if the barrel shoots where it looks , the gun will work fine.......lack of penetration is a sometimes issue with soft foster slugs but they mostly kill well anyway
have shot some brennake slugs thru my sweet sixteen and frankly , it kicks like a mule...to much for a 12 year old IMO
if his Remington is a 20 gauge I'd have him use that reguardless of the full choke several of my hunting pards in Mn use 20 s with standard foster slugs and they have no problem killing deer
