Point of impact trouble shooting

G'day team,

Had an interesting issue during a short range session yesterday. Was re-testing a load that had previously performed well for me (~0.5moa). This was following a brief hiatus from reloading/range shooting so I had taken the action out of the stock to give everything a clean and check it all over. Some info:
- Caliber: 7mm Rem Mag
- Rifle: Mauser M12 Max in factory wooden stock
- Load: 162gr ELDX / AR2225 / Norma brass on 3rd firing / Federal large magnum primers
- Issue: Point of impact shifted significantly but consistently over 10 rounds

Image below shows the last 7 rounds fired in order of the horribly drawn numbers in each of the quadrants. The first three shots fired also fell in the groups shown here relative to the target but were on a different piece of paper. Strings were fired in 3, 3, 4 with ample time between each for barrel to cool. I checked the barrel after the last string (4 shots) and it wasn't hot enough for concern. A mate and I have discussed the result and we think this may be down to the stock/action being seated imperfectly after I had cleaned and checked everything; however, I'd like to crowd source all of your thoughts for anything else to consider if it happens again. Anyway, let me know your thoughts - very interested to hear.

Cheers,
Elliott

Attachment 254882

First thing to eliminate with POI shifts is scope and scope mounts. Take scope off, re tighten everything, reinstall and sight in. Until this is eliminated as a potential source of variation I wouldn't touch anything else.

What Makros said.
Just tightening the scope rings and bases onto the picatinny rail on my .22lr shifted my POI 0.3mil right last week.

Maybe take it apart again and take extra care putting it back together again,could be crumb of bedding under block etc... not familiar with your particular bedding system but a moving group or double grouping is OFTEN something sitting in place for few shots then moving somewhere else for few then moves again.

Cheers team! First thing I did was check scope rings and bases. They were all torqued up to spec. Should have said that in the post sorry.

@Micky Duck that's where we got to as well and have started with that. Couldn't find anything specific to put the blame on but may not have been careful enough first time around.

Also.did you perhaps change positions between groups???

After looking at the group, I was thinking the same as what Micky wrote, before I read it. Reading your statement about the scope being correctly torqued, it sounds like you would have also checked torque on the action screws. Other things to check is:

Are you using a suppressor and was it done up tight?
What scope are you using?, is it near limit of adjustment in either plane? could it have loose internals?
It doesn't look like inconsistent reloading, but possibly worth checking with some factory ammo to eliminate that variable.
What sort of rest were you shooting off, this can affect POI.

@Micky Duck perhaps inadvertently but but not significantly and the POI was changing within groups. Certainly something I can be more conscious of though.

@longshot suppressor was done up tight. Scope is a vx3 - I'll definitely check limits of adjustment, thanks! Not sure how to check internals? Factory ammo check is a good idea. I've got some I can use. And shooting off bipod and rear bag, which I've done for a long time without this issue.

There isn't much you can (or should) do to check internals, check scope with a boresighter between shots and listen for rattle noises is the only things I can think of. Or try a different scope.

Some scopes, usually the cheaper ones, can get a bit wobbly inside if you have to wind either adjustment all the way one way or the other.


I haven't done much range shooting with a bipod, I usually use front and rear bags, but I have heard that a bipod on a hard surface can sometimes cause issues.

Were you shooting with a head or tail wind?

@Bol Tackshin yep there was actually a reasonable tail wind. Reckon that could have affected it?

It's possible, and the shift in POI may indicate changing wind conditions which is why they are clustered. Tail wind typically pushes the group upwards. What was the speed, and what was the range? If it is 100m. Then the wind would have needed to be pretty strong.

@Bol Tackshin Yeah it was 100yd and wind was blowing but was in a relatively sheltered valley. Load is quite warm - got them going over 3000fps.

Shooting off bi pod or bags? Whats the barrel channel like?

@blip bipod with rear bag. What do you mean re barrel channel? Checked that it was free floating and clear of debris etc if that's what you mean?

Looks to me that each group is not too bad taken alone, but the poi of each group is changing. Two possibilities.
1, If you have scrubbed the bore with abrasive to clear carbon and copper the barrel may take several shots to settle down.

2, the internal ajustments of some scopes can stiffen up with disuse as the lens corrections are spring
tensioned. Try winding the turrets fully
each way a few times and then back to your original zero setting, then retest for group tightness and finally for poi consistency. Good luck.

Hmmm.... what was the interval between shots, and barrel profile? I'm wondering if barrel heating over the string could have something to do with it. Also, have you fired tighter 10 shot strings before?

@Woody thanks mate will try the scope trick. I use bore tech eliminator and have used it for a while. Never had this issue but scope definitely possible as it was in the cupboard for a while!

@Bol Tackshin have fired tighter 10 shot strings for sure. Usually similar to the group in the bottom right quadrant. I don't think heat was an issue? Plenty of time between strings and ways regularly checking barrel for heat. Also live I Dunedin so it was (naturally) cold and a bit windy, which helps. That said, can always be more conscious about timing and temp next time I head out.

I had a look at the schematic for the M12 thinking maybe you'd left a detachable recoil lug out on reassembly . . . , but as far as I can see it's a fixed recoil lug, albeit quite small in area.

So another possibility is a crack or split in the stock in the area of the recoil lug or action screws.

Take the action out of stock & check there’s no shite in the recoil lug recess. When you reassemble & torque action screws check that the mag box is not trapped or binding. It should be free to move slightly. If the action is binding on mag box this will definitely cause double grouping or random poi shifts

Quote:

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Originally Posted by Edunn

@blip bipod with rear bag. What do you mean re barrel channel? Checked that it was free floating and clear of debris etc if that's what you mean?

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Yep check its clear how flexy is the fore end as well

I've had this a couple of times in magnums.

First time it was the the recoil lug in the stock (Tikka) and it would do this based on the uneven mating and recoil action on the barrel against the lug - even though it felt like it was all in place and seated correctly. First time it was uneven lug wear - swapped to a Gunworks lug solved the issue.

Second time was a movement between the rail screws to the bases. This is about 10 years ago so please forgive me on the details I think stock the screws were M4 and came with the rail and they sat in the holes but there was movement. All most imperceptible - but you could feel it ever so slightly . We re-tapped the holes to M5 new screws with lock tight on the rail and we were good.

From there no issues. But my personal first port of call is optic - I think you need to first you need to test with a known and dependable optic you know is good. Remove that from mix and then look at these two areas.

CD

Greetings,
Two more things. Does the VX3 scope have adjustment for parallax. If not slight differences in head position may be shifting the group. I assume you are testing an 100 metres. You may want to try a test at 200 metres. Also are you shooting with your bipod on a hard surface you may want to try a bit of padding on the bench under the bipod like a bit of old sleeping mat.
GPM.

Are you using a rail with any MOA on it? If your scope is wound down too far off center of scope adjustment for a long period it can cause the springs keeping tension on the erector tube to fail eventually.

Another thing. Your aiming point may be too small and black is not ideal. You should be able to clearly see the 4 quarters of your aiming point around your cross hairs so you can make sure they are the same. At 100 metres the 50 metre NZDA targets are good, a red circle on a white background. You can clearly quarter the interior white circle. There are good reasons why these targets are used for competition,
GPM.

Thanks for all the advice and thoughts. Plenty of stuff to try when I next get a chance. Baby number two arrived during the week so will be out of action for a bit but will be sure to report back when I sort the issue out.

congrats