Tim, I'm going to write this as if your prob wasn't fixed and I'll let you know what I believe your prob was. You're fixed, I understand, but I think your prob might have been a tad diff.
The double stop design on the Mossy 500 relies on two shell stops to cycle the shells as it pumps. One holds the round in the tube and the other grabs the shell as it pops out of the tube on cycling. That allows the gun to only feed one shell at a time. If your one stop was all that was the problem, 99% of the time you would just have had crud in the rail slot behind the stop. You can flip the gun over in your lap and pump it slowly and see where the hull sticks and won't let go. For that, you'd just pop the rails out and q-tip the slots clean, lube, and then reassemble. If that's NOT the prob, your next thing to check would be the shell lifter. The lifter is one of the parts that work those two stops. Here's the key to what I think was your problem. The lifter rides on those two pins in the receiver sides....and if one of those pins is even halfway out, the lifter grabs the bolt like a vise and it can't go back or forward. This was on another forum I frequent a few years back and another fellow had it happen a few months back, too. You can experience that sort of lifter jam by simply taking a ball point pin or a small dowel and push one of the lifter pins out of its hole with the gun assembled. Push it out so it's slightly inside the receiver. Now try the pump. See what I mean? It grips the bolt solid so it can't go forward. One fellow thought it was a design flaw and said so once....it's not, it's a slight part adjustment that has to be performed every time you reassemble your gun from cleaning. That lifter effects the stops....and if it's out of one pin hole or the other, it will miss that step. Your bolt not moving forward, to me, is the key symptom. More on that in a few secs. Making a fresh pot of coffee.
Now on your stock bolt problem....that's another problem entirely and potentially the most dangerous problem that you can have with a Mossy. A few years back, folks started using aftermarket pistol grips and aftermarket stocks that had wrong bolts. Trying a Mossy pistol grip with the full stock bolt for example. That bolt is too long, as you noticed, and it sticks forward into the path of the trigger. With the safety off and the trigger blocked, it stops the gun from working. The natural reaction is to simply pull the trigger a bit harder....and when you do, you hear a tiny "CLICK" inside the gun as the safety connector tab breaks off the trigger. At that second, your safety is rendered useless. You put it on and think it's safe and it's not. It's loaded and in battery. The only fix is to replace the trigger group or replace the trigger itself. The trigger group is the easy way since not many folks I know want to work on the trigger replacement itself. To avoid it, you either washer the bolt so it's not so much inside the gun or you cut it a tad if the stock can't be washered.
Back to that lifter thingy from earlier. The pin out of the hole thingy, a lot of folks think of as a design flaw since you supposedly can drop your gun on its side and have one pop out. If you do, you need to work it back carefully in the loading port til the pin pops back in....or strip the gun. It's not a design prob so much as a maintencance prob, though. The lifter gets compressed every time you remove it and replace it after cleaning. That lets the pins go a bit slack in the holes. It's a spring. You need to carefully spread the lifter arms every time you reassemble it to avoid that prob. Properly adjusted, you'll find an exact spot where the lifter will work perfectly without hitting the receiver sides....and at the same time it won't hit the bolt on every pump. When you hit that one exact spot, and it's easy to get each time you reassemble, you'll be surprised how much quieter the gun is and how smooth it can be.
This got a bit long. Watching Bill Oreilly and he gets me bloviating
Edit: Guys, I REALLY apologize for the typos and mispellings in this one. I'm sort of on pain pills at the moment. I even proofed this one twoice vefore posting it, too.
I give up.
