Stock tuning

[mks]

I have theorized about tuning by modifying the stock before, so I would like to show some results that demonstrate that it works. First, the graph below shows the rise in point of impact (POI) at 100 yards with increasing powder charge with my old McMillan stock after I installed a recoil pad, but before further modifications. It had about half a pound of lead glued into the buttstock about midway between the heel and toe.

Note that the regression equation shows that POI rises on average about 0.45" per grain of H4198. With variation in muzzle velocity among shots in a group, this contributes to vertical dispersion, and it would be nice to tune the rifle to eliminate this dispersion.

Varmint Al's animations of a recoiling rifle show that the wrist deflects downward, which allows the action to rotate counterclockwise, which eventually whips the muzzle angle upward, allowing slower bullets to exit the muzzle with a higher trajectory. My hypothesis was that the weight in my gun being fairly low in the buttstock might be causing the wrist to deflect upward, or at least not deflect downward quickly enough to force the necessary muzzle motion to compensate for muzzle velocity variations. So I dug out the original weight, and because the rifle was significantly under HV weight limits, added more weight, all as high as possible in the butt. I was not so disciplined as to measure everything precisely, but the main weight was made by filling about 6" of 1" copper tubing with molten lead and gluing this tube directly to the skin of the buttstock inside the heel. Probably about one pound total. To bring the rifle up to just below the HV limit, I glued a few more ounces of lead just below the main weight. Below is the POI with variations in powder charge for the modified rifle. POI at 100 yards falls about 0.11" per grain of H4198.

Sometimes it is better to be lucky than good, but this response is almost perfect for compensating for different muzzle velocities at 200 yards. Slower bullets strike the 100 yard target higher, but converge to almost the same POI as faster bullets at 200. Calculated from the regression equation and external ballistics of a typical 30BR bullet, a group with ES of 50 fps should have vertical dispersion at 200 yards of just 0.0074," not counting other sources of error.

This was confirmed at Gallatin this past weekend, when I tried a very low load of 32.8 gr of H4198 (bad mistake) and had to finish my last two targets at 200 with my old load of 34.6 gr. Between these two loads, there was no detectable change in elevation at 200.

I can't really say whether the hypothesis is proven about wrist motion, because wrist motion was not measured. But what showed up on the target suggests that rifle dynamics were favorably changed without modifying the barrel in any way. Just another alternative to muzzle devices and barrel contours like Tim shared.

An interesting question now is whether a moveable weight system could be devised to adjust the range of convergence from 200 yards to 100 yards. Seems like moving the weight downward, but not all the way down to the midpoint, might do this.

I am just a rookie BR shooter, but it has been fun applying some engineering principles to rifle tuning. Comments and advise welcome.

Cheers,
Keith

 

[Boyd Allen]

This is original and a big deal! Thanks for posting it. I would also mention that this would seem to validate the later model stocks, that have thick wrists and short (top to bottom) butts.

 

[Nodak7mm]

Keith,

I'm with Boyd, this is kool info and thx for posting it. Please continue your "out of the box" thinking and keep us posyed.

Rod

 

[abintx]

Keith ...

Here's another UNIQUE way to Stock Tune: http://bulletin.accurateshooter.com/2009/11/benchrest-tip-optimize-your-rifle-position-on-the-rests/. Bet you never thought of this!

 

[mks]

Here's another UNIQUE way to Stock Tune: http://bulletin.accurateshooter.com/2009/11/benchrest-tip-optimize-your-rifle-position-on-the-rests/. Bet you never thought of this!

Art,
Yes, that article was part of the inspiration for this focus on the stock. Something worth trying, and possibly easier than moving weights, would be to move my stock in the front rest to tune for 100 yards. I think forward would be the right direction. Joel says he tunes out vertical at 600 yards with rest position, so it seems plausible that one could tune out vertical at 100 and 200 with two different positions. I already know the right 200 yard position for my gun. I just need to find the 100 yard position.

Cheers,
Keith

 

[mks]

This is original and a big deal! Thanks for posting it. I would also mention that this would seem to validate the later model stocks, that have thick wrists and short (top to bottom) butts.

Boyd,
Interesting questions. What seemed to work in this case was getting the weight high, as close to the bore line as possible. The heel on my old McMillan has a drop of about 5/16" below the bore, while a new Edge has a drop closer to 1/2". The larger drop might actually be counterproductive.

A thicker wrist that reduces flex in the sideways directions, but still allows a fair amount of deflection vertically (so it can hinge downward) might be the best design goal.

Cheers,
Keith

 

[henrya]

Hi Keith,

I wonder how much of the effect you saw might come from the rifle moving differently in the bags and not from flexing the stock?

Henry

 

[Boyd Allen]

Tune with ring height?

 

[mks]

Hi Keith,

I wonder how much of the effect you saw might come from the rifle moving differently in the bags and not from flexing the stock?

