Alright I met your wishes for posting the UMP tonight. Bare with me here my camera was having a hard time focusing and I was still under the twitchy effects of an energy drink... Anyways the instructable is for mostly experience builders.

It features a removable magazine, fake charging handle, top and bottom rail with a foregrip attachment, and overall awesome looks. This version uses a full stock but feel free to make a flimsy open one.

Also behold the product of German and Russian engineering- The UMP-45u. Lol I swear though doesn't it look like an AK-74u combined with a UMP? It's not included in the instructable but it shouldn't be hard to figure out how to mod the original to this. Anyways on to the instructable.

Piece count complements of silentassasin
conectors
yellow:63
white:20
orange:44
light gray:14
gray:19
red:27
blue:4
purple 3d:2
rods
green:155
white:67
blue:18
red:2
yellow:4
black:1
bendy gray:2
bendy yellow:1
misc
blue spacer:20
gray spacer:2
full hinge:2
black part of hindge 2
tan clip :3
blue clip: 5
broken orange:1
y clip:25

I would rate this gun as...
A war gun (main)- 3.5/5 It's pretty good but it isn't super powerful for its size. It does have a high cap (for dark greys) removable magazine which is very useful and it was perfectly reliable for me while I had it built.
A war gun (secondary)- 4.5/5 It would serve as an excellent back up for power weapons. I would use the compact version with the same magazine though. The regular version is larger than it needs to be for a battle.
Something fun to play with- 5/5 By far one of my funnest weapons. It looks awesome. It works amazingly. What's not to love?

Step 1: Base Body

Alright sorry I couldn't be bothered to take this whole thing apart. It would have taken me forever to take pictures and upload the instructable (I still have homework to do and dinner to eat). Take you time and look at it carefully.

1-3. Different views main barrel and receiver area. Sorry I couldn't split it up more.
4-5. Make this piece and add it in here.
6-7. The muzzle. Sorry I was too lazy to take this apart too. It shouldn't be too hard to figure out from both of these angles.
8-9. Fake real barrel? The bullet travels through here but it isn't the real barrel on the real gun so...
10. Attach the muzzle and whatever you want to call it.
11-12. This is the bottom rail and how you add it.
13-15. Make this part (yellows don't need to be cut I just had them on hand and make the gun look better) and connect in these places
16. Then connect both parts to get this.

Alright the hardest part is done.

Step 2: Back and Trigger Assembly

This is the back area that connects the stock, handle, and main body together.

1-3. Here's the handle. I tried giving it a realistic angle but it made it somewhat flimsy ( it will shake a little) so fell free to make a solid 45 degree one. Also if you don't care for the looks of it then don't add the comfort part and just make another layer to go in between the two layers of the handle
4-5. Make these parts and then connect to the handle like so.
6. Connect to the back of the main body part like so. Don't forget to swing up the other side of the handle to finish the connection.
7-8. Make these parts and add like so.
9-10. Basic true trigger. You can also use a ball joint without tape I just had this taped part ever since I made the AK-74

Alright that wasn't as bad was it? Prepare for some slightly harder things.

Step 3: Last Details

Alright we're going to add details like sights among other things to make it look more UMP-ish.
1-6. Here's the rail and back site parts. I got lazy so the front of the rail is a little messy. You'll need a yellow rod. The sights use mini pieces for precision but feel free to remove them. The back mini doesn't need to be replaced by anything as the green pieces should line up with the bendy up front. You'll notice that the rail may bend upwards so I just stuck a rod through the hole (you can stop laughing now) and tuck it in the crack of the orange connectors (really stop laughing...) I guess you could use whatever color rod.
7-8. The front sites. Again I use mini pieces for precision but you can replace the mini with a green rod or something.
9-12. This is the fake charging handle and the mag unlocker. When a preloaded magazine is inserted, you press on the white rod which will remove the piece blocking the ammo. It's in a sort of safety mode now where most knex newbs won't figure out how to fire the gun if someone else picked it up in battle. Just pull the white rod back out and the ammo will load.
13-15. And here's the trigger guard. It's a little wobbly because I literally made the new handle and guard about 5 minutes before I decided to post so sorry. Feel free to figure out your own.

OK Now we just have to add the stock and we can figure out how to make this thing fire!

Step 4: Stock

Pretty simple step. Like I said earlier this uses a full stock instead of a realistic (well for the UMP I think this looks realistic in general) stock. Turns out I'm not half bad at making them. It's pretty sturdy and it does its job. Unfortunately the firing pin goes over the top of it so you'll have to rest your cheek on the very back to use the sights. Sorry I didn't break it up more but I figured you would be able to figure it out without too much problem.

1-2 Here is most of the stock. They are exactly the same with only two layers being 5 layers apart connected by a series of orange and yellow connectors. Here is also view of the back
3. Connect to the back of the gun like so.
4-6. This is the top of the stock. Credits go to Mepain for this part from his sniper. I used this only to make the top flat without using a lot of connectors but you can use grey rods if you want to. Notice that only one side connects fully while at the back it only slides into the very end of the slots. Don't worry it will hold there steady.
7-8. Oh yeah I forgot about the foregrip attachment. Uh pretty simple to make and add. It doesn't connect unto the rails you slide it in seeing how it's four layers rod to rod while the rail connections are separated three rails apart so the rods will go around the slots. It should stay in firmly unless you push or pull on it.

Step 5: Firing

Alright we're almost finished! The gun fires pretty decently. Even with the slanted mag it's 99.9% reliable and I have never had a jam of any sort. If you find that the muzzle gets in the way (which it shouldn't if you're using enough power) then just take of the extra part that makes it look rounded.

1-2. Here's the firing pin. You probably won't need the taped part which is basically a tan and I think a blue ring? Anyways I like the rubber band better. If you wrap it around the orange I think it'll hold it together. The rubber band makes the gun a little less loud when firing. The red connector is just to grab onto when you cock it as the orange will be in the back of the gun and hard to reach. Attach them about here. Move them further up for a little more power.
3-5. Here's the magazine. You can make it as big as you like but this is the most realistically sized one. It holds about 14 dark greys comfortably although you could get 15-16 if you're careful. Put the green rod in where you see it to hold block the ammo.
7-9. You then insert the magazine AK style as in you insert it at a steep slant and bring the white rod into the slots of the blue connector and then swing the magazine up so that the other side locks into position. This should be a sturdy enough connection so that it isn't pushed out by the mag pusher depending on how many pieces of ammo you load. Then when inserted press carefully (you may need to push up on the white rod to make it line up correctly or it'll wreck the magazine) on the white rod to push out the green rod then pull it back out.

Proceed to cock the weapon and then fire. The fun part is that you can make several preloaded magazines and swap them out fairly quickly. Enjoy.