Zero shift from skim bedding

[Tim K]

I'm still not sure I believe it. I bought a new stock for my little Savage .22 trainer a few weeks ago. It's walnut (looks like an AICS) and decently made. There was initially no change in zero from my old Tacticool, and that zero was years old. My confidence in that zero was 100%.

I skim bedded the action into the stock and painted it last weekend because I had nothing better to do. I didn't bother to check zero, assuming it wouldn't change. When I shot it at the .22 side match at last Saturday's PD match, I could barely hit the dirt. It was driving me nuts, so I took it out this afternoon to see if the zero had changed.

Unbelievably, it was shooting .9 mils high at 100 yards. Windage was good. I'm torquing the action more than before, so maybe something really did change. Accuracy was still good as it's grouping 5 into 3/4" at 100. If I'd had more ammo and my torque wrench, I might have tried some different values to see if I could get it to wander around.

Anyone else ever seen a change this large from something this minor?

[BPTactical]

Yes Tim and I would suspect the torque is your causative factor. The role of the bedding is to provide a stress free 100% contact between the receiver and stock.
A 1/4x28 screw torqued to 56 in/lbs equates to over 1500 lbs of clamping force.
As a result it doesn't take much torque to hold the action securely.
If the torque is excessive or uneven it can certainly cause a shift due to stressing the action.
You mentioned painting, did you paint the bedding? Any paint there will compromise the fit between the bedding and action.

[Tim K]

I did indeed paint the bedding. There also just isn't much area that can be bedded, so there's not a lot of contact.

I backed the torque down from 25 in-lb to 20 in-lb. The rear screw in particular continues to tighten after contact is made between the action and the stock. Something is moving. I'd never bothered with a torque wrench before bedding the action, but I know I was using much less. As soon as it felt snug, I quit.

I should investigate to see what it is that compressing and either try to fix it or just reduce the torque value. I should also try un-torquing and retorquing the action bolts to the same 20 in-lb and see if the zero continues to move. Or maybe I'll just never take it out of the stock again...

Any other ideas, Bert?

[BPTactical]

Is it pillar bedded?

[Tim K]

Sort of. The action has some pillars, but I don't think they touch anything. I'll post up some pics when it's not 5 AM.

[Madusa]

Are you upset that your zero moved that much? Maybe I'm old school but I would rezero and go to work.

[Tim K]

Are you upset that your zero moved that much? Maybe I'm old school but I would rezero and go to work.

No, I'm worried my zero wont be repeatable when I pull the action from the stock. Zeroing a subsonic .22 is a huge pain in the ass because you need essentially windless conditions to do so. Those conditions may not always be present between the time the action is removed and the next match.

Also, I don't like the idea that I may have zero shift of such large magnitude simply from over torquing the action bolts. Wood compresses over time, and my zero may wander as the effective clamping force decreases over time.

I'm going to look at the pillars tonight and see if I can bed them with Devcon.

[C Ward]

Anything that changes the tension on the action is going to effect zero . The torque on the screws especially . All actions flex , small thin rimfire actions more so . There is an article on 6BR about tuning the accuracy of a Savage centerfire rifle with the action screw torque .

Find what it likes and record it . Better yet leave it in the stock unless it gets wet and blow it out with shop air .

[Tim K]

Find what it likes and record it . Better yet leave it in the stock unless it gets wet and blow it out with shop air .

Good advice. I'll see if my OCD will let me take it.

[XC700116]

Wood compresses over time, and my zero may wander as the effective clamping force decreases over time.

I'm going to look at the pillars tonight and see if I can bed them with Devcon.

I think this is a real valid concern, and please post the results as I'm getting ready to put mine in the same stock and start the build up on it soon.

To me the more you can do to "firm up" the bedding, the more repeatable and less problematic in the future. As C Ward pointed out the action isn't overly robust to begin with and I could definitely see how it would loosen up over time in a wood stock. Heck I did some minor stock work on my mom's hunting rifle last week (late 60's Winchester Mod 100) and was shocked at how sloppy the action/stock fit has become over time.