..JPG (59.5 KB, 5 views)