––MODS: If you see fit, please grace this with a tack ––

I started looking at this project about 6 months ago when I sent in my form 1 and could not find a really good source of information as far as DIY in one place. I have gathered a wealth of information and documented the process from start to finish and am sharing it here as a "One-Stop" tutorial for anyone else wanting to build one of these awesome weapons. I will start out by stating that I am an engineer and have worked with many complex guns and build/conversion projects and I rank this one's difficulty as moderate as it requires a lot of hand fitting plus machining skills and equipment. That being said, you will know if you are comfortable with taking on the project yourself after reading this and if not, I have listed contact info of places that offer conversion services. NOTE: I am in no way affiliated with any of the companies listed, I am just giving them the credit they deserve for helping out with information on this project. Also I apologize for the quality of some of the images as some were taken with my phone. So without further ado, let's get started.

FIRST ORDER OF BUSINESS; Before doing ANY kind of barrel work, be sure to have an approved ATF 5320.1 in your hands.

I started with the barrel. Remove factory barrel by punching out the single roll pin located underneath the serial number. The barrel will then slide out the rear of the receiver along with the ejector blade. Pay close attention to how this fits for reassembly. I used a bandsaw to cut the barrel to approximately 7.95".



Once cut, I used a lathe to turn the length to UMP spec of 7.870" and reduce the end OD to 0.578" for 0.500" from the end.


NOTE: I chose .578x28 RH threads to match my suppressor in lieu of the M16X1 LH traditionally associated with HK. All dimensions given are for 578x28 RH thread pattern.

I then threaded the end 60°@ 28 TPI back 0.400" from the end at a depth of 0.040" and finished with a chasing die to clean the threads. I then used an 11° crowning tool to recess the crown and hand polished to perfection.


The next part required a lot of research to find factory specifications and ended up drafting the official dimensions myself. I used a 1/4" 4-flute end mill to cut the 5 vent slots on the forend of the receiver.


I used the existing hardpoints to step off the beginning cuts. You will have to sand the burrs from the edges to achieve a factory-finished look. I recommend 800 grit finished out with 1500 grit using oil medium.




Now let's get to the new stuff.
LIST OF REQUIRED PARTS:

-UMP lower (4-POS, Navy, or FBI)
-UMP magazine (USC mags will work in UMP lowers but fit loosely)
-UMP stock
-Rear block adapter (I used Block B from HDPS)
-Stock axle pin

922(r) compliance parts are available through the vendors listed at the end of this writeup. Pick any 3


Remove the forward disconnector from the UMP lower FCG and replace the axis pin. This is done with a punch and light hammer. Lower will not mate to the USC upper with it installed. Next, you will have to open the forward mounting hooks to accommodate the USC upper's larger diameter pins. I used a centered 0.275" drill and sanded to fit. This is the first of many tedious hand-fitting steps in this project so go slowly and be patient. You will also need to remove the rear buffer block from the lower by lifting the small tab over the retainer detent and sliding the block rearward.


The next machining step is to open the mag well in the upper receiver to accommodate the UMP double-stack mags. To do this, dry-fit the UMP lower to the upper and trace out the areas covered by the mag well to get the well bounds. Then you will need to remove the lower and dry-fit the magazine within these bounds and trace around it to get your inlet bounds. Be sure to set your depth to the exact thickness of the polymer in the center of the crosspiece, else you will cut into the tracks for the bolt causing major function impairment. Use the same 1/4" 4-flute end mill that you used for the forend vents to do this.


After the cut, it will look something like this. Because the different lowers mate slightly differently, exact specs cannot be assigned so you just have to trace out your mag well.


A better look at the bolt tracks that you DO NOT cut into. Depth is imperative––you only get one shot at this so NO DREMELS.


Now you will need to mount the stock to the mounting block. I chose to use Stock Block B from HDPS. To do this, dry-fit the stock into the block, lining up the outside of the axle tube. If you look down the hole, the inside diameters probably will not match up so you will need to use the block hole as a guide and bore out the stock hole with a 11/64" hand drill. Detailed instructions on this process are included with the HDPS blocks.


