The 120 mm Altay gun by MKE is a L/55 caliber (variant of Korean K2 gun). It‘s too long and heavy with severe recoil for M60. A 120 mm gun outbalances the tank, changed turret and enhanced torsion bars needed, reduced ammo rounds.
120 mm cannon length:
L/55 = 6.6 m (Rh Leo 2A6, MKE Altay)
L/44 = 5.3 m (Rh Leo 2A4, Merkava) /
5.593 m (M256 M1A1 & K1A1)
105 mm gun length (M60A1/A3):
M68 = 5.55 m
120 mm Sabra Mk II (M60T):
MG253 = 5.3 m
Israel‘s M60T Sabra Mk II upgrade uses the compact MG253 120 mm L/44 gun by IMI. Afaik MKE hasn‘t developed a L/44 version yet. It‘s not simple as shortening the length and be done with it. Koreans use licensed US M256 L/44 guns with a length of 5.593 m on K1A1/A2 tanks.
Modernized 105 mm MBTs are still useful assets. Since 2014 ROKA has improved over 1.000 K1 tanks (late 80/90s) to K1E1 standard (FCS, BMS, IFF, night sight thermals, front/rear cameras) and now upgrading to K1E2 with modern gunner sights, NBC positive pressure system, APU, air con etc.
105 mm K1E1 upgrade to K1E2: