Recoil & Stock Design

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IanP
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Recoil & Stock Design

#1 Postby IanP » Sat May 31, 2014 11:14 am

This subject interests me greatly and I'm hoping to learn something as well as contribute what I have picked up along the way. I initially struggled with recoil and it required a total re-think on my gun handling and setup just to get back to shooting consistent groups.

How much recoil do we generate in our different classes of F-Class

All calcs performed using equations found here: http://en.wikipedia.org/wiki/Free_recoil

1. FS using 155gr bullets at 2960 fps gives 5.39 fps and 7.96 ft-lb

2. F T/R using 200gr bullets at 2700 fps gives 5.66 fps and 9.05 ft-lb

3. FO using 230gr bullets at 2850 fps gives 6.31 fps and 13.62 ft-lb

Note: The above figures are for comparison only as all use the same burn rate for the powder used. I have used powder quantities that I load with to achieve the velocities listed. Same burn rate means the recoil for the 230gr bullet is over stated as the 300WM case uses 74gr of powder for the velocity given.

I assume max weight for each class of rifle. Also note that the figures given assume free recoil, like a rifle would be suspended on string. The fps value is the speed the rifle would come back toward you and ft-lb value is the force of the recoil.

Subjects of Interest (stock design & setup)

1. High C of G Vs low C of G, (Centre of Gravity).

2. Measured weight over the front and rear bags.

3. Distance between the bags, (length of forestock).

4. Best places to add weight.

5. Offset stocks and counterweights.

6. Slope of mound.

7. How to take max advantage of the stock riding on the bags.

8. Define terms like torque, etc, so we all understand.

General Info

Its generally accepted that torque is a much smaller force in play when compared to recoil. Yet some shooters see the forestock lifting and twisting off the front bag under recoil and call this effect torque.

Interestingly the rifle lifts and twists to the RHS for right hand shooters and twists and lifts to the LHS for left hand shooters. Its actually muzzle lift under recoil deflecting the shooter's shoulder that causes this happen. Hence left and right is observed for right and left hand shooters.

RH rifle twist induces torque to the right so the right handed shooter does in fact have torque working against him as well but its mostly just the shoulder deforming under recoil. Solution is to position body so that the shoulder deflect causes the least resulting lift. Really important for F T/R shooters shooting off a bipod.

Ian
Last edited by IanP on Sun Jun 01, 2014 9:46 am, edited 4 times in total.
__________________________________________
A small ES is good. A small SD is better. A small group is best!

Longranger
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#2 Postby Longranger » Sat May 31, 2014 1:53 pm

Interesting topic Ian. My thoughts on stock design are now tending towards the Eliseo tube stock design. You can set these up so the thrust line is straight back, eliminating/reducing muzzle jump. The recoil pad can be pretty much be inline with the bore centre line.

The downside (if it really is one..) is that the scope will be fairly high in relation to the bore line. To me, that really isn't a problem and in some respects has advantages. Controlling torque may possibly be better managed using (gasp!) a bipod that is well designed.

I am still learning the ropes in this game and I am in the process of building a rifle specifically addressing recoil and torque issues. Proof will be in the results obtained, but I am confident it should work well, at least for F standard anyway.

williada
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#3 Postby williada » Sat May 31, 2014 10:49 pm

Getting the rifle to shoot accurately is a trade off between many factors. Shooting on our ranges has progressed from using rear locking actions to a myriad of gear in F Open. But some of the lessons from yesteryear are transferable as the targets shrink because we are searching for that edge as what is old is new again. Contrary to what a lot of shooters think the muzzle lift is your friend at long range where matches are won. For short and mid range the centre of recoil close to bore centreline and centre of gravity would have my vote.

A little history.
Rear locking actions such as a No.4 were renowned for compensating the spread of velocity by throwing slower projectiles higher, with the faster ones exiting the muzzle in a lower position. Due to gravity at long range they would intersect at a compensating range, nominally 1000 yards. At 300 yards some shot well but at the mid ranges the vertical was pronounced. We used to stiffen the action with a plate on top and attach the sight to it to get the No. 4 rifles to compensate at 600 yards. But then the longer ranges would suffer. While many people thought it was the springy action that just accentuated the upward thrust of the barrel, we knew that the fulcrum point on the stock was lower by virtue of design, than front locking actions and also contributed significantly to the upward lift of the barrel. From extensive barrel testing I did for a manufacturer in the 1980’s, the length and taper of the barrel also affected the amount of lift. Before Shultz and Larsen changed hands, they had the profile right for the .308W factory ADI round at about 2800 fps. Barrels of 1-14 were used with great success to 600 and 1-12 for the longs using the ADI factory 144 grn. I used to also adjust a butt plate to change the fulcrum point for a compensating range on my test range at home. But was aware different mounds affected the compensation because the position on the mound changed the recoil pattern on some ranges. So setting up on the mound is critical these days with scoped rifles. Like barrel tuners used today, we used to also move the sight blocks forward and aft utilising feeler gauges in .005” movements for appropriate distances or added a few shotgun pellets behind a grub screw in the front of the foresight block. This was well within the rules but not intended for display. The top shooters often prepared a short and a long range rifle. This practice correlates with ¼ and ½ turns on adjustable tuners in fine tuning today, aside from a fundamental weight designed to reduce the amplitude of the muzzle.

Could I also say that front locking actions like the Mausers could also produce the reverse effect with compensation and give negative compensation leading to greater spread as the range got longer but be magnificent in the mid ranges. I would also say that a neural compensation was desirable across all ranges but you would tend to lose shots at 6 o’clock with dips in muzzle velocity usually caused by crook primers or variance in batches of factory rounds. So our groups would aim at the high centre to play the law of percentages if you tuned for a neutral barrel. These tended to be barrels where the projectile left the muzzle at the top or bottom of the swing in amplitude. Whereas with a positively compensating barrel the slow projectile left the muzzle at a higher point than the faster projectile and vice versa for a negatively compensating barrel. These same tuning results can be utilised to increase the super centre count. The concept of nodal tuning and low sd’s do not guarantee tight groups at long range as we have all experienced, but the integration of positive or neutral compensation increases the probability of tighter groups at specific ranges.

Now we can reload, use tuners etc. and a wide range of components to lower the sd of velocity to achieve general accuracy goals which in my mind is equal to a neutrally compensating barrel. That being said, there is another factor to consider. I am of the opinion that the rifling imposes a torque where the centre of recoil is not necessarily in line with the centre of gravity as longer barrels can flap at the end like your wrist on a stiff forearm. The groups of these barrels tend to line in an arc if you change velocities. In the past, such barrels shot better when reduced in length. Now the best solution is a faster burning powder with reduced charge to move the peak of the pressure curve towards the breach. Slower burning powders tend to waggle that muzzle in a counter intuitive way. I agree that generally having the centre of recoil close to the centre of gravity will reduce muzzle lift and make bag handling easier. But the rearward thrust is greater with centres of gravity and recoil aligned. You certainly don’t want a cheek piece interfering with the recoil even if we can assume the projectile has left the barrel before it has recoiled backwards by 0.5”. With the big boomers it can make for a punishing day and vice versa for the little tackers. So if you are using a big boomer, I would prefer a little muzzle lift to ease the rearward recoil for comfortable shooting or use a “sissy” bag as they used to be called, to take the recoil seeing we cannot use muzzle brakes. It can be compared to riding a horse all day. You are not as sore at the end of the day, riding a horse that has a steeper croup (rump), than a flatter croup. It’s like comparing a jumper to a thoroughbred in terms of confirmation. It’s horses for courses. However, having some barrel lift in the vertical plane is good because advantage can be taken with positive compensation with the aid of a tuner to decrease the vertical and there is a tendency to use less windage because of the pattern that is thrown, particularly at long range rather than that round or triangular nodal group you see at short range in testing.

As far as gunsmithing tricks, I found that a ½ degree leed gave less barrel lift than a 1 ½ degree lead. The downside of the shallow leed was a poorer powder burn and I tended to jam projectiles in these leeds to overcome this problem. Any leed does not last much more than 600 rounds. Like the crown these things need to be nipped up. The amount of torque was considerably less with 1-15 barrel with a ½ degree lead for .308W 155grn. This setup was great to 500 yards. Of course the desired pattern of shots can be markedly be changed by indexing the barrel. Because most barrels exhibit a curvature over their total length, they have to be fitted with this barrel curvature in the vertical plane to take full advantage of compensation characteristics. Might I say that across the whole course a 1-13 twist for the .308W is the best compromise for twist rate. The bullet stability theory as per distance is the same for all calibres. Plenty has been written on it now. But people with more resources will specialize in gear for short, middle and long range. It seems to be the name of the game now. Just food for thought and discussion.

