Metal clamp split rings will work but make changing the barrel difficult. They can damage the barrel and the pump. If they slip then your back to beating up the slots. Make a plastic bushing that sits between the pump and the body that limits the stroke. Black delrin would look good and wear well. You could also make a Delrin half round block that would connect to both pump arms. Position it in front of the feed tube so it will hit in the feed tube to limit the stroke. You could also drill and tap the aft end of the pump at the 6-o-clock position for a set screw. The screw could be adjusted to limit the pump stroke. Even a zip tie loop around the feed tube and the pump to hammer screws would work and not require tools or modifying the gun.
I've gotten spoiled by the pump return spring on my Phantom and truly miss that feature on my Bushmaster. I would want to add a pump return spring to the mix so I would go with a stop screw/stud at the 6-o-clock. I would give the set screw a rubber tip and a use it also as a spring guide. This just might be the next mod I do to my SC Bushmaster. Ill post pics when I get to it.