Stock Rigidity

[justjeff]

Dave,

My next stock will fix it all. 20kg of 32mm alloy plate, 200mm wide forend. Torque? What Torque? Just as well the rules say the gun only has to be man portable, not how many men.

Jeff

[trevort]

Hold on a minute, are you guys trying to tell me that picking the most aesthetically pleasing stock might not be the way to win matches


Sent from my iPhone using Tapatalk

[Old Trev-39]

To all,
I am of the opinion that keeping weight as low as possible is the way to go. Something that has more weight above centre line ( take bore line) tips easier than something with less weight up high. If I remember correctly, I was talking stock design with Peter Smith one day and he said, (I Think) that a rifle rotates around mass centre line and not bore line. I will stand corrected if Peter reads this.
Cheers.
Trevor.

[Tony Z]

Hi Trev. True that high mass above the bore line is easier to tip if the force was applied directly and externally on that upper mass. As Rod stated the mass above the bore line has resistance enertia to the torque of the bullet being spun up, the centerline of the bore being the axis of rotation of that applied force. If the weight of the scope and mounts were placed at 90 degrees to the right of the boreline of a RH twist barrel, it would still have the inertia as a stationary mass, but also a weight resistance to the torque due to gravitational pull that the torque has to now lift against gravity. Vertically above the bore line does make it somewhat easier for the torque to force a shift. The ideal position makes it impractical and ugly.

The two ways to eliminate torque are by applying a counter force or resistance. In this application weight to the right side of rotation is a counter force. Levers to resist torque (torque arm is the mechanical term) about the axis of rotation are the left side of the fore end and the comb of the butt.
Weight as a whole is resistance through inertia. The more weight you add like in a HG or rail, the less effect torque has to move things about.
While all this torque thing is happening, the rifle is shifting rearward with the projectile still in the bore. Recoil is in a minimal stage as gas is yet to vent and give the full effect. Torque is at its maximum. So the question is, is stock rigidity required to tame recoil or torque or the wave motion of primary vibration of less than a few hundred htz set up by bolt thrust and a drooping barrel straightening under pressure?
From dealing with this stuff through engineering every day i know that a U channel section is the worst structural form for resisting twist apart from flat bar. A channel with closed ends is better but still not great.
A fore end is traditionally an open U channel so its resistance to twist or torque is poor in comparison to its resistance to being bent across or through its section.
The materials used to make stocks are often great for rebounding where stocks i have had when set up with something that can really spin up a big bullet react gastly on bags when fired. This gets back to the start where it is my belief the material in a relatively conventional type stock and not the obscur construction claims is what counts where it may react ugly in the course of a shot fired, but the ugly is repetitious. Ultimately this still can and will result in accurate rifles. But not always.

My take on it is that if you want to build something more resistant to twist you need to put a roof on the channel, ie box/rectangle or use a cylinder like in drive shafts.
Have a very close look at Tinker Toy and what Davidson did and what he used and the mechanical principals he employed when building that stock. Closed geometric section with a weight bias to the right side and the bore line low and close to the plate that sits in the bag. He could have very simply bolted the action to a very light offset and skeletonised alloy channel in place of the tubes, but chose not to for a very good reason.

[dg]

They say that you can’t argue with physics and that a picture is worth a thousand words (in this case probably a few thousand).

As an aid to highlighting and making the torque/inertia situation for any rifle design being easier to understand or visualize, if someone was able to post force diagrams (both static and dynamic) (using the x,y and z axis(with the z axis being the bore axis)), I am sure it would simplify this discussion.

Additionally for the mathematically inclined, it would also enable appropriate calculations to be employed, which would definitely highlight the dominant and less dominant forces which induce and react to torque and inertia for any rifle shape or design.

Without doubt a high scope and mounts will slow inertial reactions, just as a steel or similar rod will do if positioned on the base of the stock. (another design feature which in my opinion has received insufficient attention both here and overseas).

Depending on the rifle design, this steel keel will also provide its own unique additional weight distribution, force reaction characteristics and bag/rest handling/management etc etc.

Usually, I prefer symmetrical over asymmetrical cross sectional weight/mass distribution, if others have different ideas and priorities, I have no problem with accepting and respecting their decisions..

Regardless of rifle design/ caliber/inherent accuracy etc, for most of us, our less than adequate reading/assessment of mirage and wind conditions etc will continue to be the greatest obstacle that needs to be overcome, for us to improve our shooting performances.

IMHO re-examination and improvement of wind indicator design and mirage interpretation would be of immense benefit to all.

cheers
dave

[Tony Z]

Is this the symmetry you're talking about Dave? Interesting design with the tube connected directly to the stock and the barreled action secured by the front lock nut/muzzle brake putting it all in tension from muzzle to action face. Action floats behind the tube and the recoil lug is essentially the lock nut at the muzzle.
The main point of interest is the winged keel. This is a departure from mine that uses a singular central guide in a conventional bunny eared bag. This one uses a complete 6 inch wide HG front bag and guide plate as the rear rest. The objective according to the builder was a 50/50 weight bias between the stock contact points. The gun is a 17 lb LG though it looks heavier and by all accounts shoots very well though it departs from conventional wisdom of a free floating barrel. By its ugly content I'd say it shot very well.

[dg]

looks the goods

luv the closeness of the bore to the front rest, low CG, wing width and position on base of butt, equal weight distribution and the majority of mass being below the bore axis

cheers
dave