Why is there POI shift with moderator?

I’ve noticed the same thing happened to me especially with some of my heavier moderators. The weight of the moderator itself does make your barrel sag or droop* Which in turn will make your point of impact drop

Harmonics probably, but even without that:

If you add a 8oz moderator to the end of a 1/2" diameter, 24" long barrel, angular deflection will be about 1/4" lower at 50yds. So a tiny amount likely, but always there.

I agree, harmonics. The whole gun bounces and shakes and vibrates with every shot. Add a weight to the end of the barrel, and you will change the vibration. Change the speed, and you change the place in the boogie-woogie dance where the pellet leaves the crown, which will change the impact location. 

Hi

The moderators also do the function of air strippers and if it is attached directly to the barrel by adding weight at one point of the barrel the POI can also vary

Regards 

Enkey 

Im Late, but like the others said it’s at first the weight and the harmonics.

i u have the chance, do some accuracy shooting with and without the moderator.

if it’s worse with the mod on u have to retune the rifle for harmonics, noticed the same if I have added a extension to my impact mod.



hope it was helpful.



Regards

Patrick

This is normal and it's not harmonics or anything exotic. It is simply the weight on the end of a not so stiff barrel and shroud fastened to the gun only at the receiver. The longer the barrel and the heavier the moderator the greater the POI change.

Other than the weight lowering the POI if you have a floppy barrel, there should be no change in POI. If there is, something is wrong. 

The way I look at it, if the Moderator changes the POI, then it’s affecting the pellet as it exits the gun. It might not be totally concentric, or something is affecting the flight of the pellet. And every time thats happened to me I either reamed out the moderator an extra mm or so, or replaced the moderator. On any of my guns if the moderator affected the POI, I wouldn’t use it unless I could fix it. 

The unsupported length of my barrel ,for example a Kral Puncher Breaker, is less than 4 inches (actually 2 inches). I highly doubt that would account for much barrel droop from the tightly shimmed barrel band to the crown. The Ronin dropped my POI 2 full mil at 40 yards.

I am usually wrong though....

The way to tell is to dynamic tune your pellet. Speed it up and slow it down to measure the difference made by changing velocity. Droop won't change your groups, harmonic difference will.

I am usually wrong... 

With this situation it follows that when I have made a shot set up with an added moderator, it is necessary to shoot with him or make a shot set up again without a moderator. 

Honestly never noticed one. Wether with or without, or Tatsu to Ronin.

It is possible that the moderator may be hit by the pellet, but if it is, the gun will not group very well. If the POI shift is due to the weight of the moderator, the gun will still group well, but simply in a different place.

It is also normal that the pellet velocity be slightly reduced when a moderator is mounted. This is caused by the column of air in front of the pellet is attenuated by the moderator lowering the differential pressure across the pellet. On my 25 cal.Crown the repeatable difference is 40 fps (940 without and 900 with).