Tracking Question

[docsleepy]

Newbie here.

My stock (a Stockade Stocks benchrest stock, fiberglass) angles downward from the pistol grip to the butt.

When I pull the rifle straight backwards, while looking through the scope, the point of aim therefore shifts UPWARD (and a slight bit leftward).

Am I supposed to attempt to adjust the angle of the rear bag (Protector) sand shape in some way so that the Point of Aim remains centered, even as I test the tracking of the rifle by pulling it backwards?

I'm not even sure HOW to achieve that, if I wished....

I tried various crabbing (anglin) of the rear rest in an attempt to get gun to at least not move left or right as I moved it backwards, but that wasn't easy either. Over a 9" move of the gun backwards, I'd say the point of aim moved about 6 inches left on the target. I guess I could shim or sand the stock or something to make it more symmetrical?

Any advice would be welcome! This is my first flat forearm "benchrest" style stock, and real rest, so I am still learning.

thanks,
gordon

 

[docsleepy]

thanks, george!

 

[goodgrouper]

Wow, if you are 6" left of target at 100 yards, there is something wrong. Bad.

It's actually in the rulebook that a benchrest stock must have a certain taper from the grip to the toe and the barrel should go up as the gun is slid back (this was actually how a lot of fellas went from the sighter to the record for many years but it has since tapered off in popularity) but there should not be any left or right movement. With properly set up bags, you should be able to run the stock back and forth with your hand and have the crosshair within an 1/8" of original point of aim, and it should be within the mothball AFTER firing the gun too.

It sounds like more time needs to be spent aligning the stock, front rest, and rear bag with the target. An inspection of the stock would also be good.

Also, if you are using a pedestal front rest, you should position it so that when you use your fine adjustment screw, the crosshair should go from the mothball on the sighter to the mothball on the record. If you have to move your windage screw when going from sighter to record, it will cost you valuable time.

Proper benchresting of the rifle is very important--especially if you are going to shoot free recoil.

Good luck!

 

[abintx]

Gordon ...

Let's start from scratch. Not all benchrest stocks are "B"enchrest stocks. Some are in name only. Here are some more recognizable BR stocks from Kelbly's: http://www.kelbly.com/kelblystocks.html. The first THREE in particular. Is the picture below the Stockade stock you're talking about?

 

[SGJennings]

If you shoot with someone else that has a Benchrest stock you guys could:

1. Set up on adjacent benches such that you believe the setup is straight.

2. Observe that their rifle is tracking straight on their setup.

3. Observe that your rifle is misbaving in the manner described on your setup.

4. Swap rifles to each others setup and observe the tracking.

5. If theirs tracks straight on yours and yours misbehaves as before on theirs, you have a crooked stock.

OTOH, if theirs misbehaves on your setup and your rifle track straight on their setup, it's the setup.

Greg J.

 

[docsleepy]

Hi --

Yes, abintx -- You have the correct picture of the stock I have. What can we glean from this?

Goodgrouper: I will go back and figure out how close it comes back to the original aim point when I move it backward and forward (I think it will be fairly close). However, when I pull it back, I don't think it does perfectly vertical. I'm not sure why. Something, somewhere must not be parallel on the stock, either forearm or the rear portion. I *thought* I got the barrel centered when I bedded it -- to my eye it is centered.

The stock is painted with a textured paint. The front width measures 2.998 and at the very back of the forend it measures 3.014. So there is an error of .016 somewhere. Laying a metal straightedge (yardstick) along it, it appears fairly straight with a tiny turning inward at the extreme 1/2" forward.

However, when I hold the straightedge along the forearm, allowing the straightedge to extend all the way back to the butt, the inside gap on one side is about 0.1" closer than on the other, so the forearm does not appear to be perfectly inline with the rear (I don't know the proper name) of the stock. Perhaps this is why I have some lateral movement visible through the scope when I pull the stock back--I am pulling it quite a bit.

A tenth of an inch at about 30 inches would suggest an error of 0.19 degrees between the centerline of the rear stock and the forearm. I probably pulled the stock back 5" when I checked the error, and if the point of aim shifted 6" at 100 yards (I'll have to go back and see if I'm grossly off on all of this!!) that would be a net change in the horizontal aiming of the gun of 0.09 degrees, and that measurement may be off a lot -- so the two are in the same ballpark.

I could probably lay up a test of duct-tape covered by teflon in stepped layers to fatten out one side of the rear stock, or carefullly sand down the other and repaint, to reduce the error if it is important. Commments?

 

[docsleepy]

If you shoot with someone else that has a Benchrest stock you guys could:

1. Set up on adjacent benches such that you believe the setup is straight.
2. Observe that their rifle is tracking straight on their setup.
3. Observe that your rifle is misbaving in the manner described on your setup.
4. Swap rifles to each others setup and observe the tracking.
5. If theirs tracks straight on yours and yours misbehaves as before on theirs, you have a crooked stock.
OTOH, if theirs misbehaves on your setup and your rifle track straight on their setup, it's the setup.
Greg J.