bsouthernau
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Re: Recoil & Stock Design

#4 Postby bsouthernau » Sat May 31, 2014 11:27 pm

IanP wrote:
Interestingly the rifle lifts and twists to the RHS for right hand shooters and twists and lifts to the LHS for left hand shooters. Its actually muzzle lift under recoil deflecting the shooter's shoulder that causes this happen. Hence left and right is observed for right and left hand shooters.

RH rifle twist induces torque to the right so the right handed shooter does in fact have torque working against him as well but its mostly just the shoulder deforming under recoil. Solution is to position body so that the shoulder deflect causes the least resulting lift. Really important for F T/R shooters shooting off a bipod.

Ian


Not sure if I've got the gist of what you're saying here Ian. Shooting left-handed off a bipod with utterly minimal contact with the rifle I find it settles several targets to the right of where it was aimed when it went bang. Is that in accord with what you were saying above?

Barry

ecomeat
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#5 Postby ecomeat » Sun Jun 01, 2014 6:35 am

Ian,
measured, or even felt recoil is obviously related to the weight of the rifle. What weight rifle was used in your calculations for recoil energy ? And are they all based on the individual weights of your own rifles ?
I am no rocket scientist, and I play with cows for a living, but I don't agree with you on your statement re right twist/right torque.
I used to think that was how it happened, and I remember posting my thoughts on this blog about three years ago, on a thread regarding offset stocks.
Someone (maybe Cam McEwan or Dave Mac ?) posted a photo of one of their offset stocks, and I basically asked the Question "why are you offset with the extra wood on the left, when the torque is clearly to the right ?"
It was pointed out to me that all right twist barrels torque left initially, and the right torque that I thought I was seeing was actually the secondary reaction, whereby I had missed the initial left torque, and what I was watching was the rifle twisting (bouncing) back to the right after it had initially twisted hard left.
So I started paying a lot more attention , including firing at home 100% free recoil.......and sure enough "they" were right.
My setup now is a 284Win in a McMillan F Class stock, with a SEB Neo front rest that has the one piece bag setup, along with a SEB Bigfoot rear bag.
With a 32" straight cylinder barrel, it weighs 9.950 kg and even though I am an absolute wimp with recoil, I don't feel a thing shooting prone. I do let it hit me, but like many others my shooting style is virtually/almost free recoil.
The rifle tracks straight back, with virtually zero muzzle lift and when I shoulder it forward to the front stop, it's almost always in the 5 ring at worst........ provided I spend the time to set it up properly and push the rifle down very hard into the rear bag.
Now I am left handed myself, but I have let a number of people....all right handed "normal" types, shoot my rifle. And the deep little indentation (heavy compaction) on the far left of the front bag is always there, no matter who shoots it.
I have never shot a left twist barrel, but I am certain in my own mind that you are wrong about Right twist barrels torquing right, for right handed shooters. I reckon they go left first, ......for anyone and everyone..... and it's the rifle bouncing off its initial left twist that you are saying is the initial right torque/twist.
Tony
Extreme accuracy and precision shooting at long range can be a very addictive pastime.

ecomeat
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#6 Postby ecomeat » Sun Jun 01, 2014 6:47 am

I just googled "offset stock rifle" and this article with a couple of photos on 6mmbr/accurate shooter came up http://bulletin.accurateshooter.com/200 ... er-rifles/
Extreme accuracy and precision shooting at long range can be a very addictive pastime.

AlanF
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#7 Postby AlanF » Sun Jun 01, 2014 7:29 am

Tony,

Yes all the offset stocks I've seen are to the left. So that's what I did. However I eventually dropped the idea because I couldn't get it to track straight back, so now I've opted for a VERY low profile centrally aligned fore-end, and it behaves and tracks well with most barrels. From my experience, some barrels are much easier to get to track well than others.

Ian,

Can you do some slow-mo video on the torque recoil thing? Maybe everyone's had this one wrong!

Dave W,

Another very thought provoking "article". Thank you.

