Being that I am generally known for wanting to try out every available option before settling down and deciding what I really want, I picked up a metal body conversion for Tokyo Marui's MP5 series. At this point I'm not sure who all makes metal body conversions, but the one I was able to get my hands on is by Mosquito Molds. It fits TM's entire line of MP5 Airsofts with the exceptions of the K and PDW models which have shorter receivers.
Appearance
The metal body came in three pieces; the left half, the right half, and a top rib that fits in where the scope mount goes. The finish on the metal is good, but doesn't quite match that on a real MP5. A real MP5 receiver is black and the Mosquito Molds conversion is more of a dark grey. With regard to markings it is very similar to the TM original plastic body, except on the right side where TM has their logo and the "Made in Japan" statement, there is a blank spot. The metal body is made of aluminum and is fairly weighty.
Installation
This is the hard part. Be sure you have a set of small screwdrivers (both philips head and flat head), pliers (including needle nose), a vise, and don't forget to keep the parts diagram from the gun's manual handy! There are a lot of parts in these things. If you can, lay a white sheet down where you're working so you can lay everything out respective to its assembled position on the gun.
For this conversion I chose my TM H&K MP5SD6. With a sheet laid down and all the necessary tools and the parts diagram handy, I set about disassembling the SD. The disassembly process took a while, but was fairly straightforward. Remember to remove the O-ring from the Hop-up lever rod before you try to pull the Hop-up lever off. I didn't notice the O-ring on the parts diagram and sat for the better part of ten minutes wondering what was holding the Hop-up lever in place. When you've got everything possible stripped from the plastic body, carefully begin to pry the two halves apart. This can be tricky since there are several metal pins tightly holding the sides together. I managed to gouge my plastic body a little when I was doing this because I wasn't being careful. Once the two halves are apart, remove any remaining parts (piston/gear assembly, barrel, wiring harness, etc.), and install them in the metal body, making sure to align the barrel with the piston/gear assembly. At this point, I decided that I wanted to convert my SD6 (collapsible stock) into an SD5 (fixed stock) so I could put larger batteries in it. I ran the wiring harness back from the middle of the body rather than forward. At this point, remove the metal pins from the plastic body and put them in their corresponding locations in the metal body. Put the two halves together after making sure one last time that everything's in place (including the center metal scope mount piece). Reassemble your weapon, hook it up to a battery, and fire a few pellets in both semi- and full-auto modes to make sure everything works properly. When I reassembled my MP5 I noticed the metal body halves were slightly warped, thus I needed to put it in a vise (with a piece of leather to guard against scratches) and tightened the two halves together until they were flush so I could tighten the body screws all the way. This had to be done at the front and the back of the metal body.
Overall Impressions
The weight of the metal body conversion is the real selling point. It makes the Airsoft feel much more like a real MP5. It balances more naturally, and what little recoil it had before has all but disappeared. However, for $300 (retail), one should get a more authentic-looking finish. The metal body tends to wear the finish off any magazines inserted into it. Also, the pieces should fit together better than mine did. Perhaps another company's conversion addresses these issues. If you know of another metal body conversion out there, please email me. I'm going to keep the conversion, as I really like the weight of the SD now, but I have a hard time recommending the Mosquito Molds conversion to others, unless you've got a lot of money to spend on converting your Airsoft, as well as a lot of time to spend making sure everything goes together properly.