The HK and RPK are bad. Guns got really screwed over in Black Ops. In Der Riese, you had a variety of usable guns for the 30+ rounds. PPSh, MG, Ray Gun, Wunder Waffe, Flamethrower and STG are all amazing.
In Black Ops zombies, the two undisputed best are Ray Gun and Thundergun. I cannot stress how important the Thundergun is, we got to Round 30 on Ascension with me not getting downed once while I had Thundergun. Badger and Jeremy got about 8?? downs each because you're only allowed one. Ray Gun is necessary at later rounds but I prefer the M16. M16 is probably the best reg gun, 10 times better than HK or RPK. Upgraded, it's just too good and having ammo off the wall is imperative. We got to round 32 I think before I got disconnected from Live, that was an easy level 40+ game.
If you hate the M16 that much, hit the box and alternate between Galil, Commando and Aug. <-- in order of how good they are.