Adj3530 - i think it might be a bit big.
From my understanding, .50 cal is 1/2 inch which is 12.7cm. Assuming 1/48 scale, you'd want an internal diameter of 0.26mm.
3/64 inch is 1.19mm
However, i can say that the Tamiya Jug in 1/48 has plastic gun barrels about 1mm in diameter and they look quite good.
i bought some tubing close to scale (0.3mm diameter) to substitute for my build, but, i'll be honest, at 0.3mm, it was just too small and looked a bit weak.
I hope this helps - it's up to you though. 0.3mm is probably most accurate internal diameter, but i suspect the external diameter will be too small to make the guns look realistic. Hence, you might want to go for something closer to a 1mm external diameter.