Purpose of Gun Barrel Sleeves on F6F and P-47

[roncl]

I was wondering if someone might be able to answer a question for me. Both the P-47 Thunderbolt and F6F Hellcat had steel (silver colored) sleeves over their .50 cal machine guns. I have seen them called blast tubes, jackets and barrel sleeves. What was the actual purpose of these sleeves? Would anyone possibly have copies of the Erection and Maintenance (E&M) Manuals for these two aircraft (F6F - AN 01-85FB-2, P-47 - AN 01-65BC-2)? I am sure they would provide a definitive answer. The P-38 also used these sleeves over their four .50 cal guns, and in the P-38 E&M manual it is stated that: “The machine gun barrels are equipped with close fitting sleeve assemblies to prevent air from blowing into the gun compartment”.

I am presuming the sleeves may have been used for the same purpose on the F6F and P-47, but I am not sure. If anyone who has the E&M manuals for these two aircraft could take a picture or scan of the respective pages and post them (or advise what the manual says), that would be outstanding.

Thanks.

Ron

[Edgar]

Before flight guns were normally cocked ready for firing; this meant that the barrel, all the way back to, and including, the breech mechanism, was open to cold (actually freezing) air, which could (and did) freeze the mechanism solid, and cause weapons to jam. Blast tubes (and fairings on cannon,) with the addition of seals, either metal or fabric, kept the cold air out until the guns had started firing.

[Slartibartfast]

Some aircraft used hot air to keep the guns functional at high altitude. Can't rightly say if the Jug or Hellcat were so equipped.

[GMK]

The muzzle blast from a 50 calibre machine gun firing is pretty intense. 'Blast tubes' extended to the wing leading edge (or beyond) enabling the positioning of the weapons themselves at the optimum position in the wing without being unduly limited by barrel length.

[roncl]

Thanks guys. As Edgar mentioned above, it would appear that the barrel jacket kept the cold air out of the gun compartments, and also protected the guns from moisture and dirt.

[roncl]

While we're on this topic, any idea why the Hurricane IIC did not have fairings for its four 20mm cannon? I think it was one of the few British Aircraft that did not have fairings over its cannon barrels.

[Jennings]

My guess would be because it was primarily a ground attack airplane, not intended for combat at high altitudes where the guns might freeze up.

[Edgar]

It's quite possible that it was thought not to be worth it, with the Hurricane being considered obsolescent when the IIC was built.

To fit the fairings on the Spitfire, Tempest and late Typhoon, it needed a special rigid casting to be let into the wings' leading edges, and a casting, in the Hurricane's wings, would have had to be cut off, to decrease drag, on the IID.

[mawz]

The Mustang Mk1A also had only limited fairings for its 20mm and was also intended for the Army Co-operation role. Low-altitude & Mediterranean theater use was probably the reason for this.

[breadneck]

Just a little tip. For you 1:72 scale lovers i just found out Master Model has released at least three turned gun barrel sets for the Grumman F6F. Am i buying...YES I AM

http://www.ebay.com/itm/171834307015?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT