Slugs mostly use the hipfire stats of a shotgun to check accuracy, so Mattocks mostly improve your velocity by a solid margin over Scatterguns. On stationary targets they have equivalent accuracy because they have the same hipfire stats. Slugs do not benefit from the Mattock's tighter buckshot spread, but also do not suffer from the Mattock's lower buckshot damage. With 100 extra m/s velocity, Mattocks are still better than Scatters, but maybe not by a big enough margin to buy them if you already have Scatterguns to toss slugs on. This also means that Mattocks and Scatters mix pretty well as you will only notice the velocity difference at the outer edge of your effective range.