Stock Rigidity

[albow]

Thanks for the photo’s Macca.

That is a nice looking stock Rob. Looks like quite a few hours work in each of them if you are hand shaping them individually.

Yet another stock to look at and get ideas from...... where do you stop

[Rabbitz]

A thought from left field, that is based in ignorance and exactly no experience in this area...

Like all things in precision shooting, repeatability is the thing. Making the rifle and shooter do *exactly* the same thing, shot to shot.

So in that light, is a measure of rigidity actually a useful thing? To explain, is the actual rigidity important assuming that the reactivity* of the unit is repeatable shot to shot?

* By reactivity I mean the action of the whole unit - recoil, vibration, movement on the bags, muzzle jump, shooters influence, etc. etc. etc.

[Tony Z]

Those are good points Rabbitz. The issue is the first few milliseconds where the rifle is moving rearward but the bullet has yet to exit the muzzle. That is the critical moment and why stocks are important. In a rimfire the rearward movement is about a tenth of an inch before bullet exit. A centerfire is considerably more. All small movements but an angle change there translates into bullet spread downrange.

[dg]

the various comments have provided some interesting reading and analysis of stock design.

in relation to minimising induced torque ( T = F * r * sin(theta)) , wider or offset forends obviously contribute to this aim.

what is the feeling or opinion on deep butts (including their positioning in a snug rear bag) possibly providing or not providing substantial torque control and or minimization?



cheers
dave

[justjeff]

Dave,

I would either have a deep butt, but with wings, like TZ does, or go to the two rail wide rear end. Even JR's gun had a wing of sorts, in that it had the butt coming down from a flat plane that rode on top of the ears of the bag. I think one of the advantages of having the wings is you don't get the back of the gun dropping during recoil, so if you don't happen to push right forward hard after a shot, it doesn't give you a vertical displacement.

Jeff

[trevort]

Guys thanks for taking the time to post this for us mugs


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[The Raven]

Rail guns have been steel or alloy construction for decades and are without doubt the most accurate rifles on the planet. Universal receivers used for decades are blocks of steel bolted to steel or concrete slabs with a barrel screwed in and are used by all bullet manufacturers for QA in test tunnels. Charles Bailey shot unheard of groups at 1K 20 years ago in the "USS Enterprise" all steel and alloy 120 pound gun right up until they changed the rules to prevent him competing.
I believe Jackos recent stocks have utilised the alloy ribs we have used since the early 2000s Alan. I would say Jacko is using very rigid stocks and has done so for some time. The question of rigidity is rather obvious with the above examples. As for resonance, carbon fibre and kevlar are about as bad, or good, as it gets if you are refering to transference. Personally i think resonance only plays a part when it's inconsistent. Same goes for flex.
It is not what it's bolt to, it's what it's bolted to does before the bullet leaves the barrel. Weight restrictions mean compromise. That compromise results in flex and it is that flex that determines the way the barrel and action and even the stock in combination flex. If you recall our discussion the other night i mentioned the guy who bolted his Stolle to a fence picket to prove that stock resonance and design was horse shit. Well yes he won a match and proved that a 50 cent pine board can perform as good as a thousand dollar piece of glue and balsa. But he is both wrong and right. If you recall i had two stocks for the Diamondcrap. The SGY and the MBR. The 30 inch barrel in the SGY stock had vertical that could not be cured. The MBR was longer, stiffer and had more rear bias resulting in better balance and less instance of the comb leaving the rear bag and becoming airborn, a trait most SR BR stocks show when put under some recoil. The same load used in the SGY shot little to nil vertical in the MBR. Same action barrel scope etc, but vast change in the way it reacted during recoil. It's not how it looks, or what its made of, its how it reacts to recoil. Then the next recoil and so on. My judgement is the SGY fore end was too short placing the weight bias too far forward when using the pedestal rest that left the butt end light and swinging in the breeze and never tracking consistently in the vertical plane under recoil. Sounds easy to fix but with weight restrictions, velocity requirements and rules confinement its not always straightforward as we both know. There was not a great deal of weight difference between the two stocks but worlds apart in geometry. The fix was geometry.

