The most logical way to shoot a plane down with infantry weapons is for everyone to shoot at once, like in a volley.
And you'd have to expend a lot of ammo to even score a hit. A target moving at 50 MPH + is hard to hit, let alone 100-150 or so. The only thing even worth shooting at it would be MGs, everything else would have too low a rate of fire or too weak of a round (SMGs). As for the PTRS, it would be pretty hard to get a good firing angle on it. Even then you're better off with a DP-28 or one of the fixed MGs.
I just hope the planes are really hard to shoot down, and we don't see people taking them down every game with a few shots from their bolt action.