Curios thing.. The ATR being 12-15mm may cause p.schock if rolling 1 higher tan needed on the to kill roll. But not a 15mm gun.
IMHO it must be changed to stun resull similar to MG and 12-15mm ordnance.
There are 3 rough sizes of ATR.
The 7.92mm (German and Polish). Though rifle calibre these have muzzle velocities of ~1200 m/s.
The British Boys (14mm) and the Soviet PTRD/PTRS (14.5mm). The Boys only had a MV of 750 m/s (Mk 1 cartridge) or ~900 m/s (Mk 2). The Soviet 14.5 had a MV of 1100 m/s.
The Japanese 20mm Type 97 (MV 800 m/s), Lahti 20mm (MV 800 m/s) and the Solothurn 20mm S-18/100 (MV 735 m/s) and S-18/1000 (MV 850 m/s) as used by everyone else.
Rifle calibre and 12.7mm to 15mm HMG have MV of roughly 800 m/s. That means that if such a weapon penetrates it will have little energy to do much damage to the interior other than to human flesh.
The German and Polish ATR had roughly twice ((1200/800)^2 = 1.5^2 = 2.25) the energy of a rifle calibre round. The Polish ATR round with a lead core was not designed to penetrate but to cause a roughly 20mm patch of the inner face of the armour to detach and fragment as spall (like a HESH round). Either had either enough remnant energy or enough fragments to do more damage than MG AP rounds.
The Boys, though better than a rifle, was the worst, barely better than a 0.50" MG round.
The Soviet PTRD/PTRS was simply the best of all the WW2 ATR, outperforming even the 20mm ATRs. Lots more energy left after penetration than any 12.7-15mm MG.
So with the exception of the Boys I would be inclined to leave it as it is.