After playing with the two guns a bit more they both seem completely fine power-wise. The M7A3 felt extremely slow to reload though, considering that it's not THAT powerful compared to an AK or Scar and the fact that you can't shoot medi-darts during that entire time.
The HK's sights seem to be more similar to the Handcannon's then the 9mm's. What I mean by this is that the sights need to actually go over your targets head instead of just under it (like the 9mm's sights). This means that it's not as easy to use at long-range as the 9mm, and not as powerful at CQB as the Handcannon. This might be on purpose, but I just thought I shoul point out the fact.
Since both other handguns weigh the same single or dual (sorta), I'd be against single and dual weighing different amounts for the HK. However, I am for the HK weighing 2 or 3 blocks. 4 just seems like too much for a gun we want to encourage off-perk usage of.
Also, a small cosmetic note, that beeping sound at the end of the M7A3's reload can be explained by a small static fuzz on the ammo counter after the mag is inserted. It can be the ammo counter "reseting" itself. I'm not sure what else that beep would denote, perhaps that the weapon is ready to fire again?