Recoil & Stock Design

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

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IanP
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#16 Postby IanP » Sun Jun 01, 2014 2:59 pm

I have revised the spreadsheet to include Free Recoil along with Torque so it gives a more complete overview of whats happening.

You dont have to know anything about maths to use the spreadsheet, just the load you use and the weight and dimension of your rifle. Download it and give it a go and let me know if its as easy as I think it is!

http://fclassdu.com/origin/wp-content/u ... ulator.xls

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

williada
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#17 Postby williada » Sun Jun 01, 2014 5:42 pm

Ian thanks for the comments and Danny too. Yes the mound slope is a major problem for duplicating a tune. Imagine if you are holding two garden hoses with slightly different pressures about a foot apart in the vertical with the low pressure on top like a positively compensating barrel. If you hold them straight ahead, the upper stream will cross the lower stream at some point. Let’s call this a compensation point. It’s like getting 10 super v’s. If you raise or lower your arms in parallel, that compensation point varies in distance. So by changing the angle on the mound you effectively change the compensation distance. Many range designers like to have the target about 5 degrees above the horizontal. So that angle is a good starting point for testing. Yet we know many ranges shoot down slope or upslope by more than that angle.

In the past, I conducted experiments with tall targets so I could explore the compensation effects of changing angle. Initially I worked on bullet free flight and charge to time bullet exit timing and used all sort to gizmos such as electronic Pressure Trace, and pressure calculator based on inertia that James Corbett lent me. What you need to do is measure the angle from the mound to the target. I did this with a small sextant or you could use a digital or manual angle tool for this purpose. For a series of angles you can adjust your load and use them for future reference. But the real advantage of an adjustable tuner comes into its own when dealing with changed mound angles for compensation purposes.

In case you are wondering why I rarely shoot these days it is because my mind has slowed for calculation purposes on the mound and I did not get to travel to England with Australian Palma team as the person who conducted the ballistic testing for them was because of ill health. At that time I was carrying a large brain tumour which was subsequently removed, followed by heart surgery and then prostate removal. Neither gun works (lol). The drugs then gave me thyroid toxicity. Since then my right eye has deteriorated because the surgery interfered with the eye and tear production and I have balance issues and suffer from deafness due to surgery. But remember a blind man can still play a piano. In no way do I seek sympathy but gain great satisfaction in passing on the knowledge I have gained. I have always been a person who has competed against himself by personal improvement and in that way I have never carried the burden of competition or the personality flaws of overly competitive people. Just happy to contribute where I can for the right reasons.

IanP
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#18 Postby IanP » Sun Jun 01, 2014 6:14 pm

Williada, Your knowledge in this sport is amazing and like me you have an inquiring mind and want to know the why and how of all things to do with it. Unlike me, you seem to be on top off the issues I'm just starting to explore. I look forward to your continued contributions!

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

DaveMc
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#19 Postby DaveMc » Mon Jun 02, 2014 5:20 am

Yes I will replicate the "Thanks Dave W" comment before proceeding. There are more than a few interesting points in there that I would love to discuss but will try and keep to the topic as much as possible.

Possibly very similar to your investigations we have studied compensation with a variety of modern rifle designs, stocks and actions, also using similar equipment including pressure trace etc and at short (down to 25m) and long range as well. Without going into too much detail and hijacking the thread here it seems that modern rifles are well suited to short range (3-500 yard) compensation over significant velocity ranges (have seen up to 100 fps). But to get a significantly large and fast enough upward excursion as velocity decreases (say 2 minutes over 80 fps) would take a fairly major redesign and significant experimentation. Whilst still in the back of our mind and discussions we quickly moved onto taking it for what it was - compensation in the right direction but only small - and worked on the other things to try and achieve long range accuracy - ultimately projectile consistency and low velocity variation. This way you can have a load that is almost perfectly compensated at 3-500 yards and still hold magnificent groups all the way through to 1000. It may well be the best of both worlds. Conversely, good compensation at 1000 would have too much vertical spread at 300 without good control of velocity variations anyway.

But onto topic - and this will come around to some of Davids other comments eventually.

Ian - The right left recoil you describe I believe has very little to do with torque but once again more about the recoil and resistive forces and directions. Draw a side view of your rifle and put recoil force vector arrow straight down the bore of the barrel towards the rear (as David pointed out this is not always completely true and with end muzzle vibrations will change slightly in direction but for purposes of this discussion it will do). Now put some resistive force arrows such as friction on both rests (base of forend and butt) going forward and shoulder position on buttpad going forward. Now draw a small circle around centre of gravity (somewhere in front of action and below bore line). All the resistive forces and inertia are below the recoil force and hence you get muzzle lift. As David also pointed out - this may not be a bad thing as expends some of the energy that would be otherwise directed into your shoulder.

