What's wicked about that? A .50BMG HMG with a decent cyclic RoF and a piddly ammo capacity. Most attack helos mount autocannons that fire far bigger rounds at far greater maximum RoFs and carry a lot more ammunition. I have no idea what use those HMPs are supposed to serve. Why not go for 1 minigun with a greater ammo cap instead? Would weigh less and take less space, and would achieve the exact same thing.

Do you mean the XM5 Electronic Fighting Vehicle System -- which is apparently just your basic electronic warfare center on an APC chassis, currently considered by/in development for the US Army -- or do you perhaps mean the XM8 Advanced Battle Rifle which was discussed most recently in this thread?

I guess Herstal first produced the 12.7mm (and 7.62) pod for customers who wished to arm transport or combat helicopters (things like Bo-105 or Kiowa) who didn't have a turret anyway. The Italian when they designed the Mangusta (the first real European attack chopper) also did not include a turret at first, and used Herstal pods as a temporary solutions when they realized in Somalia that guns were a lot more useful than missiles in your average post-Cold War peacekeeping operation. For the Tigre, the French and German were going to make more or less the same mistake (you know, "Block Fulda Gap or Die"). Both French and German anti-tanks version weren't supposed to get a gun turret. However the French recon/support version and the export version were given one, and it's likely the French will finally have turret on all their Tigre. The German, on the other hand, seemingly want to keep the door fully open for a future "sniping turret" conceived by Mauser, and will wait for it using (again) Herstal pods as a temporary solution.

Of course modular pods have a lot smaller ammo capacity than an integrated turret. Chosing a smaller caliber allows to take more ammo. Considering they're mostly used against infantry, it's not going to make a lot of difference. Besides, two .50 pods would carry a total of 500 or 800 bullets. That's not that far from the amount carried by most attack helicopters : 750 rounds for the AH-1 Cobra and South African Rooivalk 20mm gun ; 450 for the Tigre 20mm ; 320 for the Italian Mangusta 20mm ; 250 for the Mi-28 Havoc 30mm. Sure, the AH-64 Apache carries 1,200 rounds for its 30mm gun (the US Army originally only asked for 320), and the Mi-24 Hind 1,470 for its 12.7mm ; but let's face it, those machines are bulky. And the gun susually jams long before running out of ammo (the story says nobody ever managed to empty the Apache load, but the M-230 Chain Gun is still a lot more reliable than the Hind minigun, would be jamming after only 200-250 shots). In the end, weigth is a decisive factor for most militaries. Pods are nowhere as heavy as a turret. And the RoF for the Herstal HMG pods is 1,100 /min, against 650 for the Apache M-230.