Alan

jacksaligari
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#8 Postby jacksaligari » Sun Jun 01, 2014 7:36 am

I think tony it may have been Allan Nielson (Albow)that may have posted the photo

IanP
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Re: Recoil & Stock Design

#9 Postby IanP » Sun Jun 01, 2014 8:22 am

bsouthernau wrote:Not sure if I've got the gist of what you're saying here Ian. Shooting left-handed off a bipod with utterly minimal contact with the rifle I find it settles several targets to the right of where it was aimed when it went bang. Is that in accord with what you were saying above?
Barry


Barry, Maybe Steve Blair can explain it better, I am guilty of just trying to kick off the discussion with some interesting info that very well could prove to be wrong. http://forum.accurateshooter.com/index. ... sg36384319

Ecomeat, all calcs for recoil were done using max weight for classes. Thanks for the info, I'll look into the direction of torque some more but what you say could very well be 100% correct. Equal and opposite reaction suggest RH twist barrel should produce LH torque reaction.

I found this! http://www.sportingshootermag.com.au/ne ... be-changed

One thing I have never seen mentioned is the torque reaction imparted on the rifle by the rotation of the projectile created by the rifling twist. This force is proportional to the square of the angular velocity, which is considerable. A projectile traveling at say 3000fps in a barrel with a 1:10" righthand twist is rotating 3600 times a second clockwise. The torque reaction kicks the stock anticlockwise (to the left) into a righthand shooter's cheekbone which can be painful after a number of shots. I reckon a rifle manufacturer would have a marketing advantage if they negated this by putting a barrel with a lefthand twist on a righthanded rifle and a barrel with a righthand twist on a lefthanded rifle. The cost would be negligible and felt recoil would certainly be reduced. What do you think?

I'm learning already that the commonly held opinion that torque was to the right is wrong!

Alan, I'm hoping to be able to get some video of my rifle suspended and in recoil, to get a visual on the forces in play. This thread is about gathering some measured data and dispelling some myths on recoil and stock design/weighting. I dont pretend to know much about it and all I have is a memory of high school physics, but with research and help maybe we can get some hard facts on what works and why!

Ian
__________________________________________

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

IanP
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#10 Postby IanP » Sun Jun 01, 2014 10:19 am

Its interesting that there does not appear to be any single source for rifle recoil and static/dynamic stability. Maybe this thread can go further than any other in providing useful info for the effects of recoil on F-Class shooting.

I have found the equation for calculating rifle torque. (If there are errors please forward a correction)

M = (1.7x10^-9 x W x R^2 x n)/T

M = torque in lb-ft
W = bullet weight in grains
R = radius of bullet diameter in inches
n = rpm of bullet
T = time bullet is in barrel in seconds

Next is you need to know how to calc barrel time.

T = (barrel length / 12) / (muzzle velocity / 2)

Barrel length in inches
Muzzle velocity in fps

I'll put this in a spreadsheet later if anyone is interested.

Ian
__________________________________________

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

IanP
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#11 Postby IanP » Sun Jun 01, 2014 11:36 am

Ok I have just completed the Rifle Torque Calculator spreadsheet and you can download it!

Discovered my 230gr bullets generate just over 1.0 lb-ft of torque!

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

Ian
__________________________________________

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

bsouthernau
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#12 Postby bsouthernau » Sun Jun 01, 2014 12:03 pm

I'm interested to see that the average speed up the barrel is half the muzzle velocity. If you look at a chamber pressure curve you see that the bullet's acceleration is far from uniform. Is it a happy coincidence that it works out like this, a rule of thumb that's pretty close, empirically measured or something else? I hasten to add that I'm not saying it isn't so, just curious.

Similarly, n isn't constant either, do you use the value at the muzzle?

Barry

DannyS
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#13 Postby DannyS » Sun Jun 01, 2014 12:54 pm

Thanks David, a very interesting read.

Cheers
Danny

williada
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A few more thoughts

#14 Postby williada » Sun Jun 01, 2014 1:47 pm

Ian, it’s good to see someone with a passion to explore and refine ideas. Something new always comes from it. The results of experiments may well come down to a discussion on free recoil or firm control of the stock which I will take the liberty to comment on now.

