Recoil & Stock Design

Get or give advice on equipment, reloading and other technical issues.

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IanP
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#46 Postby IanP » Tue Jun 03, 2014 9:15 am

Interesting reading!

I dont think too many F-Class shooters can use a "Pavey" grip as very few hold the forestock with their left hand. I use the Pavy grip, (no E) to hold a BR style vertical pistol grip firm enough to pull the stock back and make contact with my shoulder. I used to push a fraction forward with my shoulder to lightly pin the forestock up against the front rest stop. I dont do this anymore with the big 30 cals as it led to inconsistent vertical. I now push forward up against the stop and then back it off the stop just a fraction. This was following Paul Janzso' advice which I found to work well.

I fully agree with the weighting of the muzzle to help counter muzzle lift and its an easy one to understand how it works.

I dont agree with the top weighting argument as being preferable to bottom weighting to help counter torque and recoil. In fact as I have said many times a low centre of gravity increases stability and I am sure this helps in riding the bags in a linear (straight line) fashion. A picture is worth a 1000 words. Take a look at this recoil stabiliser made by Anschutz and tell me if it would be more stable mounted upside down on the top of the rifle. http://ahg.anschuetz-sport.com/index.ph ... duktID=360 Scroll down the page to find the picture of the stabiser attached to the forestock.

The Anshutz stabiliser is used for 3 position shooting so it encounters a different dynamic than riding bags but it requires even greater stability. As F-Class shooters we are stuck with having a scope, mounts and ramp mounted on top of our actions and in a central position. I fitted a light scope with ultra-light rings and an aluminium ramp to reduce the weight in this area. I then fitted a weight on the bottom of the stock just in front of the action screws to bring my rifle up to max weight. This increased the stability of my setup by quite a bit and I shot my highest consistent scores using this setup. I dont think it is a coincident.

I consider stability a very big issue and thru experimentation based on increasing the stability of the rifle, found something that worked. Low centre of gravity and bottom weight bias is much more than simply having a low profile forestock and suggesting that this increases the effect of a high mounted scope to counter recoil. If it were otherwise then the Anshutz stabiliser would be designed to be attached on top and not on the bottom.

I also found when dealing with the increased recoil of the heavy 30 cals that the bags both front and rear needed to be firmer to reduce friction. Any friction or the stock catching on the bags under recoil led to flyers. Teflon tape on the stock helped with straight line recoil but resulted in increased felt recoil at the shoulder. Shooting the big 30s with heavy bullets has been quite a learning curve for me and I think has made me a better shooter. It certainly sparked my interest in the underlying science of recoil and how it works and how it can be reduced or at least better controlled.

Ian
__________________________________________
A small ES is good. A small SD is better. A small group is best!

pjifl
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#47 Postby pjifl » Tue Jun 03, 2014 10:37 am

I will touch on many things.

Another Percy Pavey Colleague ! This dates a few of us !
I had the honor of shooting and scoring with him at a Brisbane Queens a LONG time ago.

I have done a lot of sling shooting and many of the newer shooters do not realize that it accomplishes many things very very well. Like consistency and rifle control. But it is difficult to relate sling shooting with shooting from a rest and that is what I do now. Just using a sling now can be difficult when my shoulders play up and shooting in a hot coat in the humid hot tropics in summer sucks big time.

A high mounted scope does two things.

1/ The centre of mass of rifles is always below bore line. This makes for 'upward muzzle' rotation when shot irrespective of the height of friction on any front or rear bags. Adding high mass helps to move the centre of mass closer to the bore line.

2/ Now for what people are calling 'Torque' - ie the leaning over of the rifle. Any mass far removed from the centre of mass has a very very much greater resistance to rotation since this relates to the square of this distance. Engineers call this rotational resistance the moment of inertia of a body. Unfortunately, a very high mounted scope will greatly increase the force on mounting screws. My 'infamous white scope' was heavy because it is incredibly difficult to make a really lightweight scope without the specially made body and internal tube extrusions available to the manufacturers. It was mounted via 3 of 1/4 UNF bolts which was fairly rigid and the height position reduced rotation significantly.

Another rotation taming device on my rifle is a muzzle weight that is short and FAT (and heavy). To most it is strange - even off putting - but they never realize that it doubles the moment of inertia of the barrel so plays a big part in reducing rotation.


Now for the main thing.

We have the separate topics of Interior and Exterior ballistics.

The same applies to recoil.

Before bullet (and powder gases) exit, the rifle is accelerating linearly and rotationaly. This lasts for about 1 to 2 ms. After bullet exit, the rifle simply continues to move back and rotate at the same rate until opposing forces stop it since the rifle is not rigidly attached to anything.

The typical heavy target rifles we use only recoil about 2 mm and rotate about 1/10 degree before the bullet exits.

Unless you are pulling the rifle very hard into your shoulder, you are effectively close to free recoil as far as the bullet is concerned because shoulder resistance is almost zero for that 2 mm of recoil. Similarly, 1/10 degree rotation will cause little rotational resistance from bags, rests or shoulder.

I believe most of what we are have been discussing is actually rifle control AFTER the bullet has escaped.

It is important of course. It makes for a more relaxed, controlled shoot. It may allow one to follow the flight of the bullet. It may help against flinching.

It may even help in the mind. And - just maybe - this is the most important factor.

williada
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#48 Postby williada » Tue Jun 03, 2014 11:59 am

Ian you will have to excuse my spelling as I wrote the piece in the wee hours. The Pavy grip was referring to right hand control and not forend control with the left hand with the hassle of the sling. Perhaps I should have worded it better in that rotational stock control with your key fingers and wrist still allows the rifle to recoil into the shoulder without having to pull it back. Like you I sneak up to the front stop and back off as I indicated earlier that this was a fault in my technique with larger calibres.

Yes stability is very important and you have to deliver the same way each time. That is essential.

In the heavy machine rest testing, I did notice improved grouping by raising vertical weights and in that way I used them to tune the machine rest so the rearward movement did not stagger as we have several moments of inertia under recoil. Of course I was also testing different barrels with different lengths and profiles. But the pattern was reliable. I judged the vertical dispersion on paper to see whether I had raised the weights high enough. So to me it was getting the balance right under recoil and effectively you are doing the same thing by dropping your bore centreline lower into the stock. It is just a matter of degree and functionality.

In fact, the vertical balance does not have to be perfect as I like a little jump for compensation purposes and yes like a yacht keel weight below the bore centreline will settle things in the bag. But everything is a trade-off. The point about a weight on top is merely a counter and functional way of balancing the forces involved but it also has to be moved fore and aft for bag handling on another plane.

I remember my dad telling me about a famous jockey who could take six lengths off a horse by riding behind or the horse's point of balance (near the whither) to make it work harder and the punters never knew he was not allowing it to run on its merits.

Let’s say everything is balanced perfectly with the gear. The problem becomes that it feels too light and responsive to body movements and our body due to changing mound positions interferes with the harmony. I think I feel more secure feeling a bit of resistance on the gear than to balance my body. Yet I feel the tail is wagging the dog in another sense. So I generally like to have the weight forward of the rifle pivot point and a little tension on the side pads of the front rest. Naturally you will get vertical if the rear bag is too soft. You still have to be comfortable with your position and the balance of your rifle and each to their own. You certainly have to change you style to get used to shooting with a high fulcrum point which has more strengths than weaknesses and provides more opportunity if you can adapt. It’s only a different way of doing things. It’s the improvement on the paper that counts.

However, I am of the opinion that what counts the most in all this assuming we can deliver the shot consistently is the fulcrum point in relation to bore centreline and it is a more responsive way of dealing with muzzle lift and bag handling given that a general rifle balance suits you.

Observations come from testing, just as you will find. You simply try them out under actual conditions of competition. We learn something new all the time. Sometimes we just start again as I have done hundreds of times. In no way would I like to see your enthusiasm and knowledge dampened. Like all experts we are drips under pressure and we really don’t have all the answers, but as records are made to be broken, why not try?

DaveMc
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#49 Postby DaveMc » Tue Jun 03, 2014 1:12 pm

Thanks for the contribution Peter and David (again). I was looking for a link to the Border barrels simulator but it seems to be pulled offline - maybe relocated?? This had the function of simulating the raising and lowering centre of gravity and changing barrel profiles and seeing effect on barrel vibrations and impulse- it was very interesting and sure you would have loved it Ian.

As far as the underslung weight from Anschutz - there are several comments. Yes it will slow torque roll but being offset (and not balanced) it will also cause a slight shift sideways (to the left) of barrel under torque. Being forward of the handgrip and centre of balance it will also hold muzzle down a little during recoil. It is also designed for 22's with very little recoil and torque and held in a different fashion to our rests.

bsouthernau
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#50 Postby bsouthernau » Tue Jun 03, 2014 1:21 pm

williada wrote:Ian you will have to excuse my spelling as I wrote the piece in the wee hours.


Nothing wrong with your spelling David. Ian was simply differentiating between Percy and himself. :D

bsouthernau
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#51 Postby bsouthernau » Tue Jun 03, 2014 1:37 pm

DaveMc wrote: I was looking for a link to the Border barrels simulator but it seems to be pulled offline - maybe relocated??


Is this what you were looking for?

http://www.geoffrey-kolbe.com/articles/ ... ations.htm

IanP
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Location: Adelaide

#52 Postby IanP » Tue Jun 03, 2014 1:55 pm

Williada, now I know you as Dave W, from now on I will call you Dave W, if that is ok with you?

Apologies on the spelling of Pavey, you were right to use an "E" for Percy and it was my poor attempt at a joke as my surname is Pavy (no E) and I was explaining my grip to you. It is confusing if you dont know my surname!

I really enjoyed reading your post as it seems we share some technique and ideas in common. I think pjifl made an interesting observation on what is being examined and discussed mostly happens after the bullet has left the barrel. While this observation is obviously correct I still reason, (perhaps erroneously) that stability doesn't only matter after the bullet is gone. Stability must play a vital part in the 2ms that the bullet is travelling down the barrel as well as after. It seems logical if we go to an extreme, that a top heavy, high c of g, narrow rifle will be biased to tip to one side or the other more than a bottom heavy, low c o g rifle. If this is not so, why then is the Anshutz stabliser mounted on the bottom? http://ahg.anschuetz-sport.com/index.ph ... duktID=360

Maybe I'm wrong and need to look at moment of inertia in some detail to understand the forces at play. To me the overriding force in play, if I have understood what has been stated, is recoil causing muzzle lift. While torque plays a part in upsetting the balance of a rifle it is a much smaller force and therefore secondary in priority to neutralise.

Dave W, commented on having a consistent vertical fulcrum point. What about having a consistent weight bias on the bags? What should that bias be? What effect does centralising mass between the bags have? I'm hoping a video will show what effect moving weight around from the top to the bottom and between the bags will have on angular displacement of the barrel and hence the grouping of the bullets.

In setting up for video recording I plan on taking advice onboard from those mentioning the pendulum effect. I now plan on using a sliding door roller setup to attach the top of the nylon strings to instead of having them fixed to one point. I will use double rings attached to the nylon to provide hinge points to attach to the rifle. This will enable the rifle to slide horizontally without excess pendulum effect while the nylon should still be easily deflected without overly inhibiting recoil effects. Any comments on this much appreciated! I will have a calibrated backing board in the picture to measure movement frame by frame when editing.

Finally can someone help with moment of inertia, is it planar movement only, (ignoring torque) we are concerned with? I will post a link on moment of inertia and if someone can advise which is applicable I would be most grateful.

http://en.wikipedia.org/wiki/Moment_of_inertia

Ian
__________________________________________

A small ES is good. A small SD is better. A small group is best!

