I've noticed that my MAC and Uzi bulge the bottom of brass a bit. Is it necessary or desirable to use something like the EWG or Lee undersize sizing dies to bring the base back down to size?

For regular handgun shooting, I don't bother to gauge my ammo, but it seems like it is a necessity for an open bolt gun. I wouldn't want a round to go off without being fully chambered.

Thoughts? Advice? So far I have just used factory ammo, but I reload everything else I shoot. The open bolt aspect just makes me nervous.

SAAMI is SAAMI. You just need a full length sizing die.

Might want to pause at the top of the stroke.

Except pistol dies don’t size the entire length when the chamber isn’t fully supported. Just look up Glock bulge.  Probably the same thing.  Anything shot from my Glocks won’t chamber reliably after being resized with Dillon dies. It’s an unsupported chamber for “reliability”, but it wrecks the brass to the point I hate shooting my Glocks at the same time as anything else since I have to keep my brass separate.

Google push through or roll sizer

That’s my point.

Yes.  

I believe the EGW sizes just a hair "Tighter" than the Lee, but you want to size the cases so there are no "Bulges" (i.e. from factory Glock barrels) to tie up your sub and induce stoppages.

If you don't want to use an under-sized die get a Lee Factory Crimp Die.  It effectively does the same thing, but after the bullet is seated.  You (potentially) run the infinitesimally small risk of separating bullet cores from jackets doing it this way.

<== Feeds MP-5s

I know nothing about buzz guns, but I do have a picky eating 9mm
Pistol.   That Lee FCD seems to have solved my issue.

It has a carbide ring that gives a final size.

https://www.rollsizer.com/rollsizer-information/

this will process your brass quickly for the bulge you are talking about.   when they come out of this machine they normally will gauge correctly without even sizing them..

it also has the volume to keep up with a sub gun..

i met the gentleman from Australia who makes them at the shot show, good dude..

I’ve loaded thousands of .380, 9mm, and .45 through Lee carbide dies and run them through my Macs and Thompson without any issues

I sent all my brass to this guy and haven't looked back. For what he charges I'll never process my own brass again.
It's like loading up virgin brass. I took a road trip and dropped of 150+ pounds of brass and picked it up  once he was done.
Super nice guy working out of his house/shop, making a living and providing for his young family.

https://blueridgebrass.com/brass-processing/

+1, been running a Lee FCD for over 20 years.  I run the same reloads that are run in my UZI SMG's with wide chambers back in everything.  MP5's and picky handguns.  My ammo runs in everything I own after getting put through the Lee FCD.

I should probably get a Lee FCD, but right now I just gage everything, and anything that fails goes in a bin marked 'Uzi only'.

Uzi don't care, it will eat anything. Glocks will lock up on stuff that doesn't pass gaging.

Yeah, before I got my brass prepped by Blueridge any 9mm was for the Uzi only. Open bolt SMG will just sash them into the chamber.
When covid was going rampant and 9mm tripled in price, any 9mm I loaded had to be good for anything I had.
I wasn't going to waste $100+ per thousand primers on shit brass. Thankfully I didn't pay anywhere near that but I looked at them at what replacement cost would have been at the time.
Hell, loaded 9mm is now cheaper than the components again

I would look at your loadings first honestly.  My UZI smg bulged some 9mm brass pretty badly when I was tinkering with different powders, I think Power Pistol was the worst.  Going back to different powders, and trying to keep it slow and subsonic, I don’t see the bulging much anymore (it does still mangle the brass, and my UZI brass does get separated for that gun only).  

Another thing might be bolt bounce?  Might be possible that the primer is being ignited, the bolt is bouncing back, and then it isn’t all the way in battery when it fires?  Not sure how possible that is, the UZI is supposed to ignite the primer prior to full chambering, some say that is complete BS, I have not studied it enough to know who is correct.

I do know that changing my loads eliminated almost all the bulging problem though.  Just a thought.

I have been reloading a long time.  And, used to help a guy with MAC10 and SWD M11. We had no issues using regular dies.  His set now has Dillon Sizer in station 1, Lee Undersized die in station #2, expand and drop powder #3, seat bullet, and crimp….no problems.  And, this works for him quite well. The U die is more a peace of mind thing I think on his part.

Personally, I have used either Dillon or Redding Pro Series dies loading 9MM for Glocks, Sigs, M&P, High Power Clone, colt 9mm AR, and now rra 9mm AR.  I have not had a problem with those dies and even brass shot out of Gen 1 & 3 with the polygonal rifling, and now Gen 5’s with the marksman barrel.  I know some people that scavenge brass as a range frequented by USPSA shooters shooting 9MM Major!  And, there comes the issues.  That brass needs to be roll sized. A guy actually pulled the carbide sizer insert out of a Dillon sizer because brass stuck in the die.  Looked like a hoola hoop on the brass.

There is no push through die for a 9MM because it is a tapered cartridge and needs to be roll sized.  The Redding G-RX die in carbide works for 40 S&W, 10MM, and 357 Sig only the body of course because they have straight walls The bulge buster by lee is an abomination.

The Lee Carbide Factory Crimp die is only meant to be used with Jacketed bullets NOT plated, coated, swaged, or cast bullets (including Powder and HiTek coated). I swages the bullet to a smaller diameter and promotes leading and stripping off plating or coating.  I have shot plenty of coke bottle looking rounds in 9mm and 45 ACP no issues!

You're right to be a little nervous when shooting non gaged reloads in an open bolt subgun. The first time you have a piece of brass that looks like a daisy from a round that wasn't fully chambered it will be an enlightening experience.

I had one once with an Uzi and had a piece of brass in my cheek for two years before it worked itself out to the surface. My doctor said if there was anything in there it would eventually work itself out and he was right.

This is another reason to wear proper safety glasses when shooting. Standard corrective eyeglasses are not sufficient eye protection. I'm left handed so my face was right there at the ejection port and got sprayed with brass fragments. The one that entered was right where my safety glasses met my cheek. The right lens of the glasses was peppered. Had I not had glasses on or different glasses, I might have lost an eye.

Take the little extra time needed and gage all of your reloaded subgun ammo. It will ease your mind.

I've never used under sized dies.