Rock solid rifles can be found in the classifieds as they have been pre-upgraded. stick with something with a V.3 mechbox for tru headache free playing (they are more durable than v.2 mechbox) such as the AK series or G36 series.
For stock guns, the following are rock solid performers:
TM SOCOM M4 EBB (full metal too)
TM Type 89
(pretty much ANY TM is a rock solid, stock performer, but the ones i listed already have metal where metal should be on the real guns. the rest of TM's line, while good, is ABS plastic)
VFC 416 (or just anything by VFC for that matter)