Joe Woosman
Member
Keith - By the picture it appears the center of gravity is below the bore centerline. How do you think this interacts with your stock forearm? My assumption was the recoil would rotate about the CG, moving away from the front rest. If that were the case I fail to understand how the forearm provides more or less positive compensation.
I've often wondered about these ultra light carbon stocks and large heavy scopes. If the CG was above the bore it seems the recoil would rotate down into the front rest making rest placement, bag fill, etc. more critical.
Thanks for your contributions.
M
mks
Guest
Joe,
The forend is like a mass on the end of a spring. When the recoil force causes rotation, the inertia of the forend resists it. This inertia is the same whether the stock rises out of the bag or not, but the location of the bag affects how high the barrel rises relative to the forend. The lower the stiffness and mass, the lower the resistance.
Thanks for asking about the CG. I hadn't measured it until now. It is VERY close to the bore. It seems to be just barely below, rather than above. That probably explains the lower than expected compensation. The original plan was to add weight on the bottom of the stock under the receiver, kind of like Dan's possum belly™
, but the walnut was denser than expected, and the gun is already at 13.5 lb. An adjustable weight far below the CG is still a good idea, I think, but I will need to lighten the stock first.Cheers,
Keith
Joe Woosman
Member
I almost thought I had it until I read your previous post where you said "It turned out to be too flexible for 100 yards and not flexible enough for 200 yards". I may just need more time to process this. Are you taking into consideration the rotation about the bore axis caused by the barrel twist?
M
mks
Guest
Although rotation around the bore does happen, it isn't necessary to include this to explain positive compensation, or the need to tune for each yardage. Have another look at VarmitAl's plot of bullet trajectory at two different yardages. On his website, he also shows animations that include stock motion. A more flexible stock allows the muzzle angle to rise at a faster rate. My stock needs more motion. One way to get it is with more flex, but because the CG is so high, I think the first thing I will try is to lower the CG.
Joe Woosman
Member
I've reviewed Varmint Al's FEA simulations many times over the years and I don't recall much data on the torque action to the setup. I would love to see an animation viewed from the muzzle end. The simulations are wonderful for visualizing the flex in the various components. Watching the stock flex from the side view I think I understand the intent of your stock design if the torque does not influence positive compensation.
The reason I questioned torque was based on two observations. Shooting varmint rifles free recoil with rounded stock designs makes it quite apparent rifles do twist in the bags. Grabbing the muzzle end of a bench gun in the bags and twisting lifts the bore in the vertical direction. Could this not contribute to positive compensation?
M
mks
Guest
Yes, absolutely. I don't know how much, though. My old McMillan stock jumped and twisted so much that other shooters would remark about it. It had enough compensation for 200 yards. Part of that could have been the twisting. I think for best consistency, though, we want to reduce twist as much as we can. Variation in side tension on the wings of the front bag affects the amount of twisting motion significantly.
FWIW, my new stock is the most stable I have had in terms of minimizing twist and other extraneous motion. It recoils sharply backward (it's a 30BR after all), but without other motions that I can tell. I am sure the low front rest to bore dimension is a reason why. The 18 twist may be part of it, too. The lack of twisting motion might be contributing to the low compensation. I'm sure the high CG, does, and that needs to be fixed.
Remember that VarmintAl's simulations are only two-dimensional (only the vertical plane). So they don't tell us anything about twist, or about horizontal motion of the muzzle. He does show on the Barrel Harmonics page two twisting modes. They are numbers 4 and 8.
Joe Woosman
Member
It seems the flexibility you designed into the forearm could dampen the twisting motion reducing compensation in that sense. Also contributing to the lack of "extraneous motion".
In Varmint Al's simulation of Esten's Rifle & Tuner, he states "The stock in the model is more bulky than the actual stock." He shows his inputted weights as 48.389 oz for the stock and 19.966 oz for the scope. Given the bulky, heavy, lower than typical butt for a short range BR rifle, it suggests a center of gravity much lower than the centerline of the barrel. When I watch the simulation, by brain is telling me the upward rocking of the action and the flexing of the forearm is due to the action being driven rearward over top of the CG. If the CG were exactly on the bore centerline, I have a hard time imagining some of the motions in the simulation. Maybe the twisting action would produce a similar compensation producing results similar to the calculated results?
M
mks
Guest
Agreed on both points. If someone were so inclined, they could test whether twist is important by putting hard side tension on a forend that twists with light tension. Or we could all chip in to buy VarmintAl time on a supercomputer to do 3D simulations.