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Why does the POI change with a Moderator on?

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fe7565

fe7565

Member
Oct 13, 2015
1,855
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VA, United States
I noticed a while ago that my moderator changes my POI. Is that the rule or the exception? 

Does the moderator become in essence the extension of the barrel? My understanding is that it's very important that the crown of the barrel is done right or accuracy can suffer. That's because if the barrel crown is not evenly round the high pressure air leaving the barrel can push the pellet off its trajectory. Am I right? So if I add a moderator some of that high pressure air still carries on inside the moderator and the baffle rings can become crowns? Can someone enlighten me?



 
Most benchrest competitors don't use a moderator, LDC, etc (I do....*). As one experienced competitor pointed out, "it's not helping you".

Makes sense as the blast of air coming off the crown has to squeeze out the front of the moderator along with the pellet. Without the moderator much of the blast exits laterally after leaving the crown.

As SD noted, consistent POI shift can be adjusted out with scope sighting. Bigger dispersion, flyers, etc, cannot. You need to test carefully both with and without the moderator.

*Needs to be quiet where I practice. Plus the thing is glued on :(
 
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Interesting , I have a Steyr LG 110 FT rifle. The rifle came with an air stripper. At first I thought that I do not need the stripper. But, as soon as I started to shoot in my rifle my groupings at short and long ranges started to drift. I was very confused. Then a friend of mine suggested that I should put the stripper back on, my groups came together and my accuracy was wonderful. I guess it has to do with either, a barrel weight and it might also change the harmonics of your barrel. 

Now there are other guns that I have, like my FX Royale 400, that I have put and LDC on, and it does not care what I hang off the end of the barrel. I have had Air strippers, to clean barrel on it, and there has been no changes on my POI, interesting.

Thanks 

Biagio 
 
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"fe7565"SDellinger, yes barrel harmonics plays a part too. But (if I understand it correctly) that's not the same as the pellet traveling over (or exiting from) uneven surfaces like a crown...but more a result of the vibrations/resonance created by the air and pellet traveling through and leaving the barrel..
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Anything you attach to the barrel, will affect the harmonics. Browning's BOSS muzzle brake, as an example, was adjustable to take advantage of this property. One would make it a bit longer or shorter to dampen the harmonics so that the bullet would exit when the muzzle was either at the top or bottom of the vibration, thus increasing it's precision. Sights, still, have to be adjusted accordingly after the group size is minimized.

The only time a moderator would not affect the point of impact (due to barrel harmonics) is if it were a shroud that was part of the breach and the barrel makes no contact what-so-ever with the shroud. The inherent problem with such a design would be that the baffles would have to have a large enough hole so that you don't get baffle strikes. That would make the thing a bit louder. 

There is the slight change in velocity that one may encounter with the moderator as well, but for reasonable ranges, the velocity variations may be greater than any increase or decrease in the average.
 
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One would make it a bit longer or shorter to dampen the harmonics so that the bullet would exit when the muzzle was either at the top or bottom of the vibration, thus increasing it's precision.
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I have heard this before. But one would think it doesn't matter where the muzzle is when the pellet leaves, as long as it is in the same place every shot (and ES is small).

If this is true then POI wander should be from inconsistent harmonic patterns,
 
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"nvelkhunter"Take a look at danhor7's video. He notes a 1% change in fps between with and without.
http://airgunnation.dev/topic/home-made-moderator/page/2/



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The velocity increase should be attributed to the moderator acting as the extension of the barrel thus allowing the pellet to speed up longer inside while the air is propelling it. But, that does not explain POI shift left or right. The POI shift is very noticeable and inconsistent for my Daystate with a moderator on. I have two different moderators. I suspect that because my MVT has a built-in chrono tube and the passages inside the air stripper act like "crowns" with uneven exit orifices. May remove the shroud and chrono to see how it does with a bare barrel.
 