Tooley wrote a very detailed, and long article over on BRC of what is required in a stock and how that led to his MBR design and subsequent clones. If you read it and look at the mathematics around the lever principles, torque and center of gravity effect, it will alter the perception of what is actually the physical requirement of a stock. It basically says, deliberately or not, the Scoville and clones geometry is wrong way Corrigan for something with recoil and long heavy barrel. A bit unfair as the US rules have very black and white measurements and angles for SR BR stocks. Nonetheless it is probably why none grace the winners lists at 1K with something running a bit of recoil (torque) like you do. If you go that route, the math says no and i wholeheartedly agree from lots of fails to my name. A stock to my mind is about right when it places the rifles balance point between the action screws when fully assembled. If the three contact surfaces are on plane vertically to the bore, the tracking is inline and not curved which is regarded as the death knell. Why i reckon stocks should be machined in a mill and not molded. Or angle adjustable as some latest offerings indicate the importance of this.

I think you may be chasing a phantom here Alan. Our range will never show the best of a rifle since the ground alterations and the illegal population growth down there in Mehico sent the planet off its axis fucking up our wind vectors. Scores are one thing, but small groups on our range are a distant memory. I've seen your guns over a long period. They're perfect. The range sucks.

There is of course a very simple fix to this Alan. Build a heavy gun. Then you will truly see how much the range sucks for 10 shots.

Wow, that's a lot to digest for someone at my level!

I'll reread several times in the hope I learn something.

[albow]

Hi Dave

The height of the butt stock definitely impacts the handling of the rifle both when being shot and recoiling along with when operating the bolt. I think the height is more important than the snug fitting bag.

The first stock I made had a flat bottom that ran between the ears of the rear bag, it tracked well and handled well in the bunny ears and my following stocks went over to using rear wings to run on the top of the ears on the bag where it handles really well too. These stocks have very little bag holding them though are relatively deep in the buttstock. The wings are 3” wide too so again the width you mentioned comes into play to offset torque at the time of shooting the rifle and operating the action.

Rabbitz

I agree about the repeatability of the stock being important and a rigid stock I would also be inclined to think should be more repeatable however I know that rigidity comes with vibration factors which is the bit I don’t really know if it would be good or bad

[stinkitup]

Not really on topic of rigidity but the stock i had for my swede ai was solids aluminium bar stock with hdpe etc. What i was going to do was try a weight as far below the stock as was reasonable and try it at the action and further forward for testing theory being a weight placed furthest as possible should help tame twist eg acts as a pendulum like a keel.

Been thinking of grabbing a whidden block for the 358 and having a play with stock..hmm in all my spare time haha but the pendulum should help with any torsion/twist.

Ryan

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[Brad Y]

My latest FO stock was very long and ran on a z style front end, similar I believe to one of Rod’s stocks. Using a 7saum it was very well behaved. Easy to watch the bullet fly in humid conditions. Even off the bench at 1k it was a pussycat. I think that way is the best in terms of design for controlling torque in a 3 inch wide fore end. I went for a flat bottom butt on the stock, and started by choosing a bag and shaping the butt to be the best possible fit.

I’m in the process of building a new stock myself. It’s going to have to work in LG 1kBR, LG 500fly and FTR. Going to be interesting. Caliber choice is going to make a big influence on if it really works off the bench. From the bipod there is only one choice of caliber so I’m relying on some construction methods from a few USA guys to get it working. Again I’m starting with the rear bag and building from there.

[dg]

I stand to be corrected on any of the following in relation to stock applied/reactive forces, design and material selection etc.

By various ideas and means, differing efforts are made to cope with or best handle the forces generated when a projectile is fired and progresses through the barrel.

When fired, the projectile and burning powder mass generate a major recoil force and a much lesser torque induced force, all transmitted through the stock.

How these forces are compensated for, depends on a myriad of factors.

Stock weight/mass, shape, weight distribution, inherent stiffness (rigidity), construction material/materials, contact position on rests, shoulder contact or not, whether the rifle is shot free recoil or has hand contact ( and subsequent additional weight and inertia applied to disseminate the recoil forces etc), etc.

Commonly used muzzle brakes can significantly reduce felt recoil forces but make no difference to generated torque forces.

