The UMP trigger group uses a sear as a secondary safety (even on the FBI or semi-only trigger groups). Now the area that the sear would fit into in the USC receiver is blocked, and the ATF has stated that cutting that area open is manufacturing a machine gun (big no-no). (since the sear is also used for the auto function on the Navy and SEF trigger groups)

If you take the sear out then you can use the trigger group fine, but you are missing a safety component.


The USC trigger group uses a different style of secondary safety (that also lessens the trigger pull weight) and doesn't require the cut. Since the USC and UMP use the same pin locations (except for the sear pin which doesn't get used by the USC parts) you can just swap the parts the retain all the safety features of the USC trigger parts!

 

+10000 well put!

 

So in essence, I could use a UMP navy lower provided I follow federal regulations regarding 922, correct?

So if I were to install a new trigger, follower, floor plate and rear block that would satisfy the 922 compliance?

 

Rear block is not part of the 922 regs. Don't need it unless your talking about a stock block adapter, which you need for the conversion but not for compliance.

 

i hate to Zombify a dead thread, i just want to make sure of something

So i CAN leave in the UMP trigger if i like it more, and just remove the Sear? (and close the hole or re-pin) It will function ok if i do that?

 

Yes, Yes and Yes. Don't cut the upper USC receiver, remove the disconnector because it will not allow you to put the USC upper on the UMP lower. You will have a heavier trigger pull. For 922 just replace the magazine parts. You will not be using the mag in any other gun or any other mag in your gun. So basically it is part of the gun, follow?

 

Holy thread revival, batman!
Kidding.. I've heard some horror stories about doing what you are considering, such as
two and three round bursts and unwanted FA bursts. Something you do NOT want to
happen at the range.
I would use the USC trigger parts and leave it at that, personally (I'm actually planning
on doing this, even though I like the feel of the UMP trigger a LOT more).

 

Yeah, wouldnt want any mis haps like that at the range...

I guess i can switch them out, they sure do look complicated though, more so than the ARs i have put together.

 

I can't find it but someone put up a fairly good guide to doing this.
Do a search, I'm sure someone will chime in and give you a link
Or just take a bunch of pictures and do it slowly, the parts are VERY similar, some parts actually identical part numbers.

 

This one?
HK SL8-1 Trigger Group Breakdown

 

I think they're talking about the auto sear. Or trip lever. Not sure. The USC trigger sucks though. Would be nice if there was a way to make it better.

 

I think they were talking about the trip lever which has to have the additional (VERY ILLEGAL) cut in the upper to function.
Don't mess with the trigger, folks. I know it's tempting to use parts of the UMP trigger group but the USC trigger group
has a specific safety that was put in to account for the lack of the trip lever by people who know more about these things
than you or I. If you have a FA event at the range because of this you could be looking at some serious legal troubles.