Saw this in the helo forum. Think your best bet may be to modify the remote mount M60s with hand grips and mount them on tubing from the ceiling in the cabin door. UH-1Bs didn't usually have the flex mount door guns.
Simmilar set up to this.
All I have seen in 1/24 scale is a M2 .50 cal on the Italeri and Hasagawa Willys jeep kits.
Here is a SeaWolf (US Navy) UH-1B with .50 cal
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