Well I've fired slugs thru a number of doubles and it's a hit or miss proposition. Seriously though, most SxS guns are lousy at it but now and again you'll find a winner. I once had a Beretta GR2 in 20 gauge that would put ordinary Foster type slugs in pretty near the same hole at 30 yards and exactly where it was aiming too.
I now have a Merkel 20 gauge that puts a slug right on the aiming point at 30 yards from the bottom barrel.
nial