The M-103 was basically an armored shell around a 120mm anti-aircraft gun. The guns in use today (120/122/125mm) are all smoothbore with no rifling. Cheaper but also cuz the shot doesn't need to spin. The shot is also lower velocity.
Anti-aircraft guns have to reach those dizzying heights, so the spinning gives you a higher muzzle velocity. This is why the 88mm AA gun on the Tiger was able to cut through any allied tank like a hot knife through butter, but the Russian 100mm gun couldn't. The 100mm was a normal gun.
The US Army 120mm was a towed monster. I've never seen a kit of it except in 1/87th scale (ROCO Mini-tanks)(which I happen to have). The carriage was broken down into two pieces cuz of it's weight. By the time it was introduced the threat to America of high altitude bombers had subsided and radar enabled any carrier/land base to have advanced warning enough to engage any bombers far from their target.
So the guns sat dormant. A battery of them was in Houfallize during the battle of the bulge and promptly ate six or seven King Tigers for lunch. The Army then quickly pressed ALL anti-aircraft guns (90mm and 120mm) into service as tank killers. With wars end, the guns went to mothballs. As the Marines and Army were considering the next tank to deal with the newest (advertised as "invincible") Soviet tank, the T-10, an admiral remembered the 120mm AA guns.
The M-103 just looks mean as hell. The USMC were the only ones to ever use it in action. Funny enough, against T-10's and T-55's in Korea. During the border incidents from 1956-1966, many tank vs tank sniping actions would take place. My last diorama will involve an M-103 in the DMZ that has just taken out a North Korean T-55.
It's got the same "boat" lower hull as the Patton and M-60. So the basis of my scratchbuild/kitbash will be an M-60 and an M-48 hull. However, both of them have six road wheels instead of seven, so I'll need to widen the hull and lengthen it.