Now draw a plan view. All forces should be straight backwards and forwards in centre of rifle and so should be the cg - at least initially (and offset stocks different of course). Also only if you have a nice square contact with shoulder and rifle. I believe initially the rifle recoils straight back but as your shoulder is pushed back, the angle of contact changes and the rifle butt plate starts to contact on the inside of your shoulder and the recoil forces are outside this line of contact (resistive force moves inward towards body) and rifle will rotate outwards - so up and out. However I think most of this happens well after the projectile leaves the barrel. Those that shoot a bipod will know if you straighten up your body (move leg position closer to centreline) you can change the recoil direction as well.

Yes torque is to the left (or anticlockwise if looking from the rear) and is the "reaction" to the action of spinning projectile to the right as mentioned in the other thread and earlier here. It is not an insignificant force and as you are calculating (typically around 7-10 inch lbs for a 7mm) over a short distance (distance from bore to LH corner of front rest - 1.7-1.8 inches) can create several lbs of thrust. Of course this is also resisted in rear rest and cheek and weight of outfit (inertia of weight outside centre line) as well. As pointed out by others it is the return bounce of this we see in the bags as a right rotation of rifle. I believe we see the bounce more than the initial torque because the weight has come off the front bags by then with muzzle lift during recoil.

NOW - before discussing stock design I guess you need to ask the question - what are you trying to achieve?

I assume at this stage that it is similar to myself:
1) Low torque roll
2) Low muzzle lift (at expense of felt recoil)??
Last edited by DaveMc on Mon Jun 02, 2014 8:43 am, edited 3 times in total.

DaveMc
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#20 Postby DaveMc » Mon Jun 02, 2014 5:50 am

On offset "dog tracker" stocks. I have had a couple of these and (Like Alan) have now gone back to conventional centerline stocks.

Whilst they do seem to help with torque I found a few issues. Firstly the weight is shifted significantly to the right (or stock to the left). This results in unbalanced bag settling (especially if stock has "rails"). I found myself constantly adjusting cant on the rest and unchecked could result in cant/accuracy issues.

Secondly (and this is less noticeable), the shift of front rest to the left results in off centre resistive forces in the front bag. Whilst potentially insignificant I do believe it makes it slightly harder to deal with tracking and balance.

Thirdly, although offsetting the stock does give extra leverage for front rest to counter torque it does not remove this force. The bounce back can be just as severe if there is muzzle lift associated as well.

They can work well but come with their own issues that need to be dealt with in design and balancing.
Last edited by DaveMc on Mon Jun 02, 2014 8:42 am, edited 2 times in total.

DannyS
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#21 Postby DannyS » Mon Jun 02, 2014 7:41 am

David , Ian and DaveMc, great posts, fantastic reading, keep it coming. =D> =D> =D>
Cheers
Danny

IanP
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#22 Postby IanP » Mon Jun 02, 2014 8:51 am

Dave, always grateful to receive your informed input and contribution. I'm sure you have considered recoil in some depth so look forward to reading more from you on this subject.

You ask the question on what am I wanting to achieve and I'll keep it brief and to the point. I am wanting to achieve consistent stability under recoil in two axis and weight my rifle/stock to best achieve it. I am wanting to use any means I can to limit the effects of recoil.

1. Find the best setup to limit fore and aft pivot/lift under recoil.
2. Find the best setup to limit torque effect.

To achieve this I want to evaluate weight distribution over the bags and consider the balance point fore and aft. Also consider if a low c of g will help counter torque reaction. What stock design will provide for balance and counter weight against torque? All this is to have the rifle recoiling back into my shoulder in a single direction without lift or torque. Is it possible under our rules and weight restriction for F-Class rifles? This is what I am wanting to explore. To understand the forces at work that need to be considered and prioritised in order to overcome or limit them.

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

bsouthernau
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#23 Postby bsouthernau » Mon Jun 02, 2014 9:42 am

My understanding - if you could call it that - is that there is nothing you can do about the actual torque being applied to the rifle. Conservation of angular momentum requires that in the absence of any external torques the rifle will attain an equal but opposite angular momentum to that of the bullet. As what we're seeking to do is minimise the rotational speed the course of action is to maximise the moment of inertia. As a general principle the way to do this is to increase the weight - but we're limited there - and have the weight distribution as far from the axis of rotation as possible.

The alternative is to devise a system of applying a countering torque to the rifle as the bullet passes down the bore. Best of luck!

Barry

DaveMc
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#24 Postby DaveMc » Mon Jun 02, 2014 10:21 am

My understanding is the same as yours Barry (although my terminology is often corrected) - so to slow the speed of torque roll and help gain "control" or lessen speed of impact on bags one way is to apply the weight (we are limited to) as far from the axis as possible but also balanced as much as possible.

