Wow, stumbled across this old thread and saw no real answer for the OP (I hope he hasn't already passed on ...lol). Technically, what the P-47 has on its barrels aren't blast tubes. Blast tubes are located ahead of a gun muzzle, when the muzzle is still inside the aircraft, to protect the structure (the Mustang uses a short one on the inboard, staggered gun). What the P-47 uses are actually a non-perforated cooling jacket or sleeve. The reason is because the barrels protrude from the leading edge of the wing which usually causes serious problems with the airflow, particularly at high angles of attack, because the standard or ventilated jackets cause a lot more turbulence. This becomes a much bigger issue with laminar airflow wings. The ideal goal is to have the guns housed entirely within the wing or, if not possible, have the protruding barrel act as little like a vortex generator as possible by making it more aerodynamic with a non-perforated jacket or a fairing. Take the laminar winged Hawker Tempest as an example: early production had protruding, faired barrels while the later production had the flush Mk V short barrel cannon. There were multiple reasons for going to the Mk V but the Mk V itself was developed as an aircraft cannon with a short barrel for aerodynamic as well as other reasons.
And now, maybe, the topic can fade back into obscurity with dignity ... lol.