FX M3 POI Shift (With vs Without Shroud)

[Triggers and 4X4s]

Where are all the FX M3 experts. I need some advice because I cannot figure out what the cause is.

AIRGUN
FX Impact M3 700mm STX pellet liner. No mods, stock standard. Floating barrel with carbon fibre inner barrel liner.

TEST CASE
I went to the range to test the “barrel indexing” theory with my M3. To do the test I removed the shroud and silencer because it is easier to loosen the nut in the front of the barrel and rotate the barrel (FX lingo, the liner) each time between shots.

My “indexing” results - The POI remained the same no matter what rotation I do to the barrel i.e. positions 12h, 3h, 6h and 9h, all the same POI. So barrel “indexing” did nothing for me.

Note: During the test nothing was overtightened, mostly hand tight or a 1/16th of a turn with a 10mm spanner (Front nut on barrel).

PROBLEM
As soon as I put the shroud and/or silencer back on the gun there was a huge POI shift. 3cm to the right and 3cm up. When I loosen the shroud, say half a turn then the POI returns to normal but the shroud is loose.

Other than a misaligned shroud (End to end) I am stumped what the cause is?

Anyone else experience this?

 

[LDP]

Adding things to a barrel changes harmonics. Same thing happens to 1” od firearms barrels if a brake or anything is added to the barrel.

 

[Triggers and 4X4s]

Thanks LDP.

Are you saying the fitting the stock standard shroud hand tightened that it is normal for a POI shift to occur. Even when the POI shift is 1.2 inches to the right and up at 25 yards.

 

[LDP]

Thanks LDP.

Are you saying the fitting the stock standard shroud hand tightened that it is normal for a POI shift to occur. Even when the POI shift is 1.2 inches to the right and up at 25 yards.

Yes. Like I said any time you shoot a rifle no matter the propellent adding anything to the barrel will most likely change poi due to the change in the barrels harmonics. Your barrel is like a tuning fork. Adding anything that changes length or weight will most likely change the harmonics. Its nothing new and has been discussed at length in the firearms industry as well as the airgun world. You can google it and find lots of info including high speed camera video showing what a barrel does under the stresses of firing. There are rare cases that adding a shroud, mod or brake doesnt impact poi or very little but thats not the common result.

In the case of your shroud it could be a larger impact if its not perfectly straight. It would cause tension that could increase the poi shift. How does it shoot for accuracy with the shroud on and off?

 

[bucketboy]

Thanks LDP.

Are you saying the fitting the stock standard shroud hand tightened that it is normal for a POI shift to occur. Even when the POI shift is 1.2 inches to the right and up at 25 yards.

Not normal, adding weight to the barrel would naturally drop it down slightly, the mod would also slow down the exiting air reducing muzzle flip, both these factors should if anything cause the poi to drop slightly.

Sounds like a shroud/ mod misalignment issue to me.

Bb

 

[Trucker3573]

I agree with LDP. Adding those other things into the equation you don’t know what that is going to do to harmonics. I don’t think it is as simple as weight pulling in one direction.

 

[bucketboy]

I agree with LDP. Adding those other things into the equation you don’t know what that is going to do to harmonics. I don’t think it is as simple as weight pulling in one direction.

Harmonics may alter the group size not poi by the amount mentioned.

 

[Triggers and 4X4s]

The grouping with or without the shroud on is perfect, pellets stack one on top of the other.

Without the shroud the windage adjustment on my scope are not perfectly on 0 (Centre), it is quite a bit out, to the right, when you adjust the scope to get the POA and POI the same.

However with the shroud on the windage adjustment on my scope is exactly on 0 (Centre).

So my thinking is that although the POI changes when I put the shroud on, it is actually more true in terms of alignment with the shroud on.

I am not worried about elevation changes, that should be normal. It is windage or horizontal shift of the POI that I do not understand.

Harmonics should not have an effect on windage change (Horizontal).

 

[qball]

I'm with @LDP on harmonic but also agree with @bucketboy that it shouldn't be that much at 25 yards.

Have you tried to turn the barrel jam nut 1/4 of a turn after hand tight? From the sound of it your barrel jam nut could be loose and when you tighten the shroud it tighten the jam nut also and shifts POI. Just a wild guess but figure worth a try. My recommendation would be to make sure everything on and for the barrel is tight and secure, not gorilla tight but a little tighter than just gentle hand tight.

 

[Triggers and 4X4s]

I'm with @LDP on harmonic but also agree with @bucketboy that it shouldn't be that much at 25 yards.

Have you tried to turn the barrel jam nut 1/4 of a turn after hand tight? From the sound of it your barrel jam nut could be loose and when you tighten the shroud it tighten the jam nut also and shifts POI. Just a wild guess but figure worth a try. My recommendation would be to make sure everything on and for the barrel is tight and secure, not gorilla tight but a little tighter than just gentle hand tight.

Good point and exactly what I was thinking at the range. I did exactly what you suggest, results were still the same ie no change.

 

[Airgun Al]

Even 2 different moderators , the same exact length and diameter, can alter P.O.I.

 

[Vetmx]

I would be more shocked if you said you had no POI change after you screwed on a shroud. And remember, your shroud also has an air stripper.

 

[Triggers and 4X4s]

I would be more shocked if you said you had no POI change after you screwed on a shroud. And remember, your shroud also has an air stripper.

Yea, I think I might be over analyzing. OCD you know

 

[bucketboy]

Even 2 different moderators , the same exact length and diameter, can alter P.O.I.

I make moderators, All of my guns shoot more accurately with a moderator than without (probably due to the pellet leaving in cleaner air) but unless there is a great deal of difference in weight or an alignment issue there should be no or minimal poi shift.

I would bet good money on the threads of of either the crush nut or the shroud assembly are off centre and pushing the barrel out of true. This is an unfortunate side effect of having a barrel system made from numerous parts.

Bb

 

[bucketboy]

I would be more shocked if you said you had no POI change after you screwed on a shroud. And remember, your shroud also has an air stripper.

I would be devastated if any of my shrouds did cause poi shift.

Bb

 

[Triggers and 4X4s]

I understand what everyone is saying but 1.2 inches to the right at 25yards is significant. This is what stumps me.

 

[JDGriz]

I understand what everyone is saying but 1.2 inches to the right at 25yards is significant. This is what stumps me.

It could depend on the baffle ID size and baffle alignment. A pellet might be pushed towards the center of the baffle by the air venting between the pellet and the baffle, if it's off-center and if the actual clearances are low (which they should be, for sound moderation).
I don't know if anyone has actually tried this experimentally with known offsets and clearances, somehow.

 

[Healthservices]

Have you tried the tensioned shrouds? If not clocked and tensioned in the same spot the poi can be several feet off!

Allen

 

[bucketboy]

It could depend on the baffle ID size and baffle alignment. A pellet might be pushed towards the center of the baffle by the air venting between the pellet and the baffle, if it's off-center and if the actual clearances are low (which they should be, for sound moderation).
I don't know if anyone has actually tried this experimentally with known offsets and clearances, somehow

If the baffles washer is thin, the pellet has already already left before uneven pressure has had a chance to push it off course, going through a thicker baffle or stripper then theoretically there is more of a chance of disturbance.

Bb

 

[Triggers and 4X4s]

Have you tried the tensioned shrouds? If not clocked and tensioned in the same spot the poi can be several feet off!

Allen

No, I have not tried them yet.

I prefer a floating barrel though.