Recoil & Stock Design

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

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DenisA
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#31 Postby DenisA » Mon Jun 02, 2014 4:34 pm

I trialled a stock that I was building a while ago (there's a thread in the pictures section). The side were square and at a guess probably a 1/2 inch high.
One of the characteristics that I didn't like about the square side fore end in my NEO (3 piece bag) was that as the rifle torqued under freecoil, the square sides grabbed the bags and jerked the whole rest each time. I don't like my front rest to move at all.
That stock torqued excessively anyway, but it highlighted the characteristic.
Having the tapered forend sides means that the heel side can rotate toward the side bag and the toe side can rotate away from the other side bag without interference.

I know that we don't want torque, but when a square plugs sits firmly in a square hole, the slightest torque will transfer.

bsouthernau
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#32 Postby bsouthernau » Mon Jun 02, 2014 4:59 pm

IanP wrote: 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


My guess - and I'm also quite amenable to being set straight on the matter - is that if the rifle has to rotate at all it would be best if it did so about the axis of the bore. In isolation a high weight would be no more or less desirable than low one but a pair of them would combine to achieve that effect. Sound plausible?

Not sure what style of vibrator you refer to here but think I see how you envisage it working. Switching it on will be the tricky bit because we only want to apply the counter torque as the bullet travels down the bore. We can turn it off at our leisure as the shot travels downrange. :lol:

Barry

IanP
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#33 Postby IanP » Mon Jun 02, 2014 5:23 pm

Using a video to help work out best rifle balance/stability.

I can take high speed video, (relatively speaking) using a Panasonic FZ200 at 240 frames per second in VGA mode. When you consider a recoil velocity of 6 fps then that should give me a capture rate of 0.3" of riflestock travel per frame. Good enough to get some idea of what the recoil reaction will look like with the stock weighted and supported in different positions.

Here is the outline for the basic tests I am considering. Please advise if this is reasonable or if it could be conducted better in a different setup. I am open to any helpful advise!

1. Support the complete rifle from two nylon fishing line "strings" at points where the rifle would normally be in contact with the bags.

2. The attachment lines would be supported vertically over the rifle.

3. The rifle would be free to move in any direction while supported by the strings.

4. Attachments to the rifle would be wide belt like slings. No wider than the bags that the rifle would normally ride on. These would be attached tightly and taped with a ring attached at the top for the nylon to attach to.

5. A string would be attached to the trigger and require a force of 6 ozs to release.

6. A full target load would be fired and the recoil recorded on video.

This exercise would be repeated using different attachment points to mimic different bag distance setups. Also weight could be taped in place to alter balance and c of g.

The video would be taken from elevation, (side on) and end elevation from behind the rifle butt in an attempt to capture both recoil and torque. This would require 2 videos to be taken of the same load and setup as I dont have 2 cameras available to do it simultaneously.

Comments please!

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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#34 Postby DaveMc » Mon Jun 02, 2014 7:01 pm

IanP wrote: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

:D


As Barry pointed out we are trying to get the centre of mass in line with the bore with a top weight to counterbalance the stock weight - These should be symmetrical for both torque and recoil initiated barrel lift (and also has a dramatic effect on barrel vibration and initial impulse). - Having a single offset weight will cause the recoil and torque forces to pivot around the centre of mass. Ignore the rests and shoulder for a while and imagine setting off the rifle in space (no gravity). Lets also ignore torque for a bit. The recoil direction is straight along bore but if the centre of mass (inertia) is below the boreline then we create an axial moment raising the muzzle and lowering the stock butt. The rifle will head out across space but start to flip end over end in its journey.

Similarly for torque - if an offset centre of mass will want to shift the bore sideways instead of spinning on its axis.

Add resistive forces such as friction on rests and shoulder all below bore centerline and you end up with all forces encouraging muzzle lift (apart from gravity holding it down). A centre of gravity above boreline will drive muzzle down.

I don't particularly like the idea of setting a full blown round off whilst suspended on fishing line. it is not really floating in space and still has suspension points similar to a rest but these suspensions will pivot and pull -causing a pendulum effect quite different to the friction forces of a rest. It also sounds potentially hairy. I think the high speed video on normal rest will show enough to help and will include the other frictional forces at play. If you want to remove the side supports and ears of rear bag it might be interesting. If you get really keen you could create a round front stock and rest to see it rotating freely.

RAVEN
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#35 Postby RAVEN » Mon Jun 02, 2014 7:06 pm

Dave I would use braid less stretch :lol:

DannyS
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#36 Postby DannyS » Mon Jun 02, 2014 7:38 pm

Still sounds a bit hairy. As an ex watchmaker, I also worry about the pendulum effect.

