Thanks.
The M1 used the venerable ROF L7 105mm gun, the M1A1 uses the Rhinemetal 120mm smoothbore.
yeah but they're theoretically both supposed to be M1A1s, mounting the aforementioned 120mm gun - as you can tell from the prominent bore-evacuators mid-way down the barrels of both weapons. Hence, I guess, my confusion at the cannons being different lengths and suppositions that it's to do with a slightly different turret-make or longer housing.
Or, perhaps, somewhat different interpretations of dimensions between model-makers - it's previously been observed in some online reviews if I recall that the Zvezda WWII kits can wind up with more 'realistic' proportions whilst the Battlefront offerings apparently go for vaguely more 'caricature-esque' or 'cartoonish' appearancces.