Great idea. It may be a while before I can test it; until school is out, I'm unable to get to the range most days that real benchrest folks are there. But that is a great idea. From the measurements I made while you were adding your comments, I suspect the error is in the stock. Wow. I would never have realized how important each little part is! I am using a fairly cheap set up and trying to learn how to measure and understand each component, and I think I just learned another one!!

How much laterally does your point of aim shift when you pull your rifle back a set (specified) number of inches?
thanks, gordon

 

[abintx]

Yes, abintx -- You have the correct picture of the stock I have. What can we glean from this?

One, that we're at least talking about a true benchrest stock initially designed to track in the bags smoothly compared to most others. But, since it sounds like you may have done the inletting and the bedding all bets may now be off. If everything is not dead on you may very well have built in a tracking error that may never be resolved. Good luck.

 

[docsleepy]

One, that we're at least talking about a true benchrest stock initially designed to track in the bags smoothly compared to most others. But, since it sounds like you may have done the inletting and the bedding all bets may now be off. If everything is not dead on you may very well have built in a tracking error that may never be resolved. Good luck.

Hi, thanks for the info! I didn't do the inletting; just bedded it, as it comes unbedded. However, it was already a relatively close fit to the action, so bedding it was quite simple. The pillars were already there. I'm not an expert, but will pillars already in and screws in the action to keep it straight, and tape wound around barrel, it should be realtively straight with reference to the stock centerline. Measurements made above however, suggest the rear part of the stock isn't perfectly in line with the edges of the forearm (which are relatively parallel within a few thousandths.

Is there any particular method to bedding beyond the above that I should have utilized?

 

[Don]

Hi --



However, when I hold the straightedge along the forearm, allowing the straightedge to extend all the way back to the butt, the inside gap on one side is about 0.1" closer than on the other, so the forearm does not appear to be perfectly inline with the rear (I don't know the proper name) of the stock. Perhaps this is why I have some lateral movement visible through the scope when I pull the stock back--I am pulling it quite a bit.

A tenth of an inch at about 30 inches would suggest an error of 0.19 degrees between the centerline of the rear stock and the forearm. I probably pulled the stock back 5" when I checked the error, and if the point of aim shifted 6" at 100 yards (I'll have to go back and see if I'm grossly off on all of this!!) that would be a net change in the horizontal aiming of the gun of 0.09 degrees, and that measurement may be off a lot -- so the two are in the same ballpark.

I could probably lay up a test of duct-tape covered by teflon in stepped layers to fatten out one side of the rear stock, or carefullly sand down the other and repaint, to reduce the error if it is important. Commments?

Yup, you found the problem, your stock is warped!

Actually it is quite common to varing degrees. Does it affect accuracy? Nobody seems to know at what point degradation takes place. Probably none at all even with the most extreme warpages because the bullet exits the barrel within appx. .030" of reward recoil, hardly enough tracking distance to allow the rifle to start enough of a sideways movement to affect anything.

If your gun is shooting to acceptable accuracy levels than do not worry about the stock warpage. If it bugs you too much to live with then try your corrective methods or contact the stock mfg. and see if he will replace.................Don

 

[docsleepy]

Don --

Thanks for the info! Well, my groups are at best about 3/8" now, and I'm still pushing to get them better. My action is just a Savage action and let me tell you, this PPC business has been a lot to learn! Numbered cases, careful records; I've learned a LOT in the past 3-4 months since the new barrel and stock arrived. Action is to get true in a few months (on a list) and far better trigger installed. New front rest....all kinds of dies, new kind of press, the list just keeps growing.

So I'll probably try a test of making a terraced steppe of duct tape on the lean side and powder it up good with Suave (after it is applied) and see if that changes the tracking and/or groups. Taking a sander to the "fat" side of the action isn't that difficult, and even if I only took out 50% of the 0.100 error, it would mean a 50% improvement in the left-right movement on tracking, which MIGHT allow me to see some change. I dunno. I think I still have other errors that I'm still working on.

How much warped are fairly expensive stocks? The ones in the $500+ range?

Thanks,
Gordon

 

[goodgrouper]

Searched the old memory banks today and went through several extensive equipment lists and your stock was in neither place! That might tell us something? Just because a stock says "benchrest" in the brochure doesn't mean it's up to Benchrest standards.

 

[docsleepy]

thanks for the info!

