The SG/5F gun is a custom-made Gas-blowback assault rifle.
It doesn’t exist in real life, but at least it’s made completely from H&K Parts.
I built the SG/5 because I wanted a gas blowback rifle. Originally I wanted a GBB SG/1, but couldn’t think of any good way to make it.
In the end, I made a rifle which is essentially an SG/1 with an MP5 receiver. I used parts from a TM G3 SG/1 and a Maruzen MP5K GBB to make the SG/5. Pretty straightforward, but the devil, as they say, is in the details. It’s a frankengun (meaning it’s made from different gun parts and doesn’t exist “in real life”) but as was pointed out to me, at least it’s all H&K.
“F” stands for “Frankengun”, in case you’re wondering.
This is not a how-to with step-by step directions. But if you are handy with a dremel tool you should be able to figure it out for yourself without much trouble.
PARTS AND TOOLS USED
- Maruzen MP5K
- TM SG/1 complete front end, including inner barrel but not hop-up mechanism
- TM SG/1 Rear sight
- TM folding PDW stock (I will someday attempt attaching a full stock)
- Brass tubing a little bigger than the outside diameter of the AEG barrel
- Brass tubing that fits inside the cocking tube (to replace cocking rod and provide support)
- Tough PVC plumbing parts (some assorted tubing, etc)
- PVC Cement
- Plastic Steel Epoxy Putty
- Razor Saw
- Bolts, nuts, washers
- Teflon Tape
- 5.96mm Drill bit
- High temperature soldering iron
- Solder flux, good quality solder (silver solder is best)
I’ll describe the biggest challenges I encountered. The rest should be pretty easy for anyone handy with tools and a dremel.
THE INNER BARREL
The biggest problem is the inner barrel. The Maruzen MP5K GBB inner barrel, like most GBB inner barrels, is not standardized in size and shape. So you can’t just buy a long inner barrel (at least not that I know of). To make the SG/1 length inner barrel I needed to join the SG/1 barrel to the Maruzen barrel.
To do so, I wrapped the 5.97mm (get as close as you can to 6mm diameter without going over) drill bit with a couple layers of thin teflon tape. Then I used it as an insert to join the two barrels (hop-up end of the AEG barrel facing OUT – it will be cut off later). Barrels were sanded and cleaned to prep them, then they were heated nice and hot and soldered together. (Silver solder is stronger for mechanical bonds, but requires hotter temperatures.) Once the soldering was done and the barrel cooled, I cleaned the flux off and out of the barrel and sanded down any lumps. I examined the barrel’s interior by looking through it at a light – I couldn’t see any lumps inside the barrel, and in fact could barely tell where the joint was made! This was very good. But after bumping it a couple times the joint broke. So I had to re-solder it, then I put a thin brass tubing over the joint and glued it there with epoxy to reinforce it and take stress off that somewhat fragile joint. (Make sure the inner barrels PLUS the reinforcing tube over it still fits inside the SG/1 inner barrel sheath!) It looked OK, so I proceeded to test it.
With the MP5K and it’s freakishly long barrel outside at a field, I test-fired it. Seemed to work fine – range is about the same as any other GBB. But I was mostly testing to make sure the BBs would fire reliably. Most GBBs have some kind of valve system that keeps the gas flowing until the BB has exited the barrel. Honestly, I couldn’t tell if the Maruzen MP5K had an equivalent or not. I needed this, since if the gun only fired a measured amount of gas on each shot (meaning only enough to propel it out the MP5K’s stock 4 inch barrel) then it was very likely the BB would not be propelled all the way out of the now very long barrel. Happily it seemed to work fine! One down note is the fixed hop-up on the Maruzen MP5K, which doesn’t allow adjustment. But it worked!
THE FRONT END
The remaining challenge is the front end. After cutting a couple bits off, trimming the inner barrel, and trimming the inner barrel sheath, it fits on the front end. However, there is no real support. The front end is really quite heavy, and on the SG/1 it is mostly supported by the tabs sticking out from the upper reciever on each end of the barrel. But we cut this part off in order to fit it onto the MP5K receiver, and there aren’t any stick-out parts on the MP5K receiver we could have attached it to anyway.
So, I wound up deciding to take a double approach. I would modify the front handle on the MP5k – making it longer – to support the front end from below. Then I would install a central tube between the two cocking tubes to support from above.
In the end I made a rigid front end piece that replaces the MP5K’s grip and cradles the front end from below (I bedded it in steel putty epoxy to make it form-fitting). It feels very rigid. The only problem with it is that the handguard for the SG/1 now barely fits on! It takes some real fiddling to fit on right, but it does go on.
The bottom support holds it on pretty well, but just to be sure I also attach the front end to the receiver via two remaining screw holes – ideally I’d use some kind of clamp but right now I just use two zip straps.
THE FRONT END – COCKING TUBE
I could have left it like this, but I wanted to use the G3’s cocking handle to work the bolt. Plus I wanted to install something to join the two cocking tubes together for more support. I wound up removing the cocking handle and rod from the MP5K and glueing a brass tube part way into the cocking tube of the MP5K.
In other words, the MP5K has both a long inner barrel sticking out of it and a hollow brass tube sticking out of it. The SG/1 front end’s cocking tube slides over the brass tube fixed inside the MP5K’s cocking tube. It doesn’t provide much support, but it connects the two nicely and leaves a hollow channel for the SG/1’s cocking rod (which requires some retrofitting for length and for fit – I kept the SG/1’s cocking handle and replaced the rod with an aluminum tube.
Paint all the tubes flat black and you’ll have something that looks pretty good. I still need to install a rubber or plastic end onto the tip of the cocking rod (so it doesn’t scratch up the bolt too much) and the cocking handle is not spring-loaded. I might not bother, since it doesn’t bother me, but an elestic or clever spring mounting would solve that problem.
THE END RESULT
All in all it looks pretty good and feels pretty solid, and it’s heavy. My gas blowback assault rifle is pretty much complete! Sometime I might try installing a full stock on it, but to be honest the folding stock has sort of grown on me.
Performance: I haven’t brought it to a game yet, and since winter is nearly here I might not have a chance until next season (this duster-running gun will not perform well in the cold). But I’ll update this page when the information is available.
Update: Nov 2003
Well, the good news is that it works. The bad news is that it doesn’t work any better than a stock Maruzen MP5K does (meaning it performs the same as an average GBB pistol). But it sounds MEAN, and the recoil is actually kind of cool – I wasn’t expecting to feel much of anything, to be honest. But it’s got a kick to it.
Firing on full auto doesn’t seem as reliable as semi. In semi shots are more accurate. In full you get less power and much more spread.
