I FEEL LIKE IM TAKING CRAZY PILLS! 0.8 Mil Vertical Zero Shift Session To Session

[SkeeterMcgiblet]

Hello all, I have a tale to tell about a vertical zero shift issues thats been driving me crazy:

I've got a .308 T3x that I topped with a Maven RS1.2 over Leupold Backcountry mounts (one piece, like Talley's but beefier) with a Silencerco Omega suppressor (QD over brake). Had no issues with zero retention with previous scope (Trijicon Huron) on same set up through multiple hunting seasons and bumps. Zeroed the Maven at my home range (7500') and tested to far target of 380 yds. Next session, no zero change. Friday went to a different range in the plains (4000') to check dope past 400 yds. Zero check on paper showed POI 0.8 mil high... weird but adjusted and verified dope to 1000 yds (all glory to Mr Form and his teachings, when the wind died I got 3 impacts in a row at 1000 yd on a 20" plate). Next session went back to home range and all of a sudden I'm 0.8 mil low... Corrected again... Went again yesterday and surprise, 0.8 mils high. Shot one more round to confirm and called it a day.

I'm at a loss due to the consistent nature of the shift and the fact the zero stays stable through each shooting session. This experience finally made me order some UM Tikka Rings but I'm still wanting to pin down my issue, partly because I don't think its a scope mount issue that I need to address. I'm not the best shooter but I feel 0.8 mil high / low 5 rd groups at 100 are well outside of my ability to mess up a prone bipod & rear bag position.

My plan now is to disassemble, check action & stock, re-mount action screws, mount new UM rings, zero, drop test and retest, and if it passes then I'll re-check dope at long range for any dialing issues.

I've got a few ideas of why this happened but would love to hear others ideas / suggestions

- Scope mounting error? Seems unlikely due to consistent shifts, no horizontal issues either, but possible
- Over-torqued scope caps crushing tube causing issues? Don't think so due to dialing (elev & windage) fine
- Parallax reset error? I don't think so but there could have been from ~400 yds to 100 yds when I first had issues
- QD Suppressor not fully seated causing shift? No baffle strike that I can tell, each session ~20-80 rounds and nothing exploded or launched
(Can has to be QD, cant fit in safe otherwise and can gets swapped to ARs)

 

[ResearchinStuff]

Check for barrel contact on the stock and the recoil lug properly seated in the action.

 

[atmat]

Check for barrel contact on the stock and the recoil lug properly seated in the action.

^^^ Likely this

What are action screws torqued to?

 

[Shraggs]

Are you certain you accounted for DA difference and current temperature?

 

[atmat]

Are you certain you accounted for DA difference and current temperature?

100y zero isn’t impacted by DA or temp.

 

[Shraggs]

Thought he was .8 high at 400

 

[atmat]

Thought he was .8 high at 400

I don’t think so:

Zero check on paper showed POI 0.8 mil high...

but I feel 0.8 mil high / low 5 rd groups at 100m

 

[SkeeterMcgiblet]

Thought he was .8 high at 400

Nope 0.8 mil high / low at 100 yds

 

[SkeeterMcgiblet]

^^^ Likely this

What are action screws torqued to?

I believe I'm at 55 in/lbs on stock "bottom plastic"

 

[Wprinkle]

Did you degrease and locTite/ paint pen everything? Stock touching barrel was mentioned..

 

[SkeeterMcgiblet]

Are you certain you accounted for DA difference and current temperature?

Yes, at 100 yds the difference is negligible

 

[SkeeterMcgiblet]

Did you degrease and locTite/ paint pen everything? Stock touching barrel was mentioned..

Yes, everything blue loctited. Will switch to paint pen for this tear down. Will check barrel channel but did run paper through and didnt feel anything

 

[BRTreedogs]

Swap the old scope back on

 

[Wprinkle]

Yes, everything blue loctited. Will switch to paint pen for this tear down. Will check barrel channel but did run paper through and didnt feel anything

The dollar bill test on barrel channel is kinda a myth.. you want as much as possible almost to the point it looks dumb. With a gap that small you get a pine needle or whatever in there it COULD cause problems.

 

[SkeeterMcgiblet]

The dollar bill test on barrel channel is kinda a myth.. you want as much as possible almost to the point it looks dumb. With a gap that small you get a pine needle or whatever in there it COULD cause problems.

10/4, I will definitely check it out when I disassemble

 

[TaperPin]

Wind, rifle bounce or crazy positioning come to mind as the most likely causes.

Could easily be a wind issue you aren’t feeling from the bench. I spend half a dozen years at a range that has a wind roller that comes more or less straight down at one set of benches - it wasn’t unusual, just what it was. When someone visits a different range and a scope is adjusted, then back at their range the scope is adjusted back, that makes me think the odds are may 20%.

Rifle bounce from bipod on a hard surface, inconsistent bipod loading, forend being overly flexible, or barrel channel gap too small. Once a year someone I chat with has a weird vertical and it comes back as a bipod/bipod loading issue. Odds are probably 30%.

We don’t know how you set up, or at least I don’t, and there is a big difference in consistency between someone’s first 1000 rounds down range and their second, third, fourth or fifth. One dude with a fantastic rifle, a lot of enthusiasm, a great private range and almost unlimited budget for ammo was having a big impact shift between targets on the same bench and same day. He was in the middle of learning any pressure (or lack of normal pressure) changes the impact. There might have also been a small parallax issue making it worse. All cleared up when he stopped trying to “reach” too far out of his natural point of aim, and focused on consistency. Odds are maybe 25%.

The other 25% chance might be scope related, but it could also be inconsistent ignition from a dirty bolt, weak firing pin, or drag from a dirty trigger. In cold weather it would be more pronounced.

That’s the odds I’d be betting. I’m looking forward to hearing what you find out.

 

[Runwilderness]

I had similar issues in the last season with my Tikka. I chalked it up to my error (I suck) but mostly I was too lazy to tear things down while waiting on a mythic Rokstok delivery (and also afraid that I’d convince myself I needed to buy a drop test approved scope now rather than wait for the WYBTS).

I just built up the Rokstok. Lots of clearance + retorquing everything. TBD but Ill update if I see a shift and I’ll be following to see what you find out with your setup.

 

[BRTreedogs]

If you swap your old scope back on and it goes back to not shifting that will tell you if it’s in the rifle or optic.