As I am sure everyone has experienced, all other factors being equal, the heavier the gun, the less felt recoil and twist (torque) is experienced by the shooter.

Also, all would understand that recoil is a reactive force due directly to and related to the projectile mass, acceleration and the amount of powder being consumed.

Torque is the reactive twisting force generated when the projectile enters the rifling, and once again the mass or inertia of the projectile and the rate of rifling twist all contribute to the amount of torque produced (probably, hard to measure or feel the difference, but the faster the twist for any projectile, the greater the induced torque).

Various engineering types and texts have calculated or given values to the amount of energy which is attributed to direct recoil and torque, all agree that, by far, the recoil force is much greater than the twisting torque, but also all acknowledge that torque must be appreciated and compensated for in stock design.

Perhaps I have missed it, but it seems that significant attention is given to stressing the importance of offset and wider forends in terms of reducing felt torque forces when firing, but little attention is given to the effect that butt size/depth and design can also have on reducing felt or applied twisting forces (Torque).

For simplicity let’s assume in our designated stock design (with butt contact depth in the rear bag of 5.5 inches from the bore axis - a common measurement in McMillan, Choate and Hogue stocks), that the front bag contact height is level with the centre/axis of the bore, which has rarely occurred in LRBR stock design.

The greater the difference in front bag contact position to the bore axis/ centerline, the less effect the front rest has on reducing applied torque forces.

Anyone who doubts this just needs to draw and analyse this situation or calculate the differences.

As generated, Torque is a radial phenomenon and is equal to the multiplication of the moment arm length by the perpendicular component of the applied force. Units are usually either N-m, N-mm or Ft-lb.

So, in the above situation, when shot free recoil, would it be correct to state ?????? that the only reactive forces opposing the generated torque would be through the combined effect of the left hand front rest contact area/point and the rear bag/rest’s contact with the base of the butt.

Therefore, given the above scenario, would it also be fair to state, that, to equal the resistance force applied by a 5.5 inch deep butt sitting in a snug fit rear bag, that the offset left hand forend would also have to be at least 5.5 inches wide (distance from the bore/axis centre line) ????????

If not, please feel free to correct any errors.

cheers
dave

[Tony Z]

Interesting points DG.
When the bullet spins up in a RH twist barrel the torque is applied in a circular motion about the bore axis driving the left side of the stock down into the front bag and lifting the barrel and right stock edge vertically. Simple rotational resistance cause and effect. There are strict rules in SR BR for the side tension on the ears of the front bag in how tightly they squeeze the fore end. Some have been caught and warned and disqualified. So you cannot just tighten up the ears to control torque. The rifle must be able to be lifted vertically from the rest without resistance.

The centerline of the bore to the left edge of the fore end is your lever. You lengthen that, you increase the leverage and reduce the barrels rise, or really the rotation. Simple but very effective.
The deep butt. Well that is another lever where the lower the comb and the butts center of gravity is from the bore line the more torque it controls. A lever that resists torque by means of its length and weight inertia.
I don't think it requires a rocket scientists degree to get any of this.

If you are building a stock without these basic principles in mind, you are effectively leaving a lot on the table. IBS 1K and 600 and even Fly are reasonably free to work with when it comes to stock configuration.

If i do ever build another LG stock it will have a full beam rail on the RH side close to the barrel. There will be no LH side except a platform extending to the left, as long as i see fit, to slide in the front bag. It is surely going to look ugly, but the weight to the right in the rail resists rotation as a counter balance and is the rigid beam to control bounce. The platform extending left under and across the barrel is your lever to counter what torque may be left. As i said ugly as hell but it will reduce weight and remain structurally sound if not stronger than current fore ends.

There was a rifle called "Tinker Toy" that did it with aluminium tubes. Great concept but it really does not need the LH side tube. A single square or rectangular section of alloy, not carbon fibre, on edge just to the RH side of the barrel would have done it and reduced weight or that weight could have been transferred back to the right for more counter balance. I have followed the life of this rifle from start until Davidson passed away. If he was still around this rifle would have won shitloads. The rare earth tuner was later removed where testing was in his own words, inconclusive. It is fucken ugly and as those that know us up here in the north, we excel at ugly.

http://www.6mmbr.com/gunweek077.html

[albow]

So, in the above situation, when shot free recoil, would it be correct to state ?????? that the only reactive forces opposing the generated torque would be through the combined effect of the left hand front rest contact area/point and the rear bag/rest’s contact with the base of the butt.

