I don't shoot slugs, so I'm speaking largely from a perspective of ignorance when it comes to air rifle slugs. However, I do have a lot of experience in loading for metallic cartridge rifles, including casting bullets and experimenting with barrels. It seems to me there are two currently limiting factors in extracting high performance from slugs in air rifles. First, power, just need more, and currently this might require more air pressure than current designs and materials allow. Second is barrel design, more easily remedied. I believe when true "slug guns" are designed, they will not use poly barrels, but traditional cut or hammer forged rifled barrels. The power and lead alloy needs to be a combination that allows consistent obturation of the slug, and sufficient muzzle velocity. If the grooves are deep enough, and the slug not extremely soft, that might require considerably more power than is currently feasible. As these heat and pressure levels increase, the barrel material and thickness will need to be considered, as heat dissipation is not something that air rifles have had to address. From an engineering perspective, this could be an interesting endeavor, but I'm not sure that it will ever result in the kind of power that many folks seem to seek. If it is actively pursued, it is a project that will outlive me, so I think I'll just continue to plink around with pellets.