 

[Don]

Taking a sander to the "fat" side of the action isn't that difficult, and even if I only took out 50% of the 0.100 error, it would mean a 50% improvement in the left-right movement on tracking, which MIGHT allow me to see some change.
Gordon

Be aware that the outer shell of most synthetic fiber stocks are rather thin and you will break thru this shell to the foam fill underneath and then will have to re-lay the outer shell portion which is laborous.........Don

 

[docsleepy]

Hmmm.... that would be a problem.....would be much easier for me to lay up another layer of fiberglass on the thin side and then sand down to get the contour I want. I know that it needs to slope toward 0.100 so choosing slightly lower than that would guarantee a significant reduction.


As you pointed out earlier, this may be a waste of time.

The vertical movement of the point of aim is huge (due to the rake of the stock) compared to the horizontal movement (due to the slight 0.2 degree error in the laterality fo the rear stock). If there is any movement during the time the bullet is still in the barrel, the vertical movement is going to dwarf the horizontal movement.

If both are equally repeatable (a key issue, but likely), then the effect of the vertical will probably even more dwarf the horizontal, since the square root of the sum of the squares for summation of probabilistic errors will even further attenuate the horizontal.

I can do a simple test by laying up some stepped ducttape with a smooth outer layer, just over the area that contacts the rear bag, lube it well with powder, check the POA movement, and then see if I can tell any difference. With my current (not fancy) system, I bet I can see no difference in less than 30 shots or more! But I sure learned more about how to judge stocks!
(And I may ask for a bit of help from the stock maker)

 

[Don]

Hmmm.... that would be a problem.....would be much easier for me to lay up another layer of fiberglass on the thin side and then sand down to get the contour I want. I know that it needs to slope toward 0.100 so choosing slightly lower than that would guarantee a significant reduction.


As you pointed out earlier, this may be a waste of time.

The vertical movement of the point of aim is huge (due to the rake of the stock) compared to the horizontal movement (due to the slight 0.2 degree error in the laterality fo the rear stock). If there is any movement during the time the bullet is still in the barrel, the vertical movement is going to dwarf the horizontal movement.

If both are equally repeatable (a key issue, but likely), then the effect of the vertical will probably even more dwarf the horizontal, since the square root of the sum of the squares for summation of probabilistic errors will even further attenuate the horizontal.

I can do a simple test by laying up some stepped ducttape with a smooth outer layer, just over the area that contacts the rear bag, lube it well with powder, check the POA movement, and then see if I can tell any difference. With my current (not fancy) system, I bet I can see no difference in less than 30 shots or more! But I sure learned more about how to judge stocks!
(And I may ask for a bit of help from the stock maker)

I think your accessment of the stock situation is spot on, everything that you have said here is correct, in my view, and you are headed down the right path............Don

 

[docsleepy]

Thanks, Don. Lotta learning going on between my ears, with huge helpings of wisdom from y'all. Pretty soon I'll probably be saving up for a better rifle and just about every week I learn something more to look for.

 

[docsleepy]

Tried a simple test to see what would be required to true up the stock.

First measured vertical and lateral point of aim movement, pulling rifle back in the bags:
With 2.625" of aft movement,
vertical point of aim at 100 yards shifted by 8.5" (paper width)
horizontal point of aim at 100 yards shifted by 4.5" (much more than I remembered!);

Added 6 or 7 strips of duct tape to the "skinny" side of the stock (left side), starting with a small piece at the rear, and then increasingly longer pieces reaching further forward, right where the left "ear" of the rear bag would touch. Thus at the rear the thickness was about .075" and tapered down to only one thickness of duct tape at the front. (see attached photo)

With this addition, the horizontal was now roughly 0.5" for the same movement of the stock as above. I stopped at that point.

So if I were to lay up some fiberglass/gelcoat similar to above, sand the contour, I would be able to true up the tracking in the rear bag.

Now whether or not this is worth doing, I don't know. I think I'll practice some groups with the temporary fix in place (dry lubed with baby powder or suave) and see if I can tell any difference.

Question: are most stocks shiny-smooth, or textured, or teflon-strip coated? I have the option to change mine if I work on it, and I don't know which is better.

 

[abintx]

Gordon ...

Don --

My action is just a Savage action

and let me tell you, this PPC business has been a lot to learn! Numbered cases, careful records; I've learned a LOT in the past 3-4 months since the new barrel and stock arrived. Action is to get true in a few months (on a list) and far better trigger installed. New front rest....all kinds of dies, new kind of press, the list just keeps growing. Gordon

Send your rifle to Fred Moreo, the Savage Guru here: http://www.sharpshootersupply.com/. He'll have you shooting .084" inch groups in no time. He did it for me, he can do it for you. Lighten your load. Art

 

[docsleepy]

Thanks, Art-- the action (but not the whole rifle) is scheduled to go there end of May for a time/true/Evolution trigger.

What exactly did he work on that helped you out so much? I might want to have him do more than what I had planned if you recommend additional stuff.

I shot a 0.25 group today -- first of my life.
I think I need a heavier rear bag. Discovered mine was still moving around.
thanks,
gordon