The mark I is the rifled slug barrel. Why would you use this with buck shot for HD?
They must have forgotten to put the rifling in my Mark 1.
As rc601962 mentioned, they make one version with a smooth bore and one that is rifled.
I can't speak to it shooting low power dove loads, although I shoot the Federal 100 value pack 1 1/8 oz. #8 shot loads from Walmart. For me, that is about as low powered as I get. I routinely cycle a variety of 2 3/4" 00 buck, both 9 and 12 pellet, through it and have yet to find any brand of buck it does not like. After trying a half dozen different ones, I quit testing. I also shoot 1 oz. slugs (usually Federal Tru-ball reduced velocity) with good results. All of these cycle reliably using the light piston, which is designed for loads less than 1.5 ounces.
I've shot all of the above, after breaking the gun in with about 400 rounds, using the heavy piston, which is designed for loads 1.5 ounces and higher. However, on my Mark 1, after a half day of shooting a shotgun course, I'll start to see cycling issues as the gun gets dirty.
It's nice to know that in a pinch (with a clean gun), I can use the heavy piston if necessary. As such, I'll use the light piston as all of my normal loads are under 1.5 ounces.
If you want a little more info on the Mark 1, I've got a write-up or two on my site that you may find interesting.
FNH SLP Mark 1 Shotgun Review
EOTech XPS2-0 Holographic Weapon Sight
FNH SLP Mark 1 Tri-Rail Extension