Vertical stringing=optics problem?

[Geezer]

So I'm sitting here with targets from my last three range sessions. Three weeks ago I was testing Vhit550 in my .300 WSM Savage, behind 150Gr Combined Technologies BST's, going 63.5 to 65.5gr in 0.5gr increments. Have five nice groups with 64.0 and 65.5gr best, well under MOA.

Thinking faster is better, I loaded up some more rounds with 65.5gr and went back last week to try them out. Shooting five shot groups, the first one was right on the aim point, the second put three in a group 2" high, and the last two 1" higher. The third group started with the first round 3" high, the second 4" and the third 5", at which point I stopped and headed home to the work bench. Figuring something was loose, I re-tightened the stock bolts, the scope mounts, and the rings, adding medium loc-tite.

So today with my nice tightened up rifle it went like this.

Shot 1 for a starting point. Same point of aim, # 2 was 1" higher. #3 & #4 0.5" up from #2. #5 higher by 0.5". #6 thru 8 in a group 0.5" higher. #9 0.5" up. #10 & 11 grouped 0.5" up. #12 & 13 grouped 1" up. #14 0.5" up. So I basically have 14 shots in a column 1.5" wide and 5" high. All aimed at the same point of aim.

All shooting for these three sessions was done from bags at 100yds with 2 minutes cooldown between shots. Weather and wind were about the same for all sessions. The scope is a Burris Fullfield II 3X9X40 with Burris Signature rings and bases. At no time did I feel any looseness in the scope mounting.

Is there any load or rifle reason possible for this stringing? Or could there possibly be something wrong with the scope internally? If I had another scope sitting around I'd swap out and see what happens, but I'm currently a one-rifle guy and don't have a spare. Suggestions?

The 65.5gr loads are the max in the latest Nosler book, are pretty stout, and show a bit of primer flattening (CCI large rifle magnum primers) but no other pressure signs.

 

[royinidaho]

Welcome aboard.....

Hmmmmm, where to start?
Let's see if I have this right:

Initial groups looked good.

Took same loads 65.5 out again and they produced a vertical string.

Did it again and got another vertical string....

Result 1.5" wide and 5" high. Correct?

First observation is that 2 minutes coold down time is not very much time. But barrel heating shouldn't make "that" much difference regardless.....

I would suggest that the vertical string is symptomatic of a condition being affected by barrel heating as the POIs go up as number of shots increases.

Check the barrel channel. If its supposed to be floated it should be completely floated. That is a business card should freely travel from front end of bbl channel to the recoil lug.

If its not supposed to be floated then the barrel should touch evenly around the barrel channel. I'm not familiar with Savage barrel bedding so I'm talking kind of generic here.

If the barrel is supposed to be touching towards the end of the stock, you could cut a piece of business card about an inch wide and fit it between the barrel and pressure point. If things improve then rebedding the barrel and action would be suggested.

Just a couple of quick things to look at. There are more but that's a start. Others will provide useful suggestions also.

Go luck and good shooting..

How about the barrel channel.

 

[Geezer]

Hey, Roy.

Savage synthetic stocks, which I have, seem to be pillar bedded. Just in case, I hogged it out until I have about 1/8th inch clearance back to the recoil lug.

Previous to the last three range sessions, I've always had tolerable groups with the 2 minute cooldown, with both factory rounds and reloads. This is way off the norm.

I do remember that the first time out with this rifle it was all over the place due to the bolts connecting stock to action being loose, but they're tight now, and the looseness stock-to-action produced what looked like a shotgun pattern, not this vertical stringing.

 

[royinidaho]

If it became a problem all of a sudden, then something must have changed......

The question is, what?

"Previous to the last three range sessions, I've always had tolerable groups with the 2 minute cooldown," That's what one would expect.

What isn't normal now?

How long after retightening the stock screws did it go haywire?

