FX M3 POI Shift (With vs Without Shroud)

[bigHUN]

Back in time somewhere early 2020 when I got my MK2 I was reading modding posts here @ AGN, and I made those changes myself as well.
First, how many orings outside of barrel in between blocks? Kind a lazy to brows for the BOM (bill of materials) but I guess 6 or 7 orings. The factory specs is NBR 70 change those to 90. A bit harder to push in the barrel but with some patience you will be good. You cannot expect a same softness/hardness oring will perform the same up her in Canada or down South in much warmer climate . Change the material to your situation.
Second, do the same inside barrel, I put an oring on the liner every 100 mm.
Third, make a small cone on muzzle end also inside liner nut, and when you tighten the concentric will sit in same centerline. Wrapping a teflon tape is almost the same but less convenience.
Forth, torque the nut the same and mark the position with a line. I made myself a two fla socket wrench for my torques screwdriver and set it to 1.70 Nm. Works for me.
There are more but need a coffee first

 

[RickH]

Every gun is different. Five rifles built and set up identically will shoot differently. One will like one pellet and the next two are awful with pellets from the same tin. True of powder burners also. Looking at a single gun, ask yourself what is different and how might that change have physically affected that gun? You removed a suppressor? How is the gun physically different? Use a different supressor? How is it different from the other? How is the gun shooting differently? Whatever you do, only make one change at a time and see how that change works. Keep track of every change and results. Don’t get in a hurry. It may take some time and quite a few shots but once you get a routine going you’ll solve these problems more easily.

Rick H.

 

[AirGunAction]

When I bought the M3 I fitted the carbon fibre barrel sleeve. My M3 came with only 2 o-rings installed on the barrel liner.

Remember the shroud screws onto the front barrel liner lock nut - Therefore If you tighten the barrel liner lock nut only slightly then the shroud will tighten the barrel liner lock nut further when you put the shroud on and hand tighten the shroud.

I think what I am experiencing is torsional deformation caused by torque force applied to the barrel liner lock nut acting on a hollow rod (The barrel liner).

This is normal but as mentioned the shift of the POI by 1.2 inches at 25 yards does not seem normal to me.

 

[Bobbybeef]

I JB welded my carbon fiber sleeve to my liner in my FX Crown and then proceeded to index and the POI varied 5/1/2" at 35 yards and I kept the group that hit closest to the center of the oray of groups which is WRONG from a reliable person. They said I should have picked the tightest group, which was actually 5 pellets in the same HOLE no deviation I was shocked so I am going to reindex at 50 yards and if I get to a group that shoots that tight, I will stop and leave it there! The gun is a 177cal. crown about a year old. I glued a sleeve onto the liner of my other crown 22cal. I shot it at 35 yards and the group was 3/8" I will index that one next. When I redo the indexing for the 177cal. I am going to take pictures and post on the forums to let people know what can be done.

 

[qball]

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.

This is definitely puzzling, can't really think of other reason for it. Are the grub screws for the barrel tight? If you look through the shroud is there anything out of place? Maybe try another liner/barrel somehow?

 

[AirGunAction]

I could find nothing unusual that I could spot visually.

Yes. My next steps are to test with a second barrel liner and shroud.

 

[njepsen]

POI shift
Ive been following this discussion with interest, and have done lots of testing also. I have also found with several rifles (PCP and powder burners) that a horizontal POI change is normal. Who says that barrel harmonics are only up and down? They are not - they are left and right also. Try holding a garden hose 3 feet from the end and turn it on. It will go all over the place - up and down as well as left and right. A barrel is not too different unfortunately.

The ideal is that the barrel weight/barrel length/bullet speed/the way you hold the rifle/anything else that affects it - to be such that the movement both up and down and left to right - is as small as possible. This means stiff heavy barrels and good bedding. (Even the firing pin impact on a 22LR will cause the barrel to start vibrating before the bullet starts to move!!)

Secondly - ideally the projectile should leave the barrel when the barrel is at the maximum up or maximum down, or max left or max right, or max combination of all of these, because the barrel spends more time at these nodes or maximums than it does at other places in its travel. Especially bad in the in the middle.
So - at these extremes - small variations in bullet speed mean that the barrel position is more repeatable, and the group size is smaller.
So anything that alters the barrel harmonics will alter the position that the bullet leaves the barrel - and hence the POI shift.

The shroud or muzzle break or tuner can/will alter the barrel position in its up and down AND left and right position.
(I'm not talking about clipping here - no projectile should touch a baffle, because that will cause a huge POI shift.)

Indexing:
Last weekend, i spent about an hour with a .22 silencer on my FX impact testing the group size at 75 yards with 1,2,3,4,5,.6 baffles installed. The silencer baffles screwed on and were removable 1 at a time. The group size was best with 3 baffles installed. The testing involved a number of 5 shot groups with each combination, before there was a repeatable result.
Ive also spent some time indexing a new FX .22 1:16 liner. My liner when new looked like the crown had been done with a pocket knife. It was horrible. When I did the indexing thing, the 12,3,6,9 o'clock groups at 50 yds were 3 or 4 inches apart. The indexing as done with the shroud and silencer in position and the jam nut tightened to the same torque each time. I then took the liner out, crowned it properly in a lathe using a microscope and a sharp tool, polished it with a cotton bud and 1uM polish and repeated the indexing. The POI shifts were half the size suggesting to me that the poor crowning previously was part of the issue.

 

[wooferocau]

POI shift
powder burners) that a horizontal POI change is normal.

I have NEVER had POI variations once a load , and POI reference is set that are non environmental ( eg wind) on ANY Powder burner i have used or experienced....

 

[coastal drifter]

I use a Donny fatboy that comes with changeable insert ends depending on caliber needed. I started with the smallest for the caliber needed 177/22 and though it wasn't clipping it had poi shifts so I went up to the next size 22/25 and problem solved.

 

[JaceSpace1369]

I could find nothing unusual that I could spot visually.

Yes. My next steps are to test with a second barrel liner and shroud.

How tight do set those 2 screw at the end of the block?

 

[Trucker3573]

I have said this before in other threads. I watch whoteewho on YouTube everyday. Many times he will break out a rig that hasn’t been shot in awhile. They almost always require a re zero. Same ammo sighted for colder bore. I don’t have much CF rifle experience but poi shifts over time seem normal from research. I don’t know I’m convinced people read too much into it. If it happens right in front of your eyes and you changed nothing maybe so. Adding anything to the barrel I would expect the poi shifts to happen right in front of your eyes. However, this example of over an inch at 25 does seem a lot. I think the more important question would be after the poi shift happens after adding things to the barrel does it group normally. If so good to go don’t stress.

 

[coastal drifter]

I have seen videos on poi shifts with powder burners ETC and one cause is how you rest or even hold the gun regardless of brand or type of gun. I tend to let the gun move as it wants and concentrate on keeping the sight on target from trigger pull to follow through. Most know but for those who might not, never rest the barrel of the gun on anything use the stock if using a bag or rest. My 2 cents and a dollar might get you a soda.