I think that two arms of 12,7 mm are better than one of 38, 44 or 55 mm. In addition not always greater caliber means greater power. The weapon in if it has much to do and nonsingle the caliber influences in its performance. Also I think that the reason that the caliber is smaller is than the weapon of the SV-51 must be able to adapt for the use of a long and short tube because when the SV-51 transform it the front part of the tube is transferred to the legs whereas the weapon is located in the main body.
Also I ask myself if the arms can be used in mode battroid?