Some notes on the SRC MP40

Just got mine. It's beautiful. Great trades and great finish. The EBB is very entertaining. The stock is very solid. And nothing wobbles.

Some notes to future owners:
1) It comes weak OTB at 280-290 fps. I suggest upgrading the spring to a max of M125 or PDI 190%. The gearbox looks sturdy enough but it is PROPRIETARY and the back (where the spring guide rests) looks a little fragile. So personally i wouldn't go past an SP/M120.

2) I'm not sure if the steel is painted or if it's carbon blackened. I saw some spots of rust so i'd suggest going over it with WD40 and a rag before storage

3) I tried loading an AGM mP40 mag with the feed hole in front (instead of at the back of the mag for the AGM) and, while it doesn't lock in place, it fed! I think with some minor modding, AGM MP40 mags will fit the SRC.

The gearbox is relatively well designed, with general reinforcements to the front of the gearbox. Of particular note is the cylinder window cut out: It's tapered in front instead of the usual rectangle. This will make the gearbox more resistant to cracking at the window (which is where cracks usually occur). There is a little gap on top of the gearbox though and the sliding clip seen on the AGM MP40 gearbox is absent. I don't know if this will affect the durability of the gearbox. But the durability of the version 3 gearbox (of which this is a derivative) is commonly attributed to this sliding clip... The gearbox seams thicker and more robust than usual, but i don't know if it's just because of the paint finish.

Inside, the internals are well greased. Of note is the piston. It has protrusions that are essential to the EBB function of the gun. If ever the piston breaks and you need to replace it, i'm not sure is you can get a piston with these same protrusions. So you might lose the EBB function.

I'm a little worried about the hop chamber. It is proprietary, but made of brass so it should last. The thing i'm worried about is when i tried to turn the hop wheel all the way "ON" and looked through the barrel, i couldn't see the nub protruding into the barrel. I have yet to test this in the field so i don't know if the hop is effective. Also, i tried putting an M120 spring and i was only getting FPS readings of 365-375. With this spring i was expecting the FPS to be around 400. I suspect a leak in the hop mechanism.

Well, i wasn't getting any hop as i feared.

I had to disassemble it to get to the hop chamber. Disassembly is similar to but not the same as in the AGM MP40.

First, separate the upper and lower receiver as per instructions.

Next, remove these two hex screws:

Now you can slide the barrel assembly off the receiver...only, it won't slide. It's so tight that you'll have to carefully hammer it off, with inevitable scratches to the sling mount. Once you slide the barrel assembly off (and after you start breathing again), you disassemble the barrel assembly as you would an AGM: unscrew the barrel nut, slide off the front sight assembly. The receiver nut is very tight and you will need to use a wrench to unscrew it. Once it's off, you'll need to unscrew the set screw on the side.

After pulling the outer barrel off, you will notice a spring. This helps push the hop assembly in place. Now here was the part where i was stumped for a while. I thought the hop assembly was just plain stuck and i tried to gently hammer it out without success. Then i noticed that you had to twist the hop assembly to get it out.

You're left with this:

It is interesting that SRC had integrated a metal hop arm with an SCS-like hop nub, molded as one piece. The SCS is the best hop nub out there and it was nice of SRC to incorporate this in the design. My only concern is that, because it's made of metal, it might damage the hop rubber. I'll just replace the nub with one from a G36 hop chamber if and when it does.

Apparently, mine came with the barrel hop window not aligned with the hop arm. This is due to the very loose barrel lock plastic c-clip. After some repositioning and teflon tape, i got the barrel and hop working great. I hope the metal hop nub doesn't shorten the life of my hop rubber though.