Scope Zero changes Drastically at 19 yards v/s 25 yards ... Thoughts ?

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sharathkumar

sharathkumar

Member
Sep 9, 2019
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Tennessee, United States
I've got a Centerpoint 3-9x40 AO scope mounted on my FX Impact. My FX is currently setup to shoot Predator Polymags (26 gr / .25 cal) at/around 870 fps.



I zero'ed my scope at 19 yards (i.e. the distance from my bench to the target) and after that it works like a laser beam at that distance.



Now, if I move myself back 6 yards - to 25 yards (i.e. distance from my door to the target) - it seems to hitting about 6" high and 1" to the right.



Initially, I assumed the 1" right drift was wind related. But, even when there is no wind - I see the same 1" right-shift in POI. 

Furthermore, if/when I immediately move back, from the 25 yard mark - to the 19 yard mark, and take a shot at the same target - there is ZERO right-drift.



I'm assuming the POI being 6" higher at 25 yards is because the 19 yard is the near-zero and not the far-zero ... Is that an accurate assumption?

[ FWIW - target and bench at the 19 yard mark are at the same level v/s target is about 1' lower at the 25 yard mark ]





But I don't have any good/working theory about 1" right-shift.



Does it make sense to anyone else?

If so - Could you help me understand how/what might be happening?





Thanks in advance! 😊
 
Sounds correct for height. You might not have your scope mounted level and over the bore or, if it is mounted correctly, you could be canting while shooting to cause the horizontal error. Are you using a level? If not, some verifiably vertical lines through your aiming point might help to be sure you aren't canting. You can use a weight on a string or a level and square to mark it.

I would also move closer to 13 yards and see if impact is on the other side of vertical from the 25 yard impact.
 
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It could also just be that you have mounted a $50 scope on a $2K rifle. No way moving from 19 to 25 yards is moving the POI 6" vertical at 870 FPS and cant has significantly more influence at longer ranges. My money would be on a bad scope.
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I tried mounting my Hawke Sidewinder 4 - 16 x 50 scope on it. But, for some reason - at 19 yards, the POI is about 2 feet below the point of aim, and the top turret is fully turned up (CCW) and has no more clicks left in it.

I tried tall/medium/short mounts - but it didn't really do much to the abovesaid behavior.



Hence, the need to have to make do with the cheaper $50 scope.

FWIW - that very same Centerpoint scope used to be mounted on my Gen 1 Benjamin Marauder - and for 2+ years it was on there - it worked Flawlessly, at all the ranges - namely 25 yards / 19 yards / 10 yards - that I put it through.



I'm having a hard time buying that moving it from the Marauder to the FX somehow suddenly made it go bad. But stranger things have happened ... So - anything is possible !! 😁
 
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On the Hawke, you may have to shim the rear scope mount, between the scope and the ring saddle, OR try a set of FX No limit rings or Sportsmatch adjustable rings, OR Burris Signature Select rings with the adjustable shim inserts. I went through Hades trying to zero a Sightmark Photon XT. A 20 MOA rail and Burris Signature select rings finally did the trick.
 
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A six inch elevation change in 6 yards seems a bit much to me. One problem with the FX guns is barrel droop which is the reason for the FX No Limit mounts. The barrel droops and you are not able or barely able to compensate for it with the scopes elevation turret. A 20 MOA mount or rings will also bring the POI up where it's usable in the scope. As the scope werqed fine on another gun I would suspect there could be a a combination of barrel droop and a barrel indexing problem as well that could account for at least some of your troubles. Check this out to see if it is a problem. Don't forget to mark the starting point so you can get back to it.

https://www.airgunnation.com/topic/indexing-barrel-liners-in-fx-impact-fx-wildcat/


 
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Even without any barrel droop, the height of the scope over the barrel will chew up most if not all of your scope's elevation range at such short target distances. A 20 MOA rail will only further increase the scope height offsetting some of what you gain with rail, so I'd also recommend adjustable rings. I personally prefer the Sportsmatch over the FX No Limits, but either will get the job done. As for the CP scope, they aren't built like tanks and it still think its likely that something has gone wrong inside the tube. I'd stick with the Hawke and good set of adjustable rings. That will allow you to optically center your scope at the desired zero range and give you the scopes full elevation range to work with.

Curious why you are shooting Polymags? Sounds like you are target shooting and the Polys are a hunting pellet. Impacts tend to prefer JSB Diabolos of one flavor or another and they are less expensive than the Polymags. I'd also make sure the barrel is clean. 
 