Brad Y
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#53 Postby Brad Y » Tue Jun 03, 2014 3:27 pm

So if finding the balance point is important with a rifle, what problems could be associated with a muzzle heavy rifle? I mean if you put a 30 inch straight barrel (with a 700gr dampener for arguments sake) on an action you are going to need one HELL of a heavy stock to get it to balance out if you grab the rifle under or just forward of the action. My rifle is built as such and works superbly at stopping muzzle jump and torque. Though in saying that, the bore does have a fairly low center of gravity- probably not quite as much as a mcmillan f class stock- and it does have a very high scope as well. I can follow through fine and dont lose the target picture at all. In fact some conditions permit watching the trail of the bullet fly downrange. After shooting FS with a palma profile 308 barrel and having the 7.5kg rifle jumping at the muzzle (same stock, scope and same rest by the way) Im fairly convinced that having alot of weight forward counteracts all the nasties that we are trying to overcome in shooting larger caliber rifles... Just my experience.

bsouthernau
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#54 Postby bsouthernau » Tue Jun 03, 2014 5:16 pm

IanP wrote:
Finally can someone help with moment of inertia, is it planar movement only, (ignoring torque) we are concerned with? I will post a link on moment of inertia and if someone can advise which is applicable I would be most grateful.

http://en.wikipedia.org/wiki/Moment_of_inertia

Ian


Ian,

I think you're confusing planar rotation with translational movement. For the purposes of this discussion I think you need read no further than the end of the second paragraph of the intro to that wikipedia entry. We don't need a discussion of second order cartesian tensors here thank you very much!! (Even though the rifle is rotating simultaneously about two orthogonal axes)

As previously, someone please correct me if I make a blunder here as it's now 45 years since I studied this stuff. Torque has been used rather loosely in this discussion and is not the same as rotation. All you really need to know for our purposes is that torque, moment of inertia and angular momentum are the rotational analogues of the more familiar force, mass and momentum in translational movement.

If you apply a force through the centre of mass of a body you cause it to accelerate and increase its momentum (mass x velocity) which is its propensity to keep going. If instead you apply the force at a distance from the centre of mass this is a torque which causes it to spin increasingly faster, an angular acceleration, thereby increasing its angular momentum (moment of inertia x angular velocity) which is its propensity to keep spinning. Moments of inertia are calculated relative to the axis of rotation and I wouldn't like to attempt it for a rifle.

Hope that's not too much of a lecture and has helped rather than confused you further.

Barry

And yes these login names can be a curse - I got your joke. Only dawned on me yesterday who williada is.

RDavies
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#55 Postby RDavies » Tue Jun 03, 2014 6:39 pm

. In fact I believe what most people refer to here is a low bore position on front bags and a streamlined butt (expect Rod Davies quip here! :lol: )


You're right Dave. I have been thinking a lot about butts lately and streamlined butt was on the tip of my tongue, just couldn't put my finger on it.
Anyhow. My new stock should be ready very soon hopefully and one of the measurements I was adamant about, was that it have a shallow or streamlined butt. I was glad to hear that the stock maker had the same idea and will only have the butt 90mm below the fore arm level. One of my stocks has a shallow drop like this and is very easy to shoot free recoil, not very sensitive with hold. My other stock, currently on my 7mm has around 160mm of drop which is good for counteracting torque but puts the COG too low. This stock is more temperamental with hold and I have been suspecting that the extra barrel whip could be part of the reason it is so finicky with loads now, compared to its previous stock.
The shallow butt stock gun will have the scope mounted much higher than normal, even higher than my current scopes. The extra high scope should hopefully bring the centre of gravity in line with the bore, as well as providing some sort of anti torque component.

