Barrel length ...affects the sight plane and swing dynamics...in general a 20" barrel isn't very productive on flying targets...a barrel that short, tends to allow the shooter to "slap" at targets vs executing a smooth swing, bang, and follow-thru. About the only way to overcome barrel length is to add weight ...but on a real short barrel like 20" you'll make it nose heavy probably..which isn't good either.
In general a 26" barrel on a semi-auto makes the gun about the same overall length as an O/U with 28" barrels .....or 24" on semi = 26" on Over Under. I wouldn't recommend going under 24" on the semi-auto.
Gun weight if your buddies friend in terms of recoil / an additional lb in the gun will reduce recoil about 20% or so depending on the shell he's shooting. But having gone thru a major shoulder re-build myself...sometimes a gun much over 8 lbs ...takes its toll on my shoulder / and even rebuilt -my shoulder likes a 7 1/2 lb gun somedays a lot better than 8 or 8 1/2" ...so its a combination of all this stuff ...and what feels right to him. I should also temper my opinions with the fact that I'm 6'5" and 290 lbs and have fairly long arms ...so all of my primary guns ( O/U's even in .410 have 30" barrels ) and when it comes to a light gun like a Benelli Super Sport semi auto I went with a 30" barrel there too - to offset the light weight ( 7.2 # ) to give me better swing characteristics.
Personally - when it comes to HD - I turn to my revolvers or 1911's in handguns ..not shotguns ...even short barrels are clumsy in my view. The other issue on defense ...is you can use any shotgun for "Defense" vs sneaking thru the dark clearing rooms or a "Fighting shotgun"...so I wouldn't focus much on HD as an issue / lots of other ways to go - and better in my view.
Focus on a shotgun that feels good to him on flying targets - 26" maybe, 28" is a good happy medium, or a 30" barrel....
In general a 26" barrel on a semi-auto makes the gun about the same overall length as an O/U with 28" barrels .....or 24" on semi = 26" on Over Under. I wouldn't recommend going under 24" on the semi-auto.
Gun weight if your buddies friend in terms of recoil / an additional lb in the gun will reduce recoil about 20% or so depending on the shell he's shooting. But having gone thru a major shoulder re-build myself...sometimes a gun much over 8 lbs ...takes its toll on my shoulder / and even rebuilt -my shoulder likes a 7 1/2 lb gun somedays a lot better than 8 or 8 1/2" ...so its a combination of all this stuff ...and what feels right to him. I should also temper my opinions with the fact that I'm 6'5" and 290 lbs and have fairly long arms ...so all of my primary guns ( O/U's even in .410 have 30" barrels ) and when it comes to a light gun like a Benelli Super Sport semi auto I went with a 30" barrel there too - to offset the light weight ( 7.2 # ) to give me better swing characteristics.
Personally - when it comes to HD - I turn to my revolvers or 1911's in handguns ..not shotguns ...even short barrels are clumsy in my view. The other issue on defense ...is you can use any shotgun for "Defense" vs sneaking thru the dark clearing rooms or a "Fighting shotgun"...so I wouldn't focus much on HD as an issue / lots of other ways to go - and better in my view.
Focus on a shotgun that feels good to him on flying targets - 26" maybe, 28" is a good happy medium, or a 30" barrel....