Backpack customs are made based on standard magazines attached to the replicas of LMGs. Therefore they have the same BB capacity. However, there are no contraindications to use a specially prepared container whose size (and capacity) will only be limited the shape and dimensions of the backpack. However Mateusz, the man behind Almost Art, stated that only backpacks which he accepts come into play. He says: "Hello Kitty or old school backpacks will not do."
As a standard, the walls of the BB container are damped with acoustic foam, and the stiffening of the backpack, similarly to the links of the feeding belt, are built of plastic. The magazine for PKM shown in the pictures is powered from the replica's battery. The same case will be with the M60 magazine that is just being created. Of course, there is also a backpack for the M249 planned.
Almost Art also plans something that does not have its own name yet. As Mateusz says it will be a "massacre" built on the basis of replicas from the top shelf. Such an original response to the minigun, but using a system not used in current airsoft replicas. The phrase "not used in current airsoft replicas" probably suggests a multi-barrel design, but the "not used" precludes in revolving solution of Mr. Gatling. What will it be? We will probably see.
Meanwhile, several photos of the backpack for the PKM:
Finally, one more remark from the author: "Please pay attention to the belt itself. I rests on my arm, which seems more ergonomic to me than a dangling belt somewhere under my arm. I does not get caught in branches so you do not need to think about it".
Photos: Almost Art/Magdalena Wielgus, https://www.facebook.com/M.Wielgus.photo/