IanP
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Location: Adelaide

#56 Postby IanP » Tue Jun 03, 2014 7:54 pm

Barry, thanks for putting me straight on the moment of inertia. I'm glad I can think of it in simple high school physics terms. Very well explained and saved me needing to undertake an engineering degree :D

Ian
__________________________________________

A small ES is good. A small SD is better. A small group is best!

bsouthernau
Posts: 562
Joined: Sun Mar 11, 2012 4:31 pm

#57 Postby bsouthernau » Tue Jun 03, 2014 8:08 pm

RDavies wrote: I have been thinking a lot about butts lately and streamlined butt was on the tip of my tongue, just couldn't put my finger on it.


And I thought this was a clean-living family-oriented forum devoted to rifle shooting. Oh well..............

IanP
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Location: Adelaide

#58 Postby IanP » Tue Jun 03, 2014 8:27 pm

RDavies, good luck with your streamlined butt. I heard of blokes needing this sort of thing as a part of the mid life crisis :D

I too have a tubular stock designed rifle (MDT) with inline AR15 style, (streamlined) adjustable butt stock. It has extremely high scope rings and heavy tactical scope. It only has a light plastic pistol grip, (Magpul) and a perforated aluminium tube as a forestock. From everything I have read on this thread so far it should be the perfectly balanced rifle with the buttplate adjustable to be central on my shoulder and in line with the bore. It has very little weight below the bore line and I attached a light aluminium Sinclair 3" wide front plate directly to the forestock tube.

This rifle has a 30" medium palma barrel and is chambered for 308W. I guess you could call it a custom military knock off design featuring qualities that should endear it to its owner. Well both my son and I shot this rifle for some time, developed some loads and enjoyed pretty good accuracy. The thing is that it was a pig to shoot as the recoil being inline was quite harsh when shot off a front rest and when used off a Davies bipod took quite a bit af man handling to stop it from jumping off the ground. Basically my son wanted a military looking rifle but neither of us liked its manners.

Long story short, the stock is now back in its box and if anyone wants to buy please PM me. We changed the action over to a conventional design Manners stock. This stock has a butt shoulder placement well below the bore line and a much larger custom Davies heavy aluminium front plate (16mm thick) with aluminium sides taking it upto 20mm thick. This stock did not require extra high scope rings so the scope is mounted lower than on the MDT. This rifle is a pleasure to shoot in FS off a front rest and extremely accurate.

The inline MDT stock was an experiment as we had other FO and FS rifles in conventional laminate stocks. We were happy to go back to conventional F-Class stock like Shehane and Mastin make!

Ian
__________________________________________

A small ES is good. A small SD is better. A small group is best!

dave
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Joined: Tue Jun 21, 2005 1:18 pm

#59 Postby dave » Tue Jun 03, 2014 8:42 pm

each to their own with stock design.

everything is a compromise, in this case overall 17lb max class weight.

maximum stock length to ensure 50/50 rest/bag weight distribution (minimises/eliminates muzzle weight effects). lowest possible bore line, parallel bag/rest contact surfaces

IMO a deep stock butt provides greater resistance to torque than a 3 or 5 inch forend,(although a 5 inch forend is used for this stock).

additional mid mounted barrel block minimises barrel vibration and proportionally increases stiffness.

Image

But, regardless of the applied theory/design/construction procedures etc, the final result needs to be the achievement of the required consistent accuracy levels.

cheers
dave g

pjifl
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Location: Innisfail, Far North QLD.

#60 Postby pjifl » Tue Jun 03, 2014 10:25 pm

That plywood stock is similar to many of my ideas.

I now believe most benefit comes from a very heavy, well anchored rear bag with the butt base taoered to fit tightly within the bag. Preferably with enough friction with the ground that it is not moved sideways by the torque. I have seem many superb shoots where it is moved each shot but the shooter has to work a lot harder repositioning and rechecking for cant variation.

Width of front contact gives more anti cant stability but seems to do less to control a recoiling rifle than the rear setup.

So a well fitting front rest seems less important to me.

Peter Smith


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