Cheers
Danny

Brad Y
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#37 Postby Brad Y » Mon Jun 02, 2014 8:26 pm

Just get a skinny round profiled stock in a hunter weight rifle in say 30-06 and fire it off a normal f class rest. Shot one off a bench with one hand under the butt and the other pulling the trigger like I shoot my 6 dasher. PRESTO you have torque, recoil and jump all at once.

IanP
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#38 Postby IanP » Mon Jun 02, 2014 8:27 pm

Dave, interesting observations and much appreciated. Not overly concerned about the pendulum effect as only want to see the initial reaction of recoil and torque overcoming the rifle inertia. The nylon could be attached to free travelling rings sliding on a rod above the suspended rifle.

An easy test for balanced weighting of a rifle around the bore axis would be to place an axle thru the bore and action and see how it rotates. Like checking a bicycle wheel and watching the valve and stem settle to the bottom under gravity. Still does not take into account stability riding on the bags where a low c of g and bottom weight may assist.

Here is a video of a rifle setup to behave in a recoiless state. Thanks to Range Rector for sending me the link.

http://www.youtube.com/watch?v=3YUTfoxX_Cc

Ian
Last edited by IanP on Mon Jun 02, 2014 8:42 pm, edited 1 time in total.
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A small ES is good. A small SD is better. A small group is best!

IanP
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#39 Postby IanP » Mon Jun 02, 2014 8:42 pm

Brad Y wrote:Just get a skinny round profiled stock in a hunter weight rifle in say 30-06 and fire it off a normal f class rest. Shot one off a bench with one hand under the butt and the other pulling the trigger like I shoot my 6 dasher. PRESTO you have torque, recoil and jump all at once.


Brad, both you and Dave are probably right in suggesting an easy first step to take in having a look at recoil at 240 frames/s video. I'll give it a go and see if I can see the forces at work.

Ideally I want to have a weight on a rail at the bottom of the stock and move it along for as much of the rifle length as possible. I want to see what effect it has on the rifle riding on the bags under recoil.

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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#40 Postby IanP » Mon Jun 02, 2014 8:52 pm

Want to take your own slow motion (high speed) videos?
Look here: http://bestslowmotioncamera1.blogspot.com.au/

Ian
__________________________________________

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

Cameron Mc
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#41 Postby Cameron Mc » Mon Jun 02, 2014 10:57 pm

Only just read this thread. This is the stock Alan N made for me. He shoots long range BR and now F Class. I tried it with a Pierce Stainless action and barrel in 284 Shehane. It controlled torque to a certain point. I went back to my trusty MBR Tooley stocks as at that time the World Champs were coming up. It is now sitting in the cupboard as a stock only.
Cam

Image
Image
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RAVEN
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#42 Postby RAVEN » Mon Jun 02, 2014 11:39 pm

Looks like a 59 caddi :)

Cameron Mc
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#43 Postby Cameron Mc » Mon Jun 02, 2014 11:42 pm

RAVEN wrote:Looks like a 59 caddi :)


Good one Richard 8)

williada
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More torquin

#44 Postby williada » Tue Jun 03, 2014 1:53 am

I would like to consider where we need to apply the knowledge gained from tests in relation to stock design which is attempting to work with recoil. The recoil itself is an energy concept and for us old guys it is measured in foot pounds.

Total recoil is formed by three waves. It is based on opposing forces when the powder turns to gas and accelerates, where the bullet accelerates from rest to muzzle velocity and where the gas escapes at the muzzle. Equal backward forces occur in the chamber, grips, butt etc. and because the rifle is hundreds of times heavier than the bullet it accelerates. In terms of F Class shooting, the kick from the recoil is the least from the maximum weight of the rifle allowed by the rules.

Of concern to us as target shooters is how deal with each of these waves. Firstly, with waves one and two, we absorb recoil while the bullet is in the barrel and anything we do that does not absorb the recoil the same way each time will change the point of impact of the bullet. The rifle will recoil upwards because the shoulder is lower than the line of force (as DaveMc has explained), which is the barrel; and to a lesser degree with free recoil techniques as the stock still has weight below the barrel line of force. (Hence Barry Southern’s explanation for an equalising weight on top to reduce muzzle jump). DaveMc has already explained the merits of the high fulcrum point and muzzle lift.

