There might be more cloned models that use this gas system, and there might be others running a similar gas system, I don't know.
Poking around at the SLP, I've spent a lot of time looking closely at the two pistons it comes with and searching the internet for common issues. Seems a fairly common issue is piston springs breaking (probably mostly on the SLP, since they're getting run hard in 3-gun). The consensus on the fix is to replace the spring with a sleeve, a modification that they're calling a sealed piston.
All this raised more questions than anything, but I think I've worked my way through it all and figured out the system, why it fails, and how we might actually be able to fix it.
First question was, "why does this piston have a piston inside it, and WTF does that spring do?" The stupid manual (that tells me not to use any "hydrocarbon" based lubricants) doesn't really cover the theory of operation of this firearm, all it points out is that it's a short-stroke gas piston system. And that's if I'm not misremembering, and read it somewhere else... So I had to figure it out myself. It's actually pretty interesting... There are two gas ports in the barrel about midway down, at the barrel ring. They feed bleed gases down into the ring to two matching holes that are angled toward the back of the shotgun. That area of the barrel ring is larger in diameter because the front of the piston assembly seals inside of it. Now, the piston assembly is actually two pistons, a spring, a screw-in spring retainer and gas rings. One piston is inside the assembly, on top of the spring. When the gases hit the assembly, they push it to the rear, but the spring allows the internal piston to move rearward, uncovering holes in the piston body to vent gases.
Simpler way of saying it - the piston assembly is what pushes the bolt rearward, the internal piston and spring are a gas regulator. Once I realized that, I started having problems with the idea of converting to a "sealed" piston. It also got me started on the path of what was causing spring failures.
The SLP comes with two pistons - one marked for light loads (under 1 1/2 ounce) and one for heavy loads (over 1 1/2 ounce). Now, anybody that reloads should understand that there's got to be more reason to swapping self-regulating gas pistons around than just the projectile weight. What is going to matter is the pressure in the barrel when the projectile passes the gas ports. A high brass load is usually going to have more pressure than a low brass load, projectile weights be damned. I'm sure dwell is going to affect things here (barrel length past the ports and projectile velocity), too.
One of the reasons I figured out the regulation thing is that the spring in the heavy piston has quite a bit lower rate (and probably less preload, but that's a guess) than the one in the light piston. I was wondering about that, until I realized that the internal piston was a regulator. Then it made sense. It would be easier to push the heavy piston open because they were trying to slow (or restrict) the cyclic rate to keep the aluminum receiver from getting beat to death by the steel bolt assembly. Another thing to consider is that the piston pushes rearward on a thick plastic sleeve with a steel rod sticking out of it to transfer motion into the bolt group. Either way, there's no way in hell I'm going to put a sealed piston in my shotgun, since it was a pretty major investment for my broke ass.
So why are people breaking springs at 3-gun matches? IMO, it's either because FN/Browning/Winchester needs to sell us a piston with a different spring - whether that's an intermediate spring rate, a spring with a variable rate, more pre-load on the spring in the "heavy" piston (I might actually try that one out) or something else, I don't know. I can think of other ways to tweak this system that should result in better reliability like different gas port configuration, but I really think a spring is going to be the solution. I think the problem with their system is that it wasn't built to take into account the fact that 3-gunners shoot target loads (usually low brass 7 1/2 shot) along with slugs one after another in the same stage.
Maybe if the manufacturer were to put a sleeve of some kind to limit spring travel, but still allow enough for effective gas regulation.
The last, and most extreme measure to try would be adjusting the amount of gas actually going to the piston. That would get pretty involved, but I could see partially sealing one of the ports, then opening it to the point that your low brass of choice functions reliably and has good ejection (if the shells are falling at your feet when you shoot, that's a bad sign, BTW) and see what happens from there.
Let me know what you think.
I'm not an engineer, but I'm pretty good at figurin' out how stuff works.
ETA: Did some more looking around. It looks like FN isn't using aluminum pistons anymore, since I can't shave the one sitting on my desk with a good knife (don't have a magnet handy). Maybe they got the spring deal figured out and I won't have to worry about it. I might give 'em a call tomorrow, if I can find time away from working on my budget 3-gun rifle project tomorrow at lunch. Want to test that sucker out this weekend, too.
