Chasing my tail with 100 yard zero

[D.Camilleri]

For over a year, maybe longer, I have been having point of impact shifts at 100 yards on my 338 rum from one outing to the next. Last week I went to the range and started off hitting about 2 inches high at 100 yards. I rezero'd and reset my turret to match. I continued shooting with some good groups with a new load and 300 gr accubonds in front of 101 grs of rl33. I went home, cleaned my gun and put it away for a week. Today I went out with the intention of going straight to 700 yards. I decided I better verify my 100 yard zero first. I took two shots, about 1/2 inch apart, but two inches high. WTH. I lined up my cross hairs on the top of a target and dialed up 12 moa and then down 12 moa and the scope tracked proper. By the way, the scope in question is a sightron S-III 8-32x56 mounted on an egw 0 degree picatiny rail. The scope is secured with Burris tactical 6 screw 30mm rings. Nothing appears to be moving that I can tell. Just before shooting last week, I bedded the recoil lug with devcon to my Bell and Carlson Alaskan full pillar bedded stock that also doesn't show any signs of moving.

I got a range with my G-7 and went to 690 yards, the ballistic solution was 13.3 moa, since I was already 2 moa high, I dialed 11.25 and fired 3 shots. There was a bit of wind, 5-7 right to left and all the bullets impacted about 6-8 inches left which is about what I would figure. The first shot was just about perfect for elevation and the next two were about 5-6 inches low. Not a bad group considering the wind.
I dialed back down to zero on my turret and cut the two holes from my first shots, so the scope tracked consistently today, but why the change from last week? I keep thinking that something is hanging up inside of the mechanism of the scope, and after I adjust it some it starts working right again. Without a solid 100 yard zero, the rest doesn't work. Am I missing something?

 

[sp6x6]

you might be on to something with scope.Could test with other scope. I had a rail loosen up that took me a few trips to catch,devconed it down after that.

 

[D.Camilleri]

I pulled the scope a few months back and checked the torque on the rail. Everything was tight. I contacted Sightron last year and they said they didn't think it was the scope. The problem is after the gun sits for several months, there is a good chance that the zero will be off after I dial for a shot. In the past, the change was always 1 moa the last two trips out it has been 2 moa. I will try and get out a couple more times this week and take a cold bore shot. I am thinking that this has to be a tracking issue, but once I dial the scope, all is well except for returning to my exact zero. I had an issue with this several years ago and I thought the problem was my 20 moa rail in that I was almost to the bottom of the travel in the elevation on the scope. I replaced the 20 moa with a 0 and it still isn't quite fixed. I contacted Sightron last year and they told me it must be a gun problem, but I don't know how it could be, the gun always groups, just doesn't return to exact zero. The funny thing is that last week after I dialed to 750, I shot once more at 100 and drilled a bullseye and that is how I put the gun away.
I am stumped.

 

[Snowrun]

Have you tried leaving the barrel fouled between shooting sessions? I have a 338 Win Mag that does not have repeatability unless I leave it fouled. Shoots high & wide when clean but tightens right up after 5-7 rounds. Doesn't really explain if you consistently have to come up 2" every time to the range, but maybe if you are chasing zero back and forth,...Just a thought

 

[D.Camilleri]

I am going to leave it fouled for the next several sessions and see what happens. The point of impact hasn't been changing while at the range though. My initial point of impact was off the last time I went out, but I expected that since I bedded the recoil lug. I bedded the lug to rule out the action moving as a source of the changing zero. I also turned the dial back to my zero point and re-shot my 100 yard target before leaving today. Right now my zero is off by 2 moa. If it stays there, then good, but I really need to move my turret so that it is at zero again. I am tempted to pull the 6.5-20 Leupold off of my 300 rum and see if the problem goes away. I might try giving Sightron a call tomorrow.

There is one other variable that I haven't checked. Last time out my 300 gr accubond load hit to the exact same point of impact as my 300 gr Berger OTM load. The accubonds that I loaded for this session were all cleaned and tumbled where the ones before were not, just resized. I need to shoot a couple of my go to Bergers and see where they hit.

 

[Timber338]

years ago I went crazy trying to figure out a problem with a 338 win mag I used to own. It had a custom barrel, and sometines it would shoot a 1/2 moa group and other times it would shoot a 2 moa group. I went through two aftermarket stocks, a new trigger and the barrel manufacturer even installed a new barrel. Still shot bad. the scope was brand new so how could that be the problem? But I finally just tried a different scope because of process of elimination. And guess what the gun shot great group after group after group. Sent the scope back to the manufacturer and they quickly found that the scope had a bad internal mechanism that controlled the position of the reticle. They fixed the problem and the scope and the rifle worked great.

