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How a moderator affects your zero or POI

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cavedweller

cavedweller

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Dec 5, 2018
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This post is more of an FYI for the newer or who hasn't learned it.

Of recent, i have learned how to click (or use the turrets for something other than zeroing my scopes)

At home I have to shoot with moderators.

Of note - the average moderator hung off the end of a barrel (not a shroud) requires 4-5 more upward clicks than without any moderator. This observation included stout barrels like my HMX1000 and my Steyr 110 and my TM1000 pistols.

This is a big deal if you are shooting HFT or hunter field target with a pistol or any situation or law that requires you to remove a moderator.

Moderators attached via the shroud don't seem to cause any deviation or drop in POI as no weight is actually placed on the barrel.

When i attached a fairly lightweight weihrauch moderator directly to my Thomas barrel i had to add 4 upward clicks to my zero at 30 yards. But when i added a heavier Zero DB moderator to my Thomas using the shroud, i had to go back down 4 clicks at 30 yards.

This Also affected my short little TM1000 pistol barrels same # of clicks or changes in elevation were noted for the Weihrauch.

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I only have the one PCP.
You bought at least one of mine from Cameron when I sold everything.
The PRod which I sometimes use with a TKO moderator.
I found that I cannot tighten it too much on the barrel as the POI shifts a little.
Maybe the pellets are lightly touching the moderator, I don’t know.
When the shift happens, I un-screw the moderator a bit and the POI returns to the proper location
 
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On either my FX Royale 500 or Boss my holdover is compensated 2 full mildots when using a mod (DFL Ronin). Quietness has its price. Oddly enough, I find both guns to be infinitesimally more accurate, not less, with the mod. Both guns have a "floating" barrel. Unsupported after connection to the block (no barrel bands).
 
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It's a really big deal on my 700mm impact too. If I use my air stripper it shifts poi up a lot and is rimfire loud. I use donnyfl yokozuna and Ryu on the guns that are sensitive and just removing 1 baffle section can change how the gun is behaving completely. It's not just weight, it's the air on muzzle exit too. It's very nuanced.
 
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Frank in Fairfield said:
I only have the one PCP.
You bought at least one of mine from Cameron when I sold everything.
The PRod which I sometimes use with a TKO moderator.
I found that I cannot tighten it too much on the barrel as the POI shifts a little.
Maybe the pellets are lightly touching the moderator, I don’t know.
When the shift happens, I un-screw the moderator a bit and the POI returns to the proper location
Click to expand...
the threaded tension on the moderator/barrel doesn't seem to matter, it is truly a weight thing...
but... If you study the picture of the Thomas, you will notice i use a silicone washer between the barrel shroud and the moderator - this absorbs some vibration at the junction. This actually tightened my groups up a bit so now i use this on ANY of my moderated airguns.
 
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Trucker3573 said:
I know my alpha wolf changes poi with a moderator. Rather positive it shoots lower but I really don’t pay attention that much. As long as groups don’t suffer with it on wherever it shifts 🤷‍♂️…doesn’t really bother me
Click to expand...

yep... the only thing that matters is that you know how to compensate with the moderator on or off.
 
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Mike_Lenardon said:
When I add a moderator to any of my guns I test it first. I check poi with and without the can. If it changes the poi then it goes back on the shelf.
Click to expand...
POI can change if barrel droops... But it can still zero and hold zero.
I own 20 moderators of all types and they will ALL cause a slight drop if attached directly to the barrel.
if attached via the barrels shroud nothing changes on my airguns as no weight is placed on the barrel.
 
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If the moderator is heavy enough to deflect the barrel then the POI will be impacted. I struggle to believe they are that heavy, however. I tested a barrel band yesterday that clamps the shroud to the airtube on a Bullshark or P35. It was apparently not built as intended and it deflected the shroud and thus barrel upward by .010. You can guess what happened to the POI. I did not have enough travel in the scope to re zero it. The supplier is making a new one. It surprised me, however, how easy this band was to install despite the fact that the fairly robust barrel was being deflected.

So maybe the weight is the issue but it's hard to believe that a relatively light moderator can deflect the barrel by even .001 inch. I could do the math but I am not currently planning on it. It seems obvious to me that the barrel can support a moderator that weighs less than a pound.

I am not suggesting that a moderator cannot change POI, I am just suggesting a different way it affects things. I think hanging the small weight out on the end of the barrel affects it's vibratory response to the pressure pulse and sliding friction of the projectile when we take a shot. That is the design principle for the barrel tuning devices. I know using my clamp on chronograph affects my POI so I see no reason a screw on moderator cannot. But I think the effect is due to a change of the barrel's vibration, not the weight deflecting the barrel.
 
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JimD said:
If the moderator is heavy enough to deflect the barrel then the POI will be impacted. I struggle to believe they are that heavy, however. I tested a barrel band yesterday that clamps the shroud to the airtube on a Bullshark or P35. It was apparently not built as intended and it deflected the shroud and thus barrel upward by .010. You can guess what happened to the POI. I did not have enough travel in the scope to re zero it. The supplier is making a new one. It surprised me, however, how easy this band was to install despite the fact that the fairly robust barrel was being deflected.

So maybe the weight is the issue but it's hard to believe that a relatively light moderator can deflect the barrel by even .001 inch. I could do the math but I am not currently planning on it. It seems obvious to me that the barrel can support a moderator that weighs less than a pound.

I am not suggesting that a moderator cannot change POI, I am just suggesting a different way it affects things. I think hanging the small weight out on the end of the barrel affects it's vibratory response to the pressure pulse and sliding friction of the projectile when we take a shot. That is the design principle for the barrel tuning devices. I know using my clamp on chronograph affects my POI so I see no reason a screw on moderator cannot. But I think the effect is due to a change of the barrel's vibration, not the weight deflecting the barrel.
Click to expand...
Interesting - I chrono mine with and without the moderators and haven’t noticed a fps change other than standard deviation. Definitely interesting topic.
 
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JimD said:
If the moderator is heavy enough to deflect the barrel then the POI will be impacted. I struggle to believe they are that heavy, however. I tested a barrel band yesterday that clamps the shroud to the airtube on a Bullshark or P35. It was apparently not built as intended and it deflected the shroud and thus barrel upward by .010. You can guess what happened to the POI. I did not have enough travel in the scope to re zero it. The supplier is making a new one. It surprised me, however, how easy this band was to install despite the fact that the fairly robust barrel was being deflected.

So maybe the weight is the issue but it's hard to believe that a relatively light moderator can deflect the barrel by even .001 inch. I could do the math but I am not currently planning on it. It seems obvious to me that the barrel can support a moderator that weighs less than a pound.

I am not suggesting that a moderator cannot change POI, I am just suggesting a different way it affects things. I think hanging the small weight out on the end of the barrel affects it's vibratory response to the pressure pulse and sliding friction of the projectile when we take a shot. That is the design principle for the barrel tuning devices. I know using my clamp on chronograph affects my POI so I see no reason a screw on moderator cannot. But I think the effect is due to a change of the barrel's vibration, not the weight deflecting the barrel.
Click to expand...
When you attach the chronograph do you attach it directly to the barrel?
I can probably figure out ways to test this but if it was just barrel whip or what some call harmonics issue then the point of impact would not just be just a straight down vertical drop?
 
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I don't think it's complicated. ANY thing you add to the barrel, or even touches the barrel changes the harmonic signature. Simply, adding or removing things from the barrel causes the pellet/slug to exit the barrel at a different point in the vibration pattern when the shot is fired.

Of course, a significantly weighted item added to the barrel can (as suggested above) also cause an actual physical 'flex' to the barrel itself.

Even while out hunting I try to always rest the forened of my rifle's stock in the same location on my shooting sticks. The more consistent in all regards that you execute shots, the more consistent the shots will fall.
 
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My clamp on chronograph clamps to the shroud of my P35s where I have noticed an effect on POI. I have clamped it both to the bottom of the shroud and the side and seen effects both ways. On a P35 the shroud is supported by a metal fitting close to the action and by a device screwed onto the end of the barrel inside the shroud. The screw on piece is aluminum but contacts the shroud via an O-ring. So if you push on the shroud you are pushing on the barrel but the force has to go through an O-ring. My "solution" to the clamp on chronograph is to use a tripod mount instead. It does not contact the gun so there is no issue. Not all my airguns are affected by the clamp on but it's easiest just to consistently use the tripod mount unless that one has a problem (like me shooting it).

I agree that the POI consistently going down is more consistent to a minute deflection than a vibratory effect. The vibratory effect should be more random. But it is completely possible that your barrel has a downward response to a small mass increase out on the end of the barrel. That may also be a typical way for a barrel to respond. I just don't think the mass is great enough to cause a deflection big enough to shift the POI. I may soon get a barrel band that doesn't push my barrel up so much and I will be interested to see the impact on accuracy and also the ability for my chronograph or the way the shroud is supported to affect POI. In theory linking the airtube and shroud should increase the stiffness of both affecting vibration as well as the ability to be mechanically deflected.
 
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Well I got bored and did some math. The equation for the deflection of a cantilevered beam is deflection = Load x Length cubed divided by 3 times the Modulus of Elasticity times I. I of a tube = 3.14 times (OD to the 4th power - ID to the fourth power) divided by 64. E is 29,000,000 for steel (I did not try to figure out what type of steel this is for, it is probably a bit low for barrel steel). I used my P35 for an example. It has a barrel OD of about .526 and I assumed my 22 so the ID is .22 inches. For a 1 lb moderator I get a deflection of .000682 inches. That would be a heavy moderator so I also looked up my Tanto, it weighs about 4 ounces.

But how much would that move the POI? That is another potential math problem but I have an empherical example I used. I know that a barrel band that moved the end of the barrel by something less than .010 pushed my POI by at least 6 inches. The 1 lb moderator would move the POI by 0.4 inches using this example. But my Tanto would shift it by .1 inches.

Based upon this I still don't think a moderator will move the POI on my P35s by enough to be obviously apparent but for heavier moderators with barrels that are unsupported for longer lengths I think it is possible there could be a noticable effect. Especially if you use a heavy moderator. I gave you the equations in case anybody wants to do this for their gun and so you could see how different parameters affect things. The weight is a simple linear effect. The unsupported length is quite short for the P35 at 6 inches and it gets cubed. So it is much more important. The diameters (internal and external) get raised to the fourth power so they are very important.

Putting on a barrel band would effectively reduce the unsupported length to near zero and greatly reduce the effect. The support added by the airtube is not infinite so it would not strictly speaking be a zero unsupported length but the I would get pretty complicated to come up with.

My phone is not very fancy but it has a calculator that can raise values to the fourth power which is the most complicated part of this so you probably have a tool that could do the calculation.
 
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