Henry

Henry,
Yours is a legitimate question, because stock deflection was not actually measured in these experiments. If stock flexing were negligible, then what would be left would be the backward motion of the stock during recoil. The buttstock would slide down the rear bag and the forend, if it didn't rise off the front bag, would side nearly horizontally. Because of this motion, bullets that exit the barrel later would have a higher trajectory. Unless we include flexing of something (maybe the bags?), this response would be the same for a given powder charge and rifle weight, regardless of how the weight was distributed. Adding more weight would slow the rifle down and reduce the effect, the opposite of what happened in these tests. The added weight would also tend to reduce the recoil force between the buttstock and the rear bag, so if rear bag deflection, but not stock deflection, were included, the effect would be opposite again. On the other hand, if the stock were allowed to rise off the front bag, it would tend to rise more as the CG of the gun moved backward due to more weight in the butt. This could potentially explain the experimental results without involving stock deflection.

Varmint Al's stock model flexed wrist down. We have used LS-Dyna, too, and one stock model flexed wrist down and another wrist up during recoil. We need to look into this in more detail to explain the difference in response. Maybe there are some other things I am overlooking, but based mostly on these finite element simulations, my guess is that stock deflection has a strong influence on rifle dynamics.

Cheers,
Keith

 

[mks]

Tune with ring height?

Boyd,
I haven't tried it, but I think it could work. Moving over a pound of scope weight up/down would change the CG height and rotational inertia of the gun significantly.

Cheers,
Keith

 

[crb]

When you change the weight of the rifle you also change the inertia. The rifle will not have moved as far on the bags when the bullet exits. This would seem to cause a poi change.

 

[Greg Culpepper]

Keith,

I have a suggestion that might simplify your testing protocol of altering stock stiffness through the wrist by altering CG with weight position. For the purpose of tuning the stock, what do you think of actually changing stock stiffness through mechanical means? One solution to this might be to cut a slot across the top 50 or 60% of the wrist and fill that space with an elastic material. The remaining wrist material would act as a hinge point. A through bolt such as used to attach a shotgun butt to its receiver could pass through the compressible material and vary its stiffness and resulting stock flex by adjusting preload with a threaded nut in a recess at the butt or a finger adjustable wheel in a window in the stock near the butt. It might not be necessary to attach the through bolt to the receiver. It might work fine to anchor the front end of the through bolt in a recess in the face of the grip behind the trigger. I might in this way be possible to vary stock stiffness and deflection for tuning without adding additional variables that would confuse cause and effect observations.

I wish you many happy hours of experimentation, enlightenment and much success on your quest. For me, uniform bullets, small extreme velocity spread and a soft front bag work pretty good at 600 and 1K. But you might change my mind.

Greg

 

[mks]

...For the purpose of tuning the stock, what do you think of actually changing stock stiffness through mechanical means? ...I might in this way be possible to vary stock stiffness and deflection for tuning without adding additional variables that would confuse cause and effect observations....

Greg

Greg,
Neat idea. If the adjustability were repeatable, tuning for multiple yardages would be a matter of resetting preload to predetermined values. Cool.

Keith

 

[henrya]

Keith,

I'll bring a saw to the next match.

;>)

 

[j mckinnie]

but

doesn't all this effect the balance of the rifle?

 

[Dan H]

I still like my Possum Belly. When the weather becomes a little more pleasant I'll do some testing w/without to see what happens. I kinda like the idea of pushing the stock further into the front rest.

Dan

 

[mks]

doesn't all this effect the balance of the rifle?

I am not sure I know what "balance" is precisely. It has been described in terms of front/rear weight distribution, which wrist deflection and raising/lowering CG would not change. But some seem to include recoil response of the rifle, such as torquing and forend lift off the front bag, which would change. These two different characteristics are somewhat like static versus dynamic balancing of wheels/tires. While it makes some sense that balance, what ever that is, may contribute to accuracy, I would be happy with an "unbalanced" rifle that hit all the dots.

Cheers,
Keith

 

[mks]

Keith,

I'll bring a saw to the next match.

;>)

Tennessee Chainsaw Massacre, Benchrest Edition!

 

[JackieStogsdill]

Keith. What are your thoughts on birdshot sloshing around in the butt of the stock?

 

[mks]

Keith. What are your thoughts on birdshot sloshing around in the butt of the stock?

Jackie,
If it sloshed the same every shot, then you could tune the gun based on that. The concern I have is that it may not slosh the same every time, which would probably cause vertical dispersion, assuming the shot moved forward and back but not side to side and the CG of the moving shot were aligned with the axis of the rifle. If the CG of the moving shot were off to one side of the axis of the rifle, inconsistency of sloshing could induce vertical AND horizontal dispersion. Better to glue it in securely, I think. Maybe some have had good luck with this, I don't know.

Also, a difficult thing with shot is keeping the weight centered in the gun. If the CG of the shot is left or right of the bore axis, then recoil forces cause a torque that would tend to scatter shots horizontally. Keeping the CG of the whole gun directly below the bore is important, I think, as is keeping the bore aligned with the stock.

Cheers,
Keith