 

[edge]

I would lean toward one of two things first:

1) a bad scope. Since you don't have a replacement let's move to item #2

2) Scope mounting screws. I did not see how long you have had this rifle, but you did up the pressure with your loads.
I also don't know which threads this barrel has, but that DOES make a difference!

I posted this on another website for Savage muzzleloaders but the problem will be the same. If your threads are 1.120 instead of 1.055, then the dimensions will be less.

Before you mount your scope ensure that the front base screw is NOT bottoming out. This causes an unbelievable amount of problems.

Here is the problem:

The Savage action is 1.30 in Diameter and the Savage Barrel threads are 1.055 O.D. IF the threads are exactly in the center of the action then the threads in the action CAN'T be more than 0.122 deep! If your front base screw extends below the base more than this amount then the screw will feel tight but it will be bottomed out and not really snug on the scope base.

The accepted method for checking for this is:

1) Without the scope base on the rifle take the screw and count how many turns it takes to bottom out;

2) Install the base and count how many turns it takes to become tight;

3) IF your answer in #2 is not AT LEAST 1/2 turn less than in #1 then your base will NOT be tight ( even though you swear that it is );

4) You will need to grind away enough of the screw to ensure that it is AT LEAST 1/2 thread short of bottoming out. ( the screw will stretch slightly when you torque it tight ).

Good luck /ubbthreads/images/graemlins/wink.gif

edge.

 

[ss7mm]

Jerry B.:

If you haven't got it figured out and or fixed yet and you want to do some reading, put "+vertical +string", without the quotation marks but with the +signs, into the search window, set it for all forums, 4 years, grab a cup of coffer, hit submit and you can read for a long time.

 

[Geezer]

Well, I've checked everything again, including the scope mount bolts bottoming (they don't) and nothing seems wrong with either scope or stock mounting. None of the stuff I can find searching relates to the extreme stringing I'm getting, nor to the fact that each round hits higher than the previous one.

Guess I'll give Burris a call and see if they have had any problems like this reported before.

 

[JR]

Get a chronograph reading..

JR

 

[Geezer]

Culprit found, I think.

Looking at everything for the 7th or 8th time, I noticed that the back of the rear ring was about 1/16" above the base. Tried jiggling it, but nothing moved. I loosened the screws that hold the rings to the base and found that the little ears the screws capture had been bent down and back. This allowed the ring to cant slightly forward, raising the rear of the scope, and the ***-Align inserts let it do that without binding on the scope tube.

The Burris standard ring attachment method seems to rely on the front dovetail to provide the support against recoil, with the rear just centered by screws and not load-bearing. Apparently, the front ring with the plastic ***-Align inserts couldn't clamp the scope tightly enough to keep it from moving forward a bit under recoil, and the back ring just pulled a bit more out of it's mount every shot or two as the scope moved forward.

Does this seem like a reasonable scenario? Any suggestions for a more secure setup?

 

[royinidaho]

Is the rear ring base salvagable?

When "upgrading" to the Burris Sig rings for a bigger kicker than the 300 WSM, I had to degrease all ring parts including the inserts. Then upon recommendations of people here, I torqued the all the screws down to just before twist off or until I was embarrassed with the pressure I was using.

The star wrench was left with pretty much a permanent full turn. I now looks like its threaded.

This is said to help one become more comfortable ring screw torque requirements.

Glad you found it. I hear that .001" on the ring is 1" @ 100yds. Kind of a rule of thumb, I guess.

 

[Geezer]

"Is the rear ring base salvagable?"

Looks like, but I've kind of lost faith in the "standard" mounts, given my experience. I may try it out again later, and use the "tighten 'til it squeals" method.

I had previously found a one-piece left-hand Weaver style base from B-Square, and slid by the local gun shop for some Weaver rings. Everything as been de-greased, loc-tited, well screwed down and is ready to hit the range today. This may just be a temporary setup, but it should let me know for sure if I have the problem solved.