The HK51 is not a variant of the MP5, although they are visually quite similar. The MP5 is a 9mm submachinegun and the HK51 is a 7.62x51 (.308) assault rifle. The HK51 is not manufactured by H&K, but is very similar to the HK53 in .223 that is H&K made. HK51s are conversions of (usually) HK91 semi-auto battle rifles, shortened and modified with after-market auto sears to be select-fire weapons. I know this because I have 2 of them: an HK51 (8.9" barrel) and an HK51K (4.7" barrel). They very closely resemble MP5s if you don't notice the larger magazines. They are a lot of fun to shoot. If you like spectacular shooting, here are a couple of photos of my HK51 at night:
one round
multiple rounds
enjoy!
flashguy