Hi, First off...Im new here and was referred by The person who I recently purchased my new 1921. So, Hello everyone! Haha...Anyways, so Out the box, it worked great with the drum, then I tried with a 20 rnd mag and had 3 shot bursts,and feed problems and jams. I reloaded, and tried again with the drum, and is just doing bursts. Also, I noticed there seems to be a slight mushrooming at the top of the boltface that can be seen thru the top when you pull the bolt back...Any ideas?

Hello and Welcome to the Forums

Is your Thompson brand new or an older used one If it's an old one, the slight mushrooming you can see on the bolt face could be wear'n'tear. These bolts are cast zinc and will, unfortunately, wear out in time.

Consistent long burst firing relies on everything being set up perfectly. Unlike a real gun, these models are a bit more fussy.

First thing I learned was that after about 30 rounds, the detonator pin and chamber would need cleaning to remove the deposits left by the firing caps. If this isn't done, rounds start to stick in the chamber causing jamming.

Next thing is that you can get away with quickly dismantling a fired cartridge, reload it quickly with a new cap and fire again, but so much dirt builds up inside the cartridge friction makes the piston stick reducing blowback.

So, first job is to strip the cartridges, thoroughly clean them then re-load paying particular attention to lubricating the cartridge bore and 0-ring.
Please take a look at the relevant articles

http://mp40modelguns.forumotion.net/beginner-s-guides-tocartridges-f15/cleaning-and-preparing-fired-cartridges-t931.htm

http://mp40modelguns.forumotion.net/beginner-s-guides-tocartridges-f15/loading-mgc-type-cartridges-t924.htm

Next then is to remove the detonator chamber and give it a thorough cleaning.

If everything else is OK, you should see an improvement straight away. If the bolt is worn, you'll notice jamming happening as the fired cartridge drops off the bolt before being ejected, can you describe how your Thompson behaved as it stopped firing

Another tip is not to over tension the Drum spring. 5 or 6 clicks of the lever should be plenty.

Give that a try and let us know how you get on please We'll get it sorted one step at a time between us

Al -
Welcome - this is a great place for modelgunners.
With your gun being new, the bolt shouldn't be deformed enough to cause problems, however, repeated jams may accelerate the wear some.
I agree with Cerwyn, try cleaning the cartridges and give it another go.

As far as feeding, it is my belief that the drum magazine is more forgiving and operates with fewer feed issues because the cartridge is not staggered like the stick mag. Sometimes the mag lips on the stick mag may need to be adjusted to ensure smooth feeding.

Post often and let us know how you get along!

JL

Cool, Ill try that...I noticed that when I reload, the piston /seal...would go in kinda tight. should the casing be oiled before re-using after cleaning?

Yes, the cartridge definately needs some lubrication when you load it up. Silicone Grease, or Silicone Oil can be used sparingly. Make sure the cap doesn't get wet.
Don't use WD-40 as this can cause the 0-rings to swell and break eventually.

The tightness you mention can be caused by an air lock in the cartridge. As you load it, the piston's 0-ring can seal so well it traps the air inside. Try pushing the piston in a little further, if it pops back up again, it's almost certainly an air lock. Best way to expell the air is to remove the piston, and keep putting it back, until it seats properly.