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Question on USC to UMP conversion

8K views 34 replies 20 participants last post by  ToyCop 
#1 ·
Hello,

I have a USC that I'm about to do the UMP conversion on. I bought a rear stub/folding stock combo (it already had the pin installed) and an FBI lower. From my research I thought that was all I needed. When I went to my LGS today and talked to the H&K armorer there, he said the stub would not work and that I needed the HDPS stock block. I went ahead and ordered a stock block but my question is, if the stub doesn't work for the conversion, what is it for?

Thank you in advance.
 
#3 ·
The UMP rear stub is used for the conversion type where the rear of the USC receiver is cut off and the UMP stub is grafted on by plastic welding the receiver. Not everyone has the skill or experience to do this properly and it's just likely that your smith has no experience doing it. The stock block conversion is much simplier and doesn't require cutting and welding the polymer receiver, and probably is the only conversion he's familiar with.

Just send your USC, lower, and UMP stub to Tom Bostic and he can do the job.
 
#11 ·
After checking out Tommy's website, I see he does great work and is highly recommended, but I cannot afford to have the fused stub conversion done. I ordered the stock block from HKparts.net and it should be here Friday.

Renegade, that UMP looks very nice!
 
#12 ·
I am just starting down the same road with my USC45. My initial thought was to go with a fuse job but as you have discovered, it ain't cheap. I have emailed Tommy Bostic with a few questions and in the meantime, I'm researching different ways to be 922R compliant while using the chopped factory barrel. Whatever option I choose, I'll let Tommy do the milling and fitting.
 
#13 ·
Hi meh92,

For the 922R compliance, you can buy an American made USC magazine. I got 4 of them from HKparts for $50 each. From my inspection, they are just as well made as the German made magazines and are quite a bit cheaper. I haven't had a chance to try them out yet, but will do so once my conversion is finished.
 
#15 · (Edited)
Hi, all. I will soon be starting on a USC-to-UMP (.45) conversion. I will do the "basic" stuff myself (rail, change the guts of the lower, stock block, etc), but will be working with a local gunsmith for the machining bits. I do have a quick question for those more experienced here.

When changing the lower, do I need the full lower (link to hkpart) or can I get a "stripped pistol grip" lower (link to hkparts), UMP lower backplate (link to hkparts) and stock block (link to hkparts)?

Thanks in advance for any insight you can offer!
 
#23 ·
Hi AmericaNL,

I bought some of my parts from Gunbroker and the stock block from HK parts. I bought a complete FBI lower and the folding stock/pin/stub from GB and the stock block from HK parts. From what I've read you can buy the stripped lower and change out the parts but I'm not 100% sure about that.
 
#24 ·
So I recently bought a USC with a 2 position FBI lower-the original owner did just that and swapped out the parts from his USC-the selector switch is pretty slopy and sometimes gets hung up. I'm concerned that the parts were not replaced correctlly--I'm shocked at how flimsy the selector switch is- no positive detent as on mP5s. Is this normal for the polymer guns?
 
#19 ·
You will need to get the wider UMP mag catch pin (along with the mag catch), the USC is shorter. Also, grab an additional trigger housing pin. The UMP has an extra hole.

You do not need the UMP backplate if you are using the HDPS stock block. It takes the place of it.
 
#27 ·
To come back to this, I have read in a couple places that people have simply enlarged the mag well on a USC lower to accept UMP mags, modified the receiver and slapped on a Hera arms stock block. This sounds like it may be a reasonable way to go, but does anyone have pictures of a WELL DONE mag well modification?
 
#22 ·
Op, I've got a Bostic fused G36 which in my opinion the only way to go on that rifle. Cost is higher but the stock block conversion is just off - length of pull too long, rifle looks and shoots weird to me so I'm glad I did that one. I will say I'd never use that gun in a serious SHTF scenario - never bad a problem but cutting and gluing the polymer housing just creates another potential point of failure in my mind (perhaps not in reality but the builders have voiced some cautions as well).

I did choose to go stock block conversion on the UMP and it's great. I'm not a purest HK collector and some won't do anything but a fuse job but the difference on the USC is pretty minimal and to me, the cost benefit of the fuse job isnt there, it looks right and shoulders great. And, I don't ever worry about the fuse job. Best, Matt
 
#32 ·
Horvat,

I just recently completed a conversion and had Tom Bostic modify the upper and a FBI Lower as needed. He charged me $375 which included return shipping and had it for like 4 days. I tried the ACR Block and stock, but changed it to the stock block B after a range trip. ACR stock cheek weld is very high, works with an EoTech 512 fine, but the irons are useless. I also had a few issues with a couple of the US made Mags, but a quick file job on the feed lips cleared that up. I also tried the US made sear from HK Parts, and that was initially causing issues, which may require a little fine tuning if you need that for your 922 parts count. I changed back to the original USC sear and have had no issues. It is a great weapon once completed. Tom is great to work with.

Firearm Gun Trigger Airsoft Rifle
 
#35 ·
My USC is at TommyBuilt for a fuse job right now. I sent the HDPS trigger and the HKParts sear for install in the pack. I plan on using a HDPS barrel when the form 1 goes through, but if the sear doesn’t work out I guess I’ll swap out some mag parts to be compliant.
 
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