Ahm.
May I present the M36, and the waffentrager, and the Japanese Ho-I and the Avenger, plus (If you count semi-open guns that can still swivel all the way around), the Birch Gun, all the milk trucks, and the Deacon.
Dipping a bit into post-war sweden also lands you the Pvkv II, III, and IV, all of which solidly fall under that envelope.
Them having a turret is perfectly fine, especially since they have a ~+5.0 difference over their non-turreted contemporaries.
The M109 and crew are perfectly fine as-is.