The H9 is probably a tungsten alloy body with tungsten powder inside. Or tungsten sliding weights.
Heavy metal (in the manufacturing sense) almost always refers to tungsten alloys. If it was tungsten carbide it would just be called carbide.
I thought I would post this here. I think it is appropriate. Garand Thumb is an active duty USAF spec ops guy.
If ye love wealth greater than liberty, the tranquility of servitude greater than the animating contest for freedom, go home from us in peace. We seek not your counsel, nor your arms. Crouch down and lick the hand that feeds you; and posterity forget that ye were our countrymen."
Namely the fact that HK lowers are fully compatible to any pmag etc, while he suggests that HK lowers require emags etc.
A bit puzzling to me, because of course he may have a civilian gun in the video, but he should have the firsthand knowledge of 2012 and later spec guns.