Drive in stock axle pin with an oversized punch and small hammer. Additional hand-fitting will probably be required to the stock's inside flat and locking catch to achieve a rock-solid lockup. Once again, be patient and go slowly. I spent over 2 hours on this part alone.


Now you will need to extract the recoil buffer from either the old UMP buffer block or the USC stock. This can be tricky because there isn't a whole lot of exposed material to grab onto and you do not want to damage it. I inserted a tapered punch into the center hole and pulled to one side while gently digging it out with a narrow flat driver. It will pop right out with little leverage.


Insert the buffer into the stock block.


You will now need to remove the detent nub from the lower that held on the buffer block. I used a file and fine sand paper for this.


Now it will all slide together just as it came apart. Here is the fitted product. Everything should be tight with no rattles or movement.


Add a full set of rails, pat yourself on the back, snap some pics, and post them in this thread to show off your accomplishment and fine new toy







Obligatory ARFCOM picture of gun with feet


Doing all labor myself and excluding the price of machinery and tooling, total cost of this project ran $2850 including the NFA tax. If you are not equipped to do all steps yourself, HDPS offers itemized labor through their website.

Special thanks to:

HDPS.org -> DefenseArms@aol.com
H&K USA -> 706.568.1906
HKparts.net -> 801.987.3494

ETA: By popular demand, I bring pics with the can. I have had to neglect you guys for a couple months due to work travel







And with other short friends...


...short, quiet friends...

Good review - welcome to the UMP club. Now you need a 9mm kit.
More info available on www.hkpro.com

FYI, you don't have to bore out the upper receiver to the dimensions of the lower's magwell opening. Doing so ends up cutting into the rails that the bolt carrier rides on. You would be better served by opening the upper receivers hole to fit the feed lips of the mag. Look at the mag. The main body comes up and Flattens out. From there it narrows down as the feed lips take the mag from a double stack to a single feed. You only need to open the upper so that to the dimensions of the top base/ feed lip intersection. I know that is hard to follow. Maybe I can get some photos up. It's hard to tell from your photos if that's what you did or not. I know you mentioned it in your post but I looks like you opened it up to the same dimensions of the lower's magwell opening and not the feed lips dimensions.

Bored out- opened up, whatever. The links to the magwell template openings aren't working for me.

IMHO a lot people open the magwell on the upper more than it needs to be. I really need to take some photos and/or measurements to explain what I mean better.

John,

Cool. That one works and is correct.

I see a lot of folks recommend throwing the UMP lower on, tracing the magwell opening onto the upper and milling out to those dimensions. Doing so opens the upper too wide and too far forward. Probably an 1/8" too wide and too long in the front.

very nice. what fore grip is that?

If you cut it to the 1" dimensions (wide) like John linked, there is nothing to hit. The cuts go right to the edge of the bolt tracks/rails. There was nothing to hit inside the receiver. However, doing as you say and montitoring the depth is certainly the safe way to go. If you blow it out to the dimensions of the lower's magwell opening (NOT RECOMMENDED) then you will be cutting into the carrier tracks/rails. In this case you would need to worry about the milling depth, as you will not only cut into both the lower rails but the upper carrier rails as well.

If you are wondering why I beat this to death, take a look at how the big name guys do it...

Nice!!!!!

Thanks for doing this... I might take on this project in the future. Nice work too!

Very nice write up, thanks!

However, what did you use to permanently block off the auto sear location in the new lower? I've seen this service offered on the HDPS and some other tutorial websites, I'm just wondering if it is absolutely necessary.

Also, did you have any trouble swapping out the FC/trigger internals from the USC to the new lower that uses a different selector switch setup?

I will be doing this soon hopefully

Although recommended by some manufacturers, blocking the sear area is not required because without machining of the upper for the sear/bolt engagement ATF does not deem it capable of automatic fire. Same with the FCG. I simply removed the sear and because it is unable to be assembled otherwise, you are GTG.

Still no suppressor pics