Some examples - High scope (e.g. heavy NFBR on ultra high mounts), offset around cg of boreline by low stock weights in butt. Also a large diameter, short but heavy tuner on front. (You may need to look at barrel diameter and stock weight to get under weight)

To decrease muzzle lift (and possibly bounce from torque roll) though I try and build centre of gravity as high as possible and close to in line with bore (this can be done with an adjustable weight system of several holes bored in from buttplate and weight can be moved from one to the other - I use a cleaning rod down bore balanced on table at muzzle to look at cg. I also try and shoulder the recoil contact point up as high as possible and close to centre line for less muzzle lift (it is amazing what this does to front bag bounce). If free recoil then cg is important and also low friction on bags (heavy friction on bags may reduce felt recoil but add more resistance on a low point and create muzzle lift as well). If friction on front bag it will drive butt down into rear bag, if friction on rear bag (or low shoulder) it will cause extra muzzle lift.

I think the balance of weight on front and rear bags is hard to define. With long forends you can get away with less weight on front rest. Typically I start with the old 2/3 - say 6-kg (front) and 3 kg on rear and build up from there. I nearly always end up adding more weight to the rear though I also experiment a little with front and rear rest placement to see which shoots best on a new rifle (video helps here)

With better balance (vertical cg in line with bore) and lessened torque roll effect I believe the exact positioning of bags becomes less critical.

Have a look at Alan Frasers barrel height in his forend. Almost zero. This helps with torque "control" (and defined lightly) as well - so does high vertical sides in the front rest. In FS you can only go 1 inch (and not as heavy a requirement) but I believe higher in F open. My 30 cal rifle stock will have low barrel and high sides (and of course a good "sissy pad").
Last edited by DaveMc on Mon Jun 02, 2014 11:05 am, edited 3 times in total.

Barry Davies
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#25 Postby Barry Davies » Mon Jun 02, 2014 10:26 am

The demand for higher ballistics ( BC ) results in increasing the "unwanted " factors.
What you gain in one area you loose in another.

johnk
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#26 Postby johnk » Mon Jun 02, 2014 10:55 am

Dave Tooley claimed the deep butt "fin" on his stock design served to modify rotational moment, but without any particular mass to it, I wonder if it isn't accentuating muzzle flip.

DannyS
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#27 Postby DannyS » Mon Jun 02, 2014 12:11 pm

I'm currently using a McMillan F Class stock which as most would know have the barrel sitting quite low in the stock but it also has very low sides, so low that very little sideways support is provided by a 3 piece bag in a seb rest. In F Standard, I don't have much weight to play with, was thinking maybe aluminium angle attached under the fore end and extending an inch up either side would provide a better ride in the front bag.

Any ideas?

Cheers
Danny

IanP
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#28 Postby IanP » Mon Jun 02, 2014 4:06 pm

DannyS wrote:I'm currently using a McMillan F Class stock which as most would know have the barrel sitting quite low in the stock but it also has very low sides, so low that very little sideways support is provided by a 3 piece bag in a seb rest. In F Standard, I don't have much weight to play with, was thinking maybe aluminium angle attached under the fore end and extending an inch up either side would provide a better ride in the front bag.

Any ideas?

Cheers
Danny


The Seb Neo seems to perform better with 1" sides on the forestock so the angle or U plate sounds like a good idea to me. Sinclair sell a U plate for attachment to narrow and hunting forestocks for use on a 3" wide bag.

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

IanP
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Joined: Tue Jun 03, 2008 11:30 am
Location: Adelaide

#29 Postby IanP » Mon Jun 02, 2014 4:17 pm

Having a "heavy" weight a distance from the bore centre to counter torque makes sense. What I dont get is why anyone would want to place it high on top of the stock instead of low at the bottom of the stock.

An identical weight placed low on the stock the same distance from the bore centre as a top weight would provide a lower c of g and increased stability on the bags as opposed to a high placed weight. It would also oppose torque equally and have gravity working in its favour as well. What am I missing here?

I got rid of heavy top mounted scopes mainly to make weight with long heavy barrels, but the lower centre of gravity also appealed to me. Look forward to being put straight here on what laws of physics apply that make a heavy top weight more desirable than a heavy bottom weight.

Ian

PS Barry, what about a battery driven counter weight, (like vibrators use) to balance torque? :D
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A small ES is good. A small SD is better. A small group is best!

RAVEN
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#30 Postby RAVEN » Mon Jun 02, 2014 4:31 pm

What I look for in stock is
Inline design parallel with recoil
Low CG
Vertical sides on front shovel
Good overall balance
Good rest with adjustable side wings

Fairly simple really
RB


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