Browning M2HBK

would this affect accuracy?

Absolutely. I haven't been able to find anything that points to this being anything other than a modification made by lovers of weaponry, there is no real call for this mod in any military unit that I can think of. It loses out on accuracy, velocity which are major requirements of an area weapon like the Browning.

As a former user and passionate lover of the old M2, it just seems a pointless exercise

I agree. You would need to be a world class weight lifter to use it as the mod indicates. So much powder would be burning outside the barrel, I question if it would even cycle.

I think somebody amputated a worn barrel just to make the pic.