The third recoil wave, the escaping gas will not determine the impact of the shot to any large degree (except for crook crown or an Obermeyer tube at the muzzle) because Elvis has left the building but it will affect how well you can recover as the rifle is still recoiling because of it, to place the next shot quickly. In a 30 calibre, a bullet is gone in about ½ inch or rearward barrel movement.

Torque is a different concept. The rifle twisting as a counter force induced by rifling, is prevented from free rotation by your head on the stock, length of pull, hand grip, fore end on the front rest etc.

It is my belief that control of muzzle jump has a greater impact on accuracy than control of torque not because we do not shoot recoilless rifles but because the shooter, the rest and the rifle absorb torque and there is not much you can do about it. As Barry Davies said, “What you gain in one area, you lose in another”. Particularly, with big boomers and high BC’s.

Firstly, the shooter as I was taught by Percey Pavey (who I used to drive to Williamstown Rifle Range on many occasions in his later years and who was part of my development as with many others) said that you must control the rifle torque with your big and ring fingers. He liked a firm, consistent grip to control torque but the back of his palm did not interfere with the recoil to his shoulder. Like bench rest stocks, his stocks did not have palm or thumb hollows which can interfere with recoil to the shoulder when your concentration lapses. For those who remember him, he shot with a high prone position and this presented a squarer more consistent angle to the butt. This is what DaveMc talked about with shoulder position. With a free recoil setup you have to assume your setup will be the same for each shot, but it will not work for the big boomers. The Pavey grip will.

Secondly, the torque can absorbed better by a maximum width front rest. But consider whether the Pavey grip would be better combined with a slick front rest as wider rests can grab and snag. A lot would depend on the positioning of the front rest in relation to rifle balance too. Consider whether the breach is closer to your rear hand as the recoil starts there. Your length of pull and body shape determine this. Like a close flag, it will have a big influence on angular displacement.

Thirdly, the greater the mass of the rifle the greater it will absorb the torque. We really can’t do much about torque because we are governed by the maximum weight in the rules and practicality. Take a rifle that weighs 17 pounds and a bullet that weighs 142 grains. So 142/7000 gives 0.020 pounds. Then the rifle weighs 838 times more than the bullet. This is a small amount of leverage that the bullet can apply to the rifle.

Finally, the outcome of the applied action of the torque is the revolutions per minute of the projectile where: R=(12/T)*V*60. e.g. Projectile travelling at 3000 fps in a 1 in ten barrel does 216,000 rpm. A projectile travelling at 2975 fps in 1 in 13 barrels does 164,769 rpm. So maybe calibre selection is more vital in controlling the amount of torque. The last thing I would say about projectile rpm is that its decay is insignificant in terms of ballistic performance.

On the other hand the most consideration has to be given to muzzle jump for both nodal tuning at the short range and mid ranges; and for minor compensation tuning at the longs given that we have pretty good control over velocity spreads with reloading. Without going into too much that I have already discussed in this thread, the benefits of a little muzzle jump are to absorb recoil of bigger calibres and maintain elevation where atmospheric conditions interfere with velocity spreads suggests it is of more importance than controlling torque not just because of rifle handling but ballistic performance downrange.

Fix the muzzle jump first, then concentrate on technique and reading conditions.

DaveMc
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#45 Postby DaveMc » Tue Jun 03, 2014 7:13 am

Another nice post Dave W and it seems an appropriate time to reiterate a point or two to follow that.

1) as far as the right bounce back off the blocks with torque - I would not worry about this too much either - indeed Elvis has left the building for that as well (if you think about it - it actually occurs when the load is relieved as bullet exits muzzle). The only thing these after effects have an impact on is your follow through.

2) Controlling muzzle lift will dramatically decrease this torque bounce anyway as stated before - in fact I have had stocks that jumped like this under free recoil but a little high shoulder put into them and they stay on the bag nicely and track well enough to see bullet flight. This is because gravity is allowed to do the work by holding down in front rest.

I also saw a post or two about wanting a low centre of gravity in the stock. I would like to make a point that I believe this is not contradictory to a high centre of gravity with relation to bore (it actually achieves a similar result). 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: ) like the trend in some BR rifles (e.g. edge type br stocks http://www.mcmfamily.com/mcmillan-stock ... t.php#edge). This effectively is achieving the same thing by bringing bore down. The difference between centre of gravity and boreline is greatly reduced (ie not much stock below so cg close to bore). ie compared to bore you have actually raised the cg.

There was a great barrel vibration simulator attached to the border barrels site that seems to have been removed but you could change barrel profiles (including front weights etc) and cg and watch effect on barrel vibrations - was very interesting re that first upward impulse with low cg relative to bore.


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