I burned through a whole lot of time and money shooting the rifle to figure out the problem. I could easily have bought a full custom rifle with all the money I spent. Lesson hard learned for me.

Anyways, maybe I'm biased from my own experience, but your problem really sounds like an optics issue. Is there any way you could mount a different scope just temporarily? If the same problem happens with a different scope, at least you've eliminated your scope as the problem...

 

[FearNoWind]

Have you scoped the muzzle to see what the rifling lookis like up there?

 

[royinidaho]

OP, I think we're in the same boat.

The rig:

375 AM
Leuy Mk IV
Bat Action
PacNor bbl

Check load is 136 gr Retumbo behind 350 SMKs

With my driving this thing will shoot around 1/2 MOA at any distance I've shot, so far.

I'm developing some modified SMKs to up the bc a bit and they shoot as well as the unmodifieds.

Then thing to hay wire.

One evening I settled on a commbination of powder, bullet and seating depth that produced a 0.33 NI 300 yard group 2 inches left of POA.

Next morning I modified this load's seating depth only a few thou. Loaded 2 with powder A and 2 with powder B. I'm attempting to reduce ES so something reasonable.

Fired first two shots and spotted impact behind the backstop. Figure I got full penetration again. It happen with that big sucker.

Mirage wouldn't let me view bullet holes. Figured I maight be off the 8 1/2 x 11 target a bit to the left so I fired the next two shots.

Went to the target and nary a hole. WTH was a little light for what I was thinking.

The target holder is 4' x 4' with the bull in the center.

Loaded up 2 of the check loads. Missed the who thing again.

For the last several groups things shift from great groups to horrible vertical string of as much as 2 MOA

I have no idea of what is going on. This thing has been true blue for several years and about 150 shots. Now all of a sudden my heart is in my socks.

Nothing is lose. All scope, rale and action screws are properly torqued.

Will try something in fhe morning. If that doesn't pan out I'll switch scopes which I doubt will make a difference but I gotta try something.

If it weren't for bad luck I'd have no luck at all.

 

[D.Camilleri]

Fear no wind, why do you ask about the muzzle bore? A buddy just bought a new bore scope last week, but I haven't had a chance to try it yet. My gun groups normally very well. It does like having a clean bore as opposed to a fouled bore. My groups usually start to open up after around 20+ rounds. In the past when my poi would change, it was only elevation, never windage. I would put the gun away after shooting at 700 or 750 and return the turret to zero, next time out I would check zero and it would be 1 moa high. Scope would track fine, but if I didn't know I was 1 moa high, it could result in a miss on a trophy when I dial the shot. The scope tracking always seems to be spot on as far as if I dial 13 moa, it moves 13 moa at least when dialing up. Maybe it isn't always moving correctly when I am dialing down to return to zero. I am still fairly low on the elevation on the scope, which gives me a lot of travel up. When I had the 20 moa rail, I only had about 4 moa down that I could dial before hitting the bottom. Now I have around 24 moa down. I need to go shoot tomorrow before work and see where I hit.

 

[RockyMtnMT]

Only thing I can think of other than scope. Are all shots cold? I had chased one of my rifles around for a long time and mine turned out to be action mounting screws loosening. Still not sure I have figured it out. I am starting to think it is just worn out. I need to get a scope in it to see if it needs a new barrel. I have been pretty rough on it over the years testing different bullets. I feel your pain.

Steve

 

[bigngreen]

Try shooting and establishing a zero, then don't move a thing especially the parallax and see if it holds zero without adjusting the optic.

 

[Pdvdh]

As has been mentioned, failing scope can cause this type of problem. Been there before - more than once. As recommended, pull your scope and try a different scope and see if the shifting POI is resolved.

Loose scope mounts or actions screws? Scope slipping in rings?

Poor bedding. Is the barrel free floated all the way to the face of the action? Stress free bedding can help maintain a stable POI over time.

I've experience this problem in Tupperware style, soft plastic rifle stocks before also. Poor stock material and/or poor bedding in the poor quality stocks has caused POI changes over time with some of my "former" rifles and "former" rifle stocks.

 

[dsculley]

Sometimes, on some scopes, it matters whether you are dialing up or down. When you dialed up for your long range shots, the scope seemed to track good. Try dialing down below your 100yd zero and then dial back up to it. Just a thought since everything else seems to be in order. If that makes a difference, then you will know for the future.

FWIW,

Dennis

 

[royinidaho]

Figger'd out my problem.

Removed the MK-4 and mounted a VX 7.

Bore sighted. Hit back stop @ 300 w/first shot.

Fired 6 more into < 0.5 MOA perfectly zeroed. 3 of the shots were in 5/16". Never did that before and this wasn't an accuracy load...

Me thinks the Mk-4 was wonky from the beginning.

We'll see how L&S treats me.