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Curious why you are shooting Polymags? Sounds like you are target shooting and the Polys are a hunting pellet. Impacts tend to prefer JSB Diabolos of one flavor or another and they are less expensive than the Polymags. I'd also make sure the barrel is clean.
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If/when I get the urge to Plink - I use the JSB MK II 34 gr pellets - which are extremely accurate out of my Impact.

But, I use the Polymags primarily for Pesting. These days, I don't really target shoot much - except as a means to figuring out how a scope-gun-combo works at different distances/positions/etc.


 
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Are you really shooting the polymags at 870 fps? Your problem really smacks of pellet spiraling. Check you velocity again )with the polymags).



.
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I chrony'ed it again tonight:

FX Impact (Power Wheel 5):

  • .25 cal JSB MK II (34 gr) - 858
  • .25 cal Predator Polymag (26 gr) - 962



FX Impact (Power Wheel 1):

  • .25 cal JSB MK II (34 gr) - 686
  • .25 cal Predator Polymag (26 gr) - 771



FX Wildcat:

  • .25 cal JSB MK II (34 gr) - 789
  • .25 cal Predator Polymag (26 gr) - 868


 
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Remove the barrel shroud and barrel tension screw, clean up the threads there, inspect the front of the barrel liner bore for any defects, then reinstall everything. Check for evidence of pellet clipping on the exit of the moderator also. Then put the gun on some type of solid vice mount and shoot it at the short distance for a group, then move the target out to the longer distance without moving the gun and shoot it again. Is there any changes to the original problem?

You could try a new third scope as well. After 3,000 shots through the Impact my scope's parallax and optical center went out of my scope, and it is a higher end (decent) tactical scope too. The POI was erratic at lower sighting distance and side focus was acting weird at longer distances. I finally figured it out when I felt a clunk turning the parallax knob, but the scope seemed good otherwise.
 
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I got a super cheep cell phone camera mount from amazon works meh kinda flickle but any how it used it to track pellet path with phone in slowmo to figure out accuracy issues i has. saw the pellets would do a looptyloo ended up being that the pellet speed was culprit, also ive had 3 CP scopes one was good others junk! On budgut scopes ive had really good luck with tasco world class 2.5 to 10x the other mag models were fickle
 
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I got a super cheep cell phone camera mount from amazon works meh kinda flickle but any how it used it to track pellet path with phone in slowmo to figure out accuracy issues i has. saw the pellets would do a looptyloo ended up being that the pellet speed was culprit, also ive had 3 CP scopes one was good others junk! On budgut scopes ive had really good luck with tasco world class 2.5 to 10x the other mag models were fickle
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I took your advice and went ahead and purchased a cheap scope mount of them on Amazon this past week. But, I could NOT figure out how to get the camera to focus through the scope-lens and actually see the pellet in flight.

Got any pointers/sage-advice to share on how you made it work ? :) 
 
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Status Update:



I cleaned out the barrel with Ballistol + Switched scope to a Nikon ProStaff 5 + Put on a 20 MoA rail + Stabilize/Rest my Impact (i.e. on my Primos Tripod) on the air-cylinder instead of barrel + Set the Power Wheel 1 … With all those things combined – I no longer see any noticeable Rightward Drift !!! 😎



Also, the POI at 25 yards is no longer the 6" higher than it was at 19 yards !!!

[ It's only about 1" high - which is kindda-sortta inline with American Airguns Projectile Calculator thinks it should be ]



If/When I use JSB MK II 34 gr – I can get dime-sized groups at both 19 and 25 yards !!

If/When I use PolyMags (FYI - now that I can load em as is into the SideShot Jumbo magazine – I think we can discount the possible inaccuracies - attributable to variances in clipping length/angle - introduced into the equation) my groups are about quarter sized at 19 and maybe a hair bigger at 25 yards (but that could be attributed to standing-rested-on-tripod v/s sitting-bench-rested)





To be honesty – I can't definitely say which particular "treatment" did the trick.

[ I figured, I'd spend the next week/two experimenting with removing some of the "variables" and figure out which one actually made the difference ]



But, to be even more honest, now that the Impact is shooting as expected – I'm a little nervous about messing with it and/or Undoing any of the gains I've made after all these frustrating weeks of tinkering with it !!

[ That whole if it's not broken - don't fix it ideology at play I guess ! :D ]
 
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