PETRAS,
Yeah,the al391 is a damn good shotgun,not to be confused with the al3901,$600? not a bad price if she's in good shape,take a close look@ the muzzle and choke tubes to check for scoring from steel shot,some guys run steel through a full choke that will damage the muzzle area and invector chokes,make sure you can remove the choke tube as well,large size steel shot will weld the choke tube to the muzzle of these guns so be careful,used shotguns are always a gamble.
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Well norske,somewhat true,however when we look@ all the different makes and models in semi auto these days,all are pretty darn good and in his range of $1500 most if not all come with fitting shims,what fits one guy off the shelf won't fit another guy w/o some adjustments,gunfit is crucial for great performance as you stated.
Barrel length is also important,but that's another topic.