I want to reload several thousand 9mm cartridges using 124gr FMJ bullets for a sub-machine gun. Is there a special process for doing this versus loading for a handgun? Thanks.

I would run a small sample through the gun to make sure what your load will function.
You might load some dummies without primers to check feeding by hand.

That said I loaded 14 boxes of 45 ACP one time without a gun to test or check the funtioning.
I went to the range with a friend that owned a Mac 10. We burned through all that ammo in one afternoon.

I would make sure they are tapered crimped well

Nothing "Special"..but as SR4759 pointed out...make up some "test rounds" to check for function. When I was loading for my full-auto collection, there would always be a few "function-fire" test runs until I settled on the full set of parameters I would use for the bulk reloading project. A couple of places you might run into an "issue" are: re-sizing...make sure your getting every case FULLY re-sized. Overall length can be critical as well. Of course, you have to have a primer/powder charge that will provide sufficient power to cycle the weapon reliably. A full auto weapon should be run with SAFE loads that toss the empties out "with authority"..not just dribbles'em out to fall next to the gun.
ENJOY..!!

Thanks for the tips guys. Some good info.

Woody