HK USC to UMP conversion process

by Erik  

This article provides information on the USC/UMP conversion. If you don't know anything about this process, I am here to tell you everything you need to know about it. First, HK does not sell the UMP, G36, or MP5 to civilians. If you are after a UMP then your only option is to do a USC to UMP conversion.

The HK USC is the civilian version of a UMP. It has a 16" non-threaded barrel and fixed stock to satisfy the NFA and 922r compliance laws. Doing a full conversion will turn the USC into a semi-auto short barreled rifle which will require an NFA tax stamp via a form 1.

Now, there are 2 ways of completing the conversion. One requires an independently manufactured "stock block." The other requires the rear portion of a real UMP receiver which is grafted to your existing USC receiver. The graft conversion is more authentic but does require some expertise to do the actually graft work. Some of the UMP "parts kits" you see are real UMP parts minus the receiver. They are often used for grafts as well.

Here is a list of the parts needed for the conversion and estimated cost:
  • USC gun $1200-$1600
  • UMP lower (FBI, Navy, or Multi) $250-$400
  • UMP stock with pin $150-$200
  • * HDPS stock block B $200 * -OR- ** rear UMP section for graft $150 **
  • 25-round UMP magazine $90-$130
  • $200 tax stamp for NFA SBR
  • 7.87" barrel or stock barrel cut and crowned $85-$325
  • Some gunsmith work $150-$650

Optional accessories
  • HDPS rail kit $125
  • Magazines $90-$130
  • Optics $50-$500+
The cost of the parts will depend on the current market and their availability. At the time of this article stocks, magazines, and lowers are all very hard to find. Prices for those items are at least 20% higher in some cases 100% due to demand. In all, expect to pay at least $2600 for a complete UMP conversion.

Remember that by doing this conversion you will need to comply with 922r and NFA laws. By using a barrel shorter than 16" you will need to file a form 1 with $200 payable to BATFE. The 922r compliance requires you have at least 3 US made parts from the list below:
  • Stock $99
  • Sear $46
  • Magazine floor plate and follower $17 each
  • Barrel $264-$325
  • Trigger $135
If you are doing a full conversion the cheapest 922r compliance parts will be the stock, sear, and barrel. If you do the magazine parts you will need to do it for every magazine you own which may be more expensive in the long run. US made stocks are hard to find but they are cheaper than authentic HK ones. I haven't ever used a US made sear, and hear they are hit or miss.

So we really only need the original upper receiver, bolt, and the lower internal parts from the USC. Possibly the barrel if you plan on cutting it or leaving it 16". Everything else will be junk. It sucks that you have to spend $1200-$1500 for those few parts but you can make some money back by selling the USC magazines and some of the lower parts. You can sell the barrel if you don't plan on keeping it. The USC lower and factory thumbhole stock should fetch around $25-$50 on ebay. A new USC barrel can sell for as much as $200.

Here is a list of things that should be done by a gunsmith or competent gun owner:
  • Swap USC internal parts with UMP lower internals parts
  • Enlarge the magazine opening in the upper receiver
  • Re-mark the safety lever (only when using the Navy or Multi lower)
  • Enlarge the UMP lower hooks to fit the USC upper receiver
  • Cut vent holes in front upper receiver (cosmetic)
  • Add a "stock catch" that holds the stock in place when folded
  • Receiver graft if using a UMP stub section
  • Engrave trust name or individual name on upper receiver for NFA SBR compliance

Some of this may be done on your own but I elected to pay a gunsmith (Tommy Bostic) to do all of it because it would take me 4 times as long to do myself. Turn around time was quick and the work done was grade A. TBostic is one of the best in the business with these conversions.

Another thing to consider would be using a gray colored USC. HK no longer produces them but for a while there were many of them floating around. Your only option at that point is to dye the receiver black to match the rest of the parts. It's a royal pain in the ass and your best option is to pay someone to do it.

A graft conversion is a method of conversion which involves using a rear section of the upper receiver from a real UMP and plastic welding it to a USC upper. I don't know much about how this is done but Tom Bostic does these and they look flawless when he is done. If you do the graft conversion the total length of the rifle will be authentic to a real UMP. It will be slightly longer than authentic UMP when using the HDPS stock block. The graft conversion is obviously more labor intensive (costing more money) and requires a skilled gunsmith who knows how to weld and refinish polymer.

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