The P-47 is fine with 'plain' tube, as the guns were in blast sleeves (stainless steel). And aircraft such as the FW190, or Bf109G with under-wing gondolas look good with the tube too.
And as you mentioned Geo, aircraft with the guns 'buried' in the wings, such as the P-51, P-40 etc, only have the muzzle showing, or the first (full scale) inch or so, without the perforations being visible, so the hypodermic idea works fine.
I normally 'superglue' the metal tube into a plastic 'sleeve', cut from suitably sized plastic tube, to fit inside the wing, and hold the 'barrel' in place in the wing opening. This also allows easier positioning and fixing, as the plastic can be cemented with ordinary poly cement, rather than trying to position and fix with CA, especially if the metal tube can't rest on the inner wing surface.