I found the 590A1 20" 8+1 too long/front heavy to wield properly and so when I bought my own A1 went with an extended 5+1 (so 6 or 7 + 1) 18-1/2". Much more handy I feel.
The standard 590 may be perfection out of the box (both with a +4 SpeedFeed stock).
Kaylorinhi, thanks for the pic.
It is of a Mossberg with a great Marine S3 bayonet -- basically an elongated Kabar for the M-16. An excellent tool (one of, if not the, best overall bayonet today). So, look at the distance of the guard from the back of the magazine tube nut extension where it should be resting. That is NOT a very secure mount and probably should not be trusted regardless of barrel or mag tube weight. Compare this to the earlier pic of the Mossberg with an M-9 and you'll see how much further back it sits on the mount.
Sorry, but the shotgun is practically limited to the earlier AR bayonets that are quite available. And their size, shape, and weight are probably better for that shotgun as well, including being shorter. By the way, NONE of these blades are anywhere near 10 inches -- that was WW II. In fact, the longest
, the S3, is 8".