Re: Another Mariner Question: How do you mount a bayonet lug
The S&J isn't going to work on the 20" 590 or Mariner.
The rear barrel lug (which is where the bayonet lug is positioned on the 20" 590A1 and the 590) is right where the mag tube clamp would need to be. The forward barrel band is also going to make getting the clamp on there difficult.
For a Mariner you are pretty much SOL unless you want to pick up a 590 or 20" 590A1 barrel and have it finished to match. I wouldn't epoxy a lug on there because you will end up popping it off if you ever actually end up doing anything with a bayonet. You may be able to have someone weld one on, but that is going to expensive and will need refinishing as well.
If you want a bayonet pick up a 590 or 20" A1.
And $100 for a knife is not bad, particularly for a good one. That would be the starting point. Now it could be said that the M9 is not a great knife, they are serviceable. You can find the knock-offs cheaper, but I have no question about the durability of my Phrobis (it is one of the early units, and the materials and construction were a little better than late production examples), which I'd definitely put above a LanCay and way beyond many of the others.
If you want to get a good bayonet for less, you could hunt down a used one on Ebay or in a surplus store. Just check the edge and make sure it hasn't been messed up. Bayonets end up doing wierd things in the .mil sometimes.