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@I_like_irons :

"The only time a moderator would not affect the point of impact (due to barrel harmonics) is if it were a shroud that was part of the breach and the barrel makes no contact what-so-ever with the shroud. The inherent problem with such a design would be that the baffles would have to have a large enough hole so that you don't get baffle strikes. That would make the thing a bit louder. "

This a very interesting point. My Daystate Airwolf MVT's 17in barrel has a fork that slides over its end inside the shroud. Then an air stipper/chrono combo follows that which also acts as a moderator. The shroud is held on the barrel by two allen screws. The breech end of the shroud fits up snug against the breech body, it may or may not touch it, or may touch it partially. So my barrel/shroud combo may or may not be "free floating". My .22 MVT "kicks" a lot more than my .25 Vulcan. I can see that during the firing in my scope-view. The MVT kicks higher and takes longer to settle down after each shot. I suspect that the barre;/shroud combo may flex during each shot where the shroud may or may not touch the breech. If it touches the breech that may affect each shot. So I moved the shroud a bit away from the breech, about the width of two sheets of paper. Will see next time I take it out to the range. But that's barrel harmonics.

Still, I think that all those internal stuff inside the shroud can affect the pellet as long as such high pressure air is behind it. Some with the Vulcan added some 1mm to 1.5mm holes near the end of their shroud that allowed some of the air that built up inside the shroud (to be later dissipated towards the front) instead to dissipate towards the rear. I think the Airwolf MVT may benefit from this also, albeit the rear of the shroud near the breech does not have any o-rings so it's not sealed towards the rear.

Stock photo of an Airwolf (not MVT) front with shroud removed and an MVT shroud where it meets the breech:


 
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"Kim"
One would make it a bit longer or shorter to dampen the harmonics so that the bullet would exit when the muzzle was either at the top or bottom of the vibration, thus increasing it's precision.
Click to expand...
I have heard this before. But one would think it doesn't matter where the muzzle is when the pellet leaves, as long as it is in the same place every shot (and ES is small).

If this is true then POI wander should be from inconsistent harmonic patterns,
Click to expand...
I think this is very true. As long as the barrel is in the same position every time it vibrates during a shot when the pellet leaves the barrel the POI should be the same. But if you use a different pellet weight, or type, or air pressure left in the tank (unregulated) varies, the harmonics probably changes with each pellet weight. This may be purely academic, and may be the harmonics does not need to be dialed in with each different pellet weight/velocity combo because the effects are negligible. I suspect that the powder burner guys would know if they need to calibrate barrel harmonics to each and every bullet type they fire.
 
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"fe7565"
"Kim"
One would make it a bit longer or shorter to dampen the harmonics so that the bullet would exit when the muzzle was either at the top or bottom of the vibration, thus increasing it's precision.
Click to expand...
I have heard this before. But one would think it doesn't matter where the muzzle is when the pellet leaves, as long as it is in the same place every shot (and ES is small).

If this is true then POI wander should be from inconsistent harmonic patterns,
Click to expand...
I think this is very true. As long as the barrel is in the same position every time it vibrates during a shot when the pellet leaves the barrel the POI should be the same. But if you use a different pellet weight, or type, or air pressure left in the tank (unregulated) varies, the harmonics probably changes with each pellet weight. This may be purely academic, and may be the harmonics does not need to be dialed in with each different pellet weight/velocity combo because the effects are negligible. I suspect that the powder burner guys would know if they need to calibrate barrel harmonics to each and every bullet type they fire.
Click to expand...
The reason you want the bullet or pellet to leave when it is at the end of the vibration (top or bottom) is that it is not moving at that point in time. A little bit earlier or later, it is not moving very fast. When the muzzle is in the middle of the "swing" it is now moving quite rapidly. The projectile leaving the same little bit early or late will have a much greater change in where the muzzle actually is. Look at how a pendulum swings. At the point where it is changing direction, the speed is quite slow. In the bottom of the swing, it is moving quite rapidly. 

Your last sentence about needing to calibrate the barrel (or load for the barrel---generally) is spot on. I have a particular 6.5 x 55 Swedish Mauser that has unusual vibrations. The vertical is about 90 degrees out of phase with the horizontal. I played with the same bullet, powder, primer, and case but only changed the charge weight in order to map what the barrel was doing. I could get it to group with a vertical spread of six to seven MOA, with a horizontal of less than one MOA, or the reverse where the vertical was about one MOA but the horizontal was now about six. In the middle, the group is round and about 2.5 to three MOA. (Note: The vertical and horizontal are actually about 5 degrees clockwise from true vertical and horizontal. That barrel is interesting.) 
 
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