[Crunchmeister]

I'm currently having a feed and velocity issue with my MP5 that's got me rather perplexed, so I'm now turning to the more experienced gun docs for some advice on what could be the issue. To see the current situation with it, it's probably a good idea to explain the history of the gun a bit.

Background:

It started life as a JG MP5-J and had has slowly been modified over time. This was a long-term project experiment gun, and it'a been great until the last work I did on it, which is admitedly major surgery.

The last "good" configuration of the gun saw it with a CA metal upper receiver & front end, stock, never-opened JG mechbox, and JG lower receiver. The hopup / barrel were the stock JG, loosely fitted in the CA body. This is a body designed to take the new style metal CA hopup that's adjusted under the bolt cover rather than the external level. However, I used it with the original JG plastic TM-style externally adjusting hopup. The gun fired a consistent 330-340 fps, was very accurate, and had a unbelievable range for such a short gun. It developed a crack on the mechbox near the nozzle at about 35k rounds, but still kept on going without issue until about 60k rounds.

So a few weeks ago, I decided I just couldn't kill this mechbox, and it was time to finally gut the gun and install a new mechbox and the proper CA metal hopup I had bought for it. While the gun was proving reliable, I just didn't trust it with the cracked mechbox. I got Deepfire reinforced 7mm gearbox. and a set of Modify modular gears (torque-up), a Deepfire metal spring guide, and reused all the other stock mechbox parts including the spring, since it shot at the velocity I wanted. The original parts, including the piston and piston head showed absolutely no signs of wear, nor did the tappet plate or nozzle. I tranfered all the wiring and shell parts (except installing a new TM cutoff lever) and parts. Compression tests showed an excellent seal, and the mechbox operated without issue. I installed a 6.03 tightbore and a Guarder clear hopup rubber, mounted the metal hopup into the body, and installed the lower, and got ready to test fire. This is where problems began.

Problem:

The BBs seem to get stuck in the hopup and none would fire. At first, I suspected the hopup rubber was installed wrong, so popped the lower receiver off and slid out the barrel / hopup assembly (gotta love the quick-disassembly CA metal body). Trying to push BBs though the hopup, they seemed to jam. After some messing around, I tried to isntall a proper CA MP5 barrel I had (came with my front end). Some BBs were feeding through, but not well. I would end up with low velocities and jams. I tried different hopup rubbers I had, both used (good condition) and new, with similar results. I've eliminated the mags as a possible issue, since they all give the same result.

I really don't think it's a seal issue inside the mechbox. I think it would be more like a seal issue between the nozzle and the hopup if it's a seal problem. Is there any way I can test this?

Another thing I considered was that because the CA hopup is different from the TM style hopup, should the loading nozzle be the same as a regular MP5, or should it be a different style? I wasn't able to find any info on this, so throwing this out to see if anyone knows.

So that's where I am and I need some help. I hope I didn't bore you too much with the long-ass post, but through I would provide thorough info on the gun and its evolution, as it might shed some light on its current problem.