After measuring all dimensions of the Lewis, both myself and Nick agreed that 76mm pipe is the best size for a scratch build.
After wrapping to simulate the barrel threads, it would be 79mm.
These are the measurements from the actual Lewis;
Main Barrel OD 80mm.
3" or 76mm Pipe with the 1.5mm wire wrap gives us 79mm OD, which is within reasonable tolerance for a scratch built T21
76mm pipe in the UK is readily available, so this allows Ryan to get as close to the 80mm OD as he can effectively.