Giving them a switch ability to use their rifles instead of PTRS would be quite simple. But you might as well just let the ptrs actually hit infantry sometimes instead of that, the only side effect would be that it will no longer miss against vehicles. In a game where panzerschreks have close to 100% accuracy I don't think an antitank rifle being accurate against tanks is an issue...
True missing accuracy against vehicles isn't a problem, in fact I'm all for 100% accuracy against vehicles considering overall RoF, Penetration, Damage, and even aim time are very poor. But I think just increasing the accuracy would be problematic due to the fact that they do 40 damage, half a Soldier's health. It'd be too inconsistent, 2 lucky shots would kill a model but 4 unlucky shots will do nothing at all.
What about increasing the accuracy tenfold, reducing damage to 20 (only 4 higher than a standard rifle), and doubling the rate of fire? Once that's complete, it can be tuned from there to do more or less DPS, but 20 is a lot more consistent than 40.
EDIT
Also Cruzz, is it possible to change the projectile from Target Weak Point to the HEAT type? Sometimes it's difficult to tell if TWP is actually working or if a regular shot is being fired, especially if it fails to penetrate. The flashier HEAT projectile type could help by showing when the TWP shot is fired.