Seneca .308 Cal, 118 Grains, Flat Point, 100ct

By US convention all standard 30 cal rifle barrels (firearms) are .300 bore diameter to the top of the lands and .308 bore diameter to the bottom of the grooves. Air rifle pellet barrels are not built to this convention and can have shallow grooves as well as a smaller bore diameter to the tops of the lands and this is because pellets are made from soft lead alloy with a soft skirt. True slugs tend to be harder alloys and lack a skirt, unless they are cavity back design. In the US we call a .308 barrel a 30 cal because it measures .300 across the lands. In the UK a traditional .303 barrel measures .303 across the top of the lands and can vary .312-.314 to the bottom of the grooves. When I make bullets for British guns I have to do more sizing to get the correct fit as their barrels were not standardized as early on as American barrels.

Airgun pellet barrels are .300-.303 sized, but .308 slug guns are actually .308 sized. These are .308, what you need is a .300 or, at max, a .303 slug.

These are 308 slugs not 30 caliber pellets big difference