Urban POI Shift Defies Extensive Testing

[mr3D]

I have an Urban that exhibits significant POI shift. POI shifts consistently with pressure, with pellets at 1700psi impacting ~15mm higher at 30 yards than those fired at 2900psi. I had previously posted about this and thought I had solved the issue by adjusting the moderator end cap, but have since moved from an apartment with very limited opportunities to go shooting into a house with a basement. I have a 10yd range in the basement that has allowed me to conduct extensive testing, but I have not been able to isolate the issue.

I can think of 5 possible causes of the POI shift.

1. Scope not aligned with bore axis before sear is released.
2. Barrel vibrations after sear is released
3. Shifting of entire gun after sear is released
4. Pressure imbalance/jetting in moderator
5. Variation in FPS

Tests were designed in an attempt to isolate the effects of each of these issues. The testing was conducted by firing two two-shot groups at a sample card, one each at 1700psi and 2900psi. Two shot groups were chosen to conserve pellets, and because the ~5mm POI shift at 10yds was much larger than the typical sub 2mm groups from that range. Throughout the testing process I ended up shooting multiple sample cards from the same configuration, so I believe the data collected should be sufficiently reliable. Currently, about 50 sample cards have been generated with different combinations of features.

The test(s) for each factor were as follows:

1. Scope not aligned with bore axis before sear is released.
A 3D printed scope mount was constructed that clamped directly to the barrel. Additionally, the barrel band was temporarily removed.

2. Barrel vibrations after sear is released
A 3D printed barrel band was fabricated that featured much tighter tolerances and more robust construction than the OEM barrel band. Additionally, 3D printed shims were placed between the air tube and barrel that applied positive upward pressure to the barrel. Testing was conducted using the shims both with and without the 3D printed barrel band.

3. Shifting of entire gun after sear is released
Without a secure gun vise, a method of testing that provided 100% assurance was not possible. Several tests were used instead, including testing the effects of light vs firm hold and attaching weights to the gun. Although not absolutely conclusive, it is not believed that the gun shifts enough during the shot cycle to cause such a large POI shift.

4. Pressure imbalance in moderator
Internals and end cap of moderator were removed.

5. Variation in FPS
Velocity was measured using an airsoft type chronograph with 3D printed adapter to maintain concentricity with bore. Additionally, a test was performed with the airgun inverted. During the inverted test, groups at 1700psi impacted lower than those at 2900psi instead of higher.

None of these tests have 100% fixed the POI impact. I have seen some variation in the spread of POI shift, but am not able to nail down any specific cause. At this point I have more or less given up, but just thought I'd throw this baffling issue out there for entertainment value if nothing else. If anyone has any thoughts on the logic or methodology of my testing I'd be happy hear some ideas!

[MB]

Surely point 5 should be accurate inverted or right way up as gravity does not care, lower psi will result in lower velocity ergo : hitting lower

1). Remove Barrel Band - free float barrel
2). Check for Clipping in Moderator
3). Put on a normal Scope Mount - Attached to breech rail and not barrel
4). Clean Barrel
5). Single load pellets don't use magazine to see if its a mag issue
6). Try different pellets - best in mine HNN FTT 14.66, HNN Terminator and JSB 14.35
7). How are you shooting? Offhand. Bench, if off bench on Bipod or rested on a Shooting Bag
. What spread are you getting FPS?
9). Check Barrel lock screws are tight.

This is based on my personal experience modifying and customizing my Urban, hope it helps

[moorepower]

I had the same issue and removed my barrel band. Problem solved. The tube was bowing under changes in pressure and it does not need to move much, when the barrel band is tugging the barrel down to start with.

[desertplinker]

watching this thread. I have an Urban that I have not shot. Barrel is out having shroud made and I will  free float barrel when I get it back. Also, since I only have 27 yards I have turned the power down.

[mr3D]

Desertplinker, free floating should work well due to the heavy contour of the urban's barrel. However, I would pay special attention to the interface between the barrel and the breech. There is only 2 inches of engagement, and if the hole in the receiver is slightly larger than the barrel it can cause the barrel to shift around, even with the lock screws tight. When I had mine configured free floated I used bits of aluminum foil as shims wrapped around the barrel, as I found slight horizontal shifts in POI could be cause by pushing the barrel from side to side.

[mr3D]

I just completed another test with the barrel scope mount as far forward as possible and a shim applying upward pressure to the free floated barrel, as seen in the pic. The result was the same 5mm of POI shift at 10 yards.

Is it possible for a barrel crown irregularity to cause variation of POI at different pressures? Keep in mind that this gun will print VERY small 5 shot groups. I have inspected the crown to the best of my ability with the molded on moderator housing and can not see any problems.

[mr3D]

Well, I've made a minor breakthrough with tonight's testing.

For no real reason, I decided to perform my usual test with the hammer spring adjuster backed all the way out. Previous testing has been conducted with the hammer spring 11 turns clockwise from all the way backed out. Instead of running the test at 2900psi and 1700psi I chose 2700psi and 1500psi. Velocity with H&N FTT 14.66 grain pellets was 665fps at 1500psi and 630fps at 2700psi. I ran two tests, and during the first test fired a third shot at a separate bullseye for 1500psi because I was unhappy with the two shot group size. As you can see, there is none of the POI shift seen at higher power settings. The gun was configured as seen in my previous photo, except the barrel was fully free floated instead of having a wedge.

I do not have a good explanation as to why there is such a dramatic difference at the lower power setting. However, I wonder if it has something to do with vibration/harmonics. at lower power settings the shot cycle is much smoother. Part of this is because with the hammer spring adjuster all the way out the hammer is fully free floating as would be seen in an SSG. Still, it seems highly suspicious that other Urban owners (or PCP owners in general) do not seem to experience my specific problem.

[MB]

Mine is shooting 780 - 800 fps with FTT 14.66 Grain poi shifts where only present when losing pressure, have a regulator in mine and no issues

[mr3D]

Mine is shooting 780 - 800 fps with FTT 14.66 Grain poi shifts where only present when losing pressure, have a regulator in mine and no issues

800 fps with 14 grain pellets seems to about the sweet spot for Urbans. I'm tempted to get a regulator for my gun. If my POI shift is due to the difference in shot cycle vibrations between low and high pressure I'd think a regulator would cure the issue. MB, Which reg do you have in your gun?

[MB]

Mine is shooting 780 - 800 fps with FTT 14.66 Grain poi shifts where only present when losing pressure, have a regulator in mine and no issues

800 fps with 14 grain pellets seems to about the sweet spot for Urbans. I'm tempted to get a regulator for my gun. If my POI shift is due to the difference in shot cycle vibrations between low and high pressure I'd think a regulator would cure the issue. MB, Which reg do you have in your gun?

I have an Alteros in mine...