As stated above. I have seen 4 SVDs (3 on helis, 1 on a dead player), yet not a single mag. Neither Reddit nor the forums report finding any.
On the other hand, I have seen more than 20 mags for VSS, and a bunch of boxes. In fact, more mags and ammo than for anything else! I feel those should spawn at helis only, or at Tisy (once you add it)