On one hand free recoil has fewer variables to contend with and we will assume tighter groups with nodal tuning. Firm control of the stock introduces the human factor with more variables but is necessary to master for compensation tuning for the long ranges. For instance a prone shooter can vary muzzle velocity by approximately 20 fps between a tight hold and a loose hold. With a firm hold the energy increases the projectile velocity. The free recoil method tends to have less barrel lift with a lower velocity of the projectile, whereas the firm control has more barrel lift and calls for sound technique and shoulder position. The very reason people’s zero’s vary using the same rifle is not the eyesight difference but their mass and position on the mound and its influence on recoil and therefore muzzle lift. I also noted a fault in my own technique in pressing the rifle hard up against the forend stop on my front rest in order to muscle the rifle into position when aiming off. There is no substitute for setting up properly.


To anyone with the resources, you may like to measure the muzzle velocity with the rifle suspended freely and with varying degrees of butt pressure up to and past your calculated recoil. You may like to vary the butt fulcrum point to examine the muzzle lift as well. I was fortunate to do this from a machine rest.

You don’t need high speed photography to capture the muzzle lift angle but a picture would speak a thousand words. What I did in the past, at 25 yards was measure the height on the target of the shot to calculate the muzzle lift angle from a machine rest. (About 40 years ago I was inspired by Robin Fulton from England and conducted similar tests shooting prone following his experiments). I then extrapolated this using software to calculate intersections of trajectories at different distances. Don’t worry about group size or coning effects at 25 yards, if you go that way, it’s the relative differences you are looking for in trajectory due to muzzle angle and velocity. If you are worried about coning effects do this experiment at 140 yards. But we need to test short to reduce the effects of other variables such as wind, light and Magnus force. The added benefit of this exercise will tell you whether your rifle is performing with positive, neutral or negative compensation as previously discussed i.e. whether your projectile is exiting the muzzle on an upswing, neutral position or downswing relative to each other.

With the .308W I started proceedings for rough ball park calculations. I used the average factory velocity and loaded three rounds, one duplicating the factory velocity, one loaded a grain below the factory velocity and one loaded 1 grain above factory velocity. Of course recent factory rounds are a bit hotter, so you have to work up for safety for that one loaded over factory specs and look for pressure signs. You can then start refining this exercise around known nodal points or start playing with rounds loaded with 30 fps average differences. In doing this, you can anticipate where you may lose shots vertically at specific ranges. These strategies were refined at 1000 yards using a machine rest a purpose built concrete bench at Rosedale. If you would not take a knife to a gunfight why would you take a negatively compensating rifle to a long range shoot? They may be on a node at one range but changes in air density have a way upsetting things when you least expect it.

There is a trend to use bigger calibres at long range to buck the wind which call for a firm hold for comfort and have the rifle positioned for the next shot as quickly as possible. The ability to shoot these well will come from excerpting the same butt pressure and same fulcrum point with your shoulder for constant velocity; and secondly for consistent trajectory due to muzzle lift. This applies in the paddock too, funny how we call the target sport, “field class”. I know that from head shooting foxes at long range at home I have had better results leaning into a Harris bipod.

In the end, as the targets are small enough now and rifles are so accurate there is greater propensity to improve your score with the ability to read conditions.

IanP
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#15 Postby IanP » Sun Jun 01, 2014 2:55 pm

bsouthernau wrote:I'm interested to see that the average speed up the barrel is half the muzzle velocity. If you look at a chamber pressure curve you see that the bullet's acceleration is far from uniform. Is it a happy coincidence that it works out like this, a rule of thumb that's pretty close, empirically measured or something else? I hasten to add that I'm not saying it isn't so, just curious.

Similarly, n isn't constant either, do you use the value at the muzzle?

Barry


Barry, thanks for your observation and you are right on the money, (from my limited perspective) that its a value thats pretty close. The same can be said for the free recoil calculations that I do with one burn rate for the powder charge.

Any ideas for an improved equation/formula would be much appreciated. If we can improve on what I have found so much the better.

Williada, I appreciate your comments very much and the measured angle achieved from free recoil was a very simple and elegant solution that I would not have come up with by myself.

Your comments about consistent butt pressure and fulcrum point are spot on!

One thing that also has an effect on the outcome is the slope of the mound and the need to have a level aim at the target. I think its pretty important not to shoot on an excessive angle, up or down to the target. In line recoil is the consistent way to go if at all possible.

Ian
__________________________________________

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


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