Well, I don't think replenishing ammo would be hard at all. And in any case, listed below are 2 references on the internet about WWII tail gunners getting ammo from other stations.
If there are no seperate ammo belts around, then I think this would be the process.
Open the top cover of your gun
Remove the ammo belt from the receiver of your gun
Pull out three arm's-length of the belt from the ammo chest and break the belt
Put the lead of your remaining belt in the receiver and close the top cover
Take the broken belt of three arms-length to the tail gunner
If the tail gunner is already out of ammo, he just lifts the gun's top cover, inserts the belt, draws the bolt cocking handle, and fires
If not yet out, he simply fires till he is out, then inserts the extra ammo per previous step
No one needs to splice belts
http://www.thehewitt.net/sam.htmlSam Hewitt 15th Air Force WWII POW
"By now, because of the loss of so many ships (B-17s) that were behind us, we found ourselves to be in the unenviable position of "tail end Charlie"! The battle was now approaching two hours, which for an air battle was rare. Our tail gunner had run out of ammunition and came up to where I was (top turret) to get some of what I had left."
http://www.southernoregonwarbirds.org/b24a.htmlGeorge R. Insley
Brief Bio:
George R. Insley …major theater of wartime operations was in Europe (ETO), from September 1943 to July 1945, with the 66th and 506th Squadrons, 44th Bomb Group, 2nd Air Division, 8th Air Force, based in Shipdon, England. He flew a total of 48 combat missions and was awarded a Distinguished Flying Cross with one star and 8 Air Medals.
"There we were, two groups, one a B-17 group and one a B-24. When the fighters showed up the B-24 leader slid our formation in under the B-17 group, for more protection as well as for more defense against the fighters. So where did they attack? Yes right in my corner, low left and last. You can see why they called it Purple Heart Corner. My tail gunner, George Federlin shot up all of his ammo and the waist gunner took more ammo to him."