Therefore, given the above scenario, would it also be fair to state, that, to equal the resistance force applied by a 5.5 inch deep butt sitting in a snug fit rear bag, that the offset left hand forend would also have to be at least 5.5 inches wide (distance from the bore/axis centre line)

Dave I know that balance and weight distribution affects the number that would be calculated for the force that the left side of the foreend and the butt stock though I think you’re pretty well correct for the basic principle. In addition to the reactive forces on the left side of the stock foreend and the butt stock in the rear bag though you will also have the scope resisting this rotational torque too given it is a large mass a distance from the bore centreline that is at rest when the shot is fired. The inertia that it holds would also assist in opposing the rotation of the rifle.

So the heavier and higher you can get your scope the more it will help. That means you need a big fat and heavy NF 12-42 BR in 3 or 4” high rings

[RDavies]

[/quote]

Dave I know that balance and weight distribution affects the number that would be calculated for the force that the left side of the foreend and the butt stock though I think you’re pretty well correct for the basic principle. In addition to the reactive forces on the left side of the stock foreend and the butt stock in the rear bag though you will also have the scope resisting this rotational torque too given it is a large mass a distance from the bore centreline that is at rest when the shot is fired. The inertia that it holds would also assist in opposing the rotation of the rifle.

So the heavier and higher you can get your scope the more it will help. That means you need a big fat and heavy NF 12-42 BR in 3 or 4” high rings
[/quote]
Exactly. You can get by without a heavy Nightforce BR scope if you lift any scope up extra high. You can either go for an extra high scope rail, scope ring risers, or extra high rings. (or use all 3 like I do). This will not only raise the centre of gravity and reduce torque, but will also put your head up in a comfortable shooting position.

[dg]

I agree tony

who cares if guns are ugly !!!

what matters, is, if they shoot consistently to the required standards.

in the past, when I had access to a workshop, i was partial to the odd ugly and different rifle design.

usually, ideas did not work to expectations and occasionally they did.

either way, experimenting, although time consuming, expensive and frustrating, was still an interesting exercise.

you might remember the “ugly 308” which Peter Kay used, to kick ours and everyone else’s backsides in the first 2012 Southern Cross Rendezvous.

that gun had a 6 inch plus deep butt and only a 3 inch central forend ( 1.5 inches either side of the bore), yet it was a pleasure to shoot, not only because it was a 308, but also because the torque management was very forgiving.

Alan and Rod

i agree that inertia can regulate the effects of induced torque.

increasing the overall mass of any gun and or the positioning of additional mass/masses as far as possible from the bore axis will definitely make the effect of induced torque less noticeable (as per LG’s) or imperceptible in HG’s.

for any torque induced situation the only means of reducing or negating the effects of torque, is to introduce an opposing radial force or forces.

similar to the concept of the ballerina, with either “legs parallel ,hands and arms held tight to the body” or alternatively “ knees bent and arms and hands extended”, speed of rotation can be increased or decreased simply by moving the mass away from the bodies axis.

but unless opposing radial force/forces is/are introduced the ballerina will continue to spin.

purely for discussion purposes, if a HG was suspended in a weightless situation (i.e. space) and fired, regardless of its mass or size, it would recoil in an opposite direction to the projectile and due to the induced torque of the projectile moving through the barrel, it would rotate anticlockwise.( if the barrel was right hand twist).

in reality however, for LG’s that are not shot free recoil, the effect of shoulder contact and hand weight and grip around the pistol grip, supplement any existing anti torque features of the rifle design.

friction, pressure and weight from the shoulder contact, deliberate hand weight and applied hand forces etc make it possible to compensate for and oppose recoil forces and also reduce stock twisting and bouncing. (in most stock designs, the proximity of the pistol grip to the butt, and the additional downwards forces, further accentuates the role of the butt in managing induced torque)


In my opinion Alan, your clever idea of the butt wings (especially the contact from the left hand wing) also provides additional torque control to the butt.

cheers
dave