Interesting question... Ecomass is a company that produces a lot of this type of material, and I've worked with them in the past.
Most of the materials out there based on plastic-matrix with metal powder the plastic is the primary mechanical component, and the metal is only added to add weight/mass to the material, and the metal is milled fine enough to pass through the gates of the injection molding machine.
I am a little surprised these materials are being used for airsoft guns, as they're generally not very strong, and they're relatively expensive for what they are. They were originally made for hybrid radiation shielding (mass for gamma, hydrogen for X-ray and neutrons).
I have had this material "in bulk" it was in the form of sheets, and was asked to machine it, it does machine fairly well, but you need to provide a lot of support, as it seemed rather easy for the material to crack and crumble. I might drop the tool speed, and keep with a low feed rate, you don't want to melt it, but you want to make sure you're getting good material removal and not putting a lot of force on the cutting face. You may want to try HSS tooling as this is typically sharper than carbide, and conducts heat better than carbide does.
You might want to check, because I would think these parts would be made of Zamak or some other die-casting alloy.
I'll dig around, and see if I still have some of that material, if so, I'll send it to you (trying to clean the garage anyways).