Other Light Weight Slugs

You’re right to think there could be issues beyond accuracy. Very light slugs in a big bore can sometimes cause the valve to open and close more abruptly, which might lead to extra wear or even damage over time, especially if the rifle is tuned for heavier projectiles. They can also result in higher velocities that push the pellet or slug past the optimal stability range, hurting consistency. It’s not guaranteed to cause damage, but if your setup is tuned for heavier slugs, it’s worth keeping that in mind before running a lot of super light ones through it.
 
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If your big bore is tuned for heavy slugs and you shoot lighter slugs you will get way fewer shots with the lighter slugs. Your mind wants to think the opposite. Lighter slugs use less power / air to get them up to a higher FPS. But with big bores, those heavy slugs take longer to get up to speed and longer to leave the barrel vs. the lighter ones which will be already out the end of the barrel by the time the valve shuts. And all that extra air is wasted. These is as others have stated the problem with exhaust valves being damaged from the "slamming" of the valve when shooting lighter slugs. They don't allow the gun to "self-regulate" like the heavy slugs do. The heavier slugs kinda slow the shot cycle down so to speak. I don't own an AF big bore but I've read where they are more prone to this problem because of the inline valve setup.
 
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If your big bore is tuned for heavy slugs and you shoot lighter slugs you will get way fewer shots with the lighter slugs. Your mind wants to think the opposite. Lighter slugs use less power / air to get them up to a higher FPS. But with big bores, those heavy slugs take longer to get up to speed and longer to leave the barrel vs. the lighter ones which will be already out the end of the barrel by the time the valve shuts. And all that extra air is wasted. These is as others have stated the problem with exhaust valves being damaged from the "slamming" of the valve when shooting lighter slugs. They don't allow the gun to "self-regulate" like the heavy slugs do. The heavier slugs kinda slow the shot cycle down so to speak. I don't own an AF big bore but I've read where they are more prone to this problem because of the inline valve setup.
That's what I was thinking I remembered....at least something along those lines. Thanks for the info. I'm getting ready to take a new-to-me Extreme Big Bore .357 out to sight in and will be using some slugs in the 150 - 177 gr range. I also may pick up some different Century springs (hammer side) and was thinking maybe trying some lighter slugs at that time. This is a later EBB from after the move to Texas, so I'm not sure exactly what spring is in it right now. I know they came with two different springs when Brent had the company, but I'm not sure with the second generation guns and I only have the one in the gun.
 
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Along with tuning the air delivery system to be more efficient with the light slugs, you're likely going to have match the speed and slug length to the barrel twist rate so it will stabilize well. (Or get a new barrel with a twist that mates well with the desired speed and length of slug.)
I won’t be changing out the barrel any time soon. I was just wanting to try some lighter slugs than what I see being used in these guns. I realize they are built for max power and that’s what I’ll be shooting it for, if I do draw another Coues deer tag in Arizona! Thanks for the perspective on slug length and speed. I’ll definitely keep that in mind.