Originally Posted by Mr.Hitman
The difference is just the trigger.. Both MP5 and M4/M16 are same shell.
That is 99.99% wrong...(the mechbox shell is indeed the same). There are several parts that differ between a M4 mechbox and a MP5 mechbox.
- selector plate
- safety bar
- trigger spring
- cylinder (depending on model)
The STS block is meant for metal body M4's. There is a list of manufacturer makes that will work with it. I strongly suspect that this list is not comprehensive of all makes...nor is it to be relied upon 100%. I suspect that this list was created from the manufacturer's "buddy's" rifles that he tried/used it on.
The STS device is a solid little chuck of machined Alum, designed to fit in around the hopup unit and press up against the mechbox shell and the front of the metal receiver. It basically transfers the "responsibility" of absorbing the impact of the piston from the mechbox shell to the [hopefully more reliable] metal receiver.
Not condoning this...but if you understand what its principles are, you could make one yourself.