The tape was used after cleaning to seal the barrels to keep the guns clean, free of moisture and also to prevent the blast air from cooling the guns too quickly. High altitude fighters and escort aircraft had electric heaters attached to the receiver or main body of the guns to keep them from freezing at altitude. As mentioed earlier some aircraft had ducted heat or even small gas fired heaters to apply heat to the guns.
Here is a shot of a waist gun with a heater attached to the receiver which is just to the right of the feed chute.

As also mentioned earlier the tape was an indicator as to whether the guns had been fired. If the the guns were not fired and the tape was still in place the guns didn't have to be cleaned or serviced the same as if had they been fired. If one of the guns tape was not blasted through when theothers were then it could also be an indication of a malfunction.
The complete guns were not removed after each flight simply for cleaning but typically the internal parts were. The parts removed consisted of the barrel, barrel extension, buffer group, bolt and return spring. There parts could slide out the back of the gun when the back plate was removed. You can see this in many shots of the gunners or ground crew cleaning the weapons and the receiver and cooling jacket would still be in the wings or turrets. In a lot of installations if the receiver was removed then the gun would have to be bore sighted again which is additional work and could be pretty involved. Later war aircraft were going to quick detachable mounts which made it easier to remove the complete guns for maintenance.
There were a lot of concerns with proper maintenance of the 50 cal guns especially at altitude. I have a can of oil that is WWII vintage called AIRCRAFT INSTRUMENT AND MACHINE GUN OIL. The Browning 50 and 30 cal ANM2 guns were pretty precice machines just like the instruments in front of the pilot. These close tollerance machines, as well as the rest of the aircraft, had to function from 50 below zero to 150 above zero at crazy airspeeds and in all kinds of extreem conditions. From the instruments to the bomb sights to the radios, engines and their support systems not to mention the air crew and what they had to do to survive.
Pretty amazing to think about really.
_________________
To donate to the PV-2D project via PayPal click here
http://www.twinbeech.com/84062restoration.htmWe brought her from:

to this in 3 months:

Help us get her all the way back

All donations are tax deductible as the Stockton Field Aviation Museum is a 501c3 nonprofit organization. Tell a friend as the Harpoon needs all the help she can get.
Thank you!
Taigh Ramey
Vintage Aircraft, Stockton, California
http://www.twinbeech.com'KEEP ‘EM FLYING…FOR HISTORY!'