You attempt to lighten the heavy and huge Stallion.
-M1 Stallion: This is a belt-fed machine gun using the enormous, thundering .60 caliber round, exactly twice the diameter of the .30 caliber or 7.62mm rounds. It is a cumbersome weapon, with a large box for the mechanism, and a narrow water-cooled barrel with copper heat sinks. The range and stopping power of the .60 caliber round is undiminished, but the gun only fires about 180 times a minute, relatively slow for a machine gun- because of its size, it might be better described as an autocannon. The gun hits in very tight groups for a machinegun, but it is heavy and can only be pivoted slowly, meaning only very skilled gunners can be accurate at long range. It normally moved by horse on a small wagon, although the new lighter model can be carried in parts by a crew, and is crewed by a team of four or five men. It is both complex (expensive) and uses a lot of metal (moving it to Very Expensive), special M1 Stallion crews will need to operate it in the field.
Your engineers have reduced the weight of the gun as much as they can while still allowing it to contain the force of the .60 caliber round. They've pushed your current steel manufacturing techniques to the limit: A number of bolts, plates, and struts are now drilled with rows of holes, the water cooled barrel is shrunk and fitted with shining copper heat sinks, the casing is half the thickness, and the action has been redesigned to fit in a smaller area, but not much. The heavy wooden carriage was replaced with one made is narrow steel plates with pneumatic automobile tires. Split into parts, a machine gun crew can now carry an M1 Stallion on foot, although they will complain about it.