Actually we just plain aren't going to have an ammo counter. Do what they did in real life, either fire until it's depleted, or reload when action is over. The most we might have is a total number of magazines/clips that you have left (So say, 6 mags for the MP40, or 12 clips for the K98k). Basically, ammo will be standard load carried by the soldier for the weapon in particular. That also means engineer classes will have less ammo (for instance, standard practice in the German army was that only infantry riflemen carried 2 rifle ammo pouches, the rest only were issued one), and MG classes will have their ammunition spread out amoungst the squad, thus requiring squad cohesion in order to keep the MG gunner supplied. In that case, all british riflemen will carry 3 Bren mags each (as in real life), and German riflemen will carry a belt of 250 rounds each. The gunners themselves will only be given the amount of ammunition that gun team would have carried themselves, so 500 rounds (IE two belts) for the German MG34/42 gunner, and 90 rounds for the Bren gunner (IE 9 mags).
That said, there are no plans to have a dedicated loader, due to the massive problems that would create for animations (one would actually have to link both players into the same animation, as well as the problem in getting a player to literally do nothing all game but lay next to the gunner and stare at an animation. Much the same way for gun crews on AT guns/heavy MGs/mortars/etc- they will be one player manned, partly to ensure that as many players as possible are taking active roles (and so that weapons teams don't take too many players away from the front lines, considering the engine limitations on player counts), but also due to issues in gameplay- people will, generally, not want to play something that involves them just staring at their screen not doing anything. Also, gun crews operated as a well oiled team that has trained together for months or even years- not casual gamers who are meeting for the first time. Thus, in order to make gun crews effective, it is much better to simply hand it all off to one player, than to try to rely on someone to only ever want to click his mouse once every 5 seconds to load a new shell in.