Poking around at the SLP, I've spent a lot of time looking closely at the two pistons it comes with and searching the internet for common issues. Seems a fairly common issue is piston springs breaking (probably mostly on the SLP, since they're getting run hard in 3-gun). The consensus on the fix is to replace the spring with a sleeve, a modification that they're calling a sealed piston.
All this raised more questions than anything, but I think I've worked my way through it all and figured out the system, why it fails, and how we might actually be able to fix it.
First question was, "why does this piston have a piston inside it, and WTF does that spring do?" The stupid manual (that tells me not to use any "hydrocarbon" based lubricants) doesn't really cover the theory of operation of this firearm, all it points out is that it's a short-stroke gas piston system. And that's if I'm not misremembering, and read it somewhere else... So I had to figure it out myself. It's actually pretty interesting... There are two gas ports in the barrel about midway down, at the barrel ring. They feed bleed gases down into the ring to two matching holes that are angled toward the back of the shotgun. That area of the barrel ring is larger in diameter because the front of the piston assembly seals inside of it. Now, the piston assembly is actually two pistons, a spring, a screw-in spring retainer and gas rings. One piston is inside the assembly, on top of the spring. When the gases hit the assembly, they push it to the rear, but the spring allows the internal piston to move rearward, uncovering holes in the piston body to vent gases.
Simpler way of saying it - the piston assembly is what pushes the bolt rearward, the internal piston and spring are a gas regulator. Once I realized that, I started having problems with the idea of converting to a "sealed" piston. It also got me started on the path of what was causing spring failures.
The SLP comes with two pistons - one marked for light loads (under 1 1/2 ounce) and one for heavy loads (over 1 1/2 ounce). Now, anybody that reloads should understand that there's got to be more reason to swapping self-regulating gas pistons around than just the projectile weight. What is going to matter is the pressure in the barrel when the projectile passes the gas ports. A high brass load is usually going to have more pressure than a low brass load, projectile weights be damned. I'm sure dwell is going to affect things here (barrel length past the ports and projectile velocity), too.
One of the reasons I figured out the regulation thing is that the spring in the heavy piston has quite a bit lower rate (and probably less preload, but that's a guess) than the one in the light piston. I was wondering about that, until I realized that the internal piston was a regulator. Then it made sense. It would be easier to push the heavy piston open because they were trying to slow (or restrict) the cyclic rate to keep the aluminum receiver from getting beat to death by the steel bolt assembly. Another thing to consider is that the piston pushes rearward on a thick plastic sleeve with a steel rod sticking out of it to transfer motion into the bolt group. Either way, there's no way in hell I'm going to put a sealed piston in my shotgun, since it was a pretty major investment for my broke ass.
So why are people breaking springs at 3-gun matches? IMO, it's either because FN/Browning/Winchester needs to sell us a piston with a different spring - whether that's an intermediate spring rate, a spring with a variable rate, more pre-load on the spring in the "heavy" piston (I might actually try that one out) or something else, I don't know. I can think of other ways to tweak this system that should result in better reliability like different gas port configuration, but I really think a spring is going to be the solution. I think the problem with their system is that it wasn't built to take into account the fact that 3-gunners shoot target loads (usually low brass 7 1/2 shot) along with slugs one after another in the same stage.
Maybe if the manufacturer were to put a sleeve of some kind to limit spring travel, but still allow enough for effective gas regulation.
The last, and most extreme measure to try would be adjusting the amount of gas actually going to the piston. That would get pretty involved, but I could see partially sealing one of the ports, then opening it to the point that your low brass of choice functions reliably and has good ejection (if the shells are falling at your feet when you shoot, that's a bad sign, BTW) and see what happens from there.
Let me know what you think.
I'm not an engineer, but I'm pretty good at figurin' out how stuff works.
ETA: Did some more looking around. It looks like FN isn't using aluminum pistons anymore, since I can't shave the one sitting on my desk with a good knife (don't have a magnet handy). Maybe they got the spring deal figured out and I won't have to worry about it. I might give 'em a call tomorrow, if I can find time away from working on my budget 3-gun rifle project tomorrow at lunch. Want to test that sucker out this weekend, too.
