I couldn't begin to tell where I found it, but somewhere on FN's site is the information I was looking for. The web site says there are 2 pistons, the owners manual says there are 2 pistons, but according to FN the late production (they don't say when) SLP shotguns have only 1 gas piston.
Since my range is only a short walk from my house I did a little familiarizing, shake down, break in and in general getting to know the SLP. The best shotgun I've ever owned, probably not. Best squad car shotgun I've ever had, maybe maybe not. Best house gun I've ever had, don't know yet. What I do know is that the SLP works with very heavy loads and very very light loads. Several hundred rounds down range so far and not a single malfunction. Not even my venerable old 870 was 100 percent, I actually short stroked it once. The SLP is capable of filling many different roles and doing them all quite well. It's not the softest shooting shotgun I've ever had but even with the very heavy loads it's tolerable. Only time will tell if this becomes my all time best, so far it is doing well.
There is only one piston in the later models (I bought my SLP two years ago). I had trouble with light loads and did the Surecycle mod (great people by the way). I haven't had a single issue since then, regardless of load.
I joined this forum to find some piston info, this thread happened to be on top.
I bought my 18" SLP back in March. It came with two pistons. According to the manual addendum the piston in the shotgun was the light piston, the one inside the box was heavy (labeled 1 1/2 OZ or heavier).
I've only been shooting slugs through the thing and having a good time with that. However, I have a boatload of Federal LE flitecontrol 00 buckshot which is 1185 fps which I just tried the other day. Doesn't come close to cycling. Target loads did not cycle, heavy target high velocity loads dont cycle. I took it apart and saw the heavy piston installed.
However when I went home and got the other piston it was marked as the heavy piston as well. Both had gold accents and looked identical.
So I sent FNH a message and then went searching for forums. But upon reading this they only ship with 1 piston now? This might be a problem for me if the "light" piston doesnt exist for me. I saw the sealing service and its nice to have the option but I was definitely not planning on that sort of thing.
I bought my 18” SLP back in 2012 and it has two gas pistons. I use the light piston most of the time but I did my some Federal slugs and some Brenneke Black Magic slugs which are both 3”. I used heavy piston instead and my SLP cycled and shot fine.