Thread: The Maths of Recoil

Originally Posted by grandpamac

Yes. Recoil increases in proportion to projectile mass but in proportion to the square of velocity. Those slugs are going a lot faster.
Grandpamac.

I'm not singling you out. I'm referring to comments like the above that are incorrect, because clearly it is the velocity of the projectile/s that are being referred to.

If Isaac Newton were to turn in his grave would he think about which way first?

While we have recoil at one end of the firearm and the momentum of the projectile at the other, the effect of the projectile on game is not as simple as the amount of momentum nor is trajectory @Puffin. In my experience a sub sonic projectile with a given momentum will not kill as quickly as a high velocity smaller projectile with the same momentum despite laws of physics

Greetings All,
My apologies to any who felt mislead by my posts. In retrospect I should have taken the time to get the differences between momentum and kinetic energy straight in my head before starting. The point of my posts was to, in general terms, show that the powder charge plays an important part in the recoil we feel and in answer to Micky's question why short barrel rifles boot more than longer barrel rifles with the same ballistics. I tried to keep away from formula to keep things simple to understand but did not do a particularly good job of that. Hopefully most of us have still learnt something in the process, I know that I have.
Regards Grandpamac

Greetings All,
On the subject of recoil I remembered a test done, years ago, to debunk the stories of deer being knocked flat by powerful modern rifles. The test was done using a wooden deer (a box filled with sand on three legs) and a .303 calibre rifle. Shots were fired into the box, from close range, and the amount the "deer" was rocked back by the impact recorded. It was not rocked back all that much. Placing the but of the rifle against the box and pinching the trigger against the trigger guard to fire it resulted in about twice as much movement of the "deer". This was likely published in Outdoor at the time and later in "The Sharp Shooter" published by Bruce Grant in the 70's. The original testing was done by Matt Grant ( Bruce's father) and others in 1947. We have not really come all that far have we?
Regards Grandpamac.

@grandpamac
Chapter 15 - Knocking Them Flat
I'd post an image of the "deer" but photos aren't uploading lately.

I think the shorter barrel length producing more recoil is also easily explained using Newtons second law, F=ma which is force = mass x acceleration.

If mass (bullet weight) is constant and MV is the same across barrel lengths a short barrel will need greater acceleration to achieve the same MV as a long barrel. Hence, force increases with the increased acceleration of a short barrel which results in more recoil

Sent from my SM-G975F using Tapatalk

Originally Posted by zimmer

@grandpamac
Chapter 15 - Knocking Them Flat
I'd post an image of the "deer" but photos aren't uploading lately.

The Sharp Shooter is a great book. The chapters on killing shots on deer should be required reading today. I don't know how many deer Matt shot in his skin hunting days but it must have been heaps.
Grandpamac.

funny as heck..I was just reading thread and thought of that very thing ...then saw it in next post LOL.....didnt they do cricket ball thrown too...and it rocked "deer" more than projectile????

one thing is for sure and for certain...the suppressor has changed the whole ball game COMPLETELY......and for the better too.
I used to flinch...and down loaded my main rifle because of it..suppressed it.now she back up to full power/velocity and flinch...what flinch???

Originally Posted by Moa Hunter

While we have recoil at one end of the firearm and the momentum of the projectile at the other, the effect of the projectile on game is not as simple as the amount of momentum nor is trajectory @Puffin. In my experience a sub sonic projectile with a given momentum will not kill as quickly as a high velocity smaller projectile with the same momentum despite laws of physics

I think that's to do with the hydraulic shock more than anythy . Not sure on the science though but no doubt someone here will know.
From my experience with my 338 lap mag and his 338 edge the science doesn't always work the way it should when it comes to PERCEIVED recoil.
I have a lightweight Hardy 338 lap mag...carbon everything...around 4.5 kilo with scope and suppressor.
The 338 edge has heavy stainless barrel, laminate wood stock, slightly longer barrel and muzzle brake and extra metal added to stock etc as runners as it's used solely for long range shooting and competition. Mine is a hunting rifle. He shoots 300gr versus my 250gr at near enough the same velocity. It must weight a good 2 or 3 kilos more than mine. On paper with all that extra weight and muzzle brake it should be significantly nicer than mine to shoot recoil wise. Hell no... I am happy to shoot mine all day but I won't shoot his using any limp excuse I can come up with.
I think the brake alone is a large part of this. Again on paper it should be more effective at reducing felt recoil But in my opinion it increases Perceived recoil immensely. The blast noise is huge and with it blowing the scenery back in your face only increases the sensation. It also has a lot more solid butt pad. Mine is very spongey and absorbs a considerable amount of recoil. Also the blast noise is obviously significantly lower with the suppressor and it doesn't blow back at you which reduces the Perceived recoil considerably.
So from my experience the math only tells half the story regarding actual recoil and perceived recoil. My lightweight 338 lap is a pleasure to shoot in comparison to his rifle but going by the math on paper it should be the other way around by a considerable margin.

Don’t forget get to add in trigger pull in your maths and finger weight.

[QUOTE=Moa Hunter;. In my experience a sub sonic projectile with a given momentum will not kill as quickly as a high velocity smaller projectile with the same momentum despite laws of physics[/QUOTE]


Ever tried a 45/70 shooting subs? They leave a pretty big hole and a quick death , especially using expanding rounds like the new Hornady offering.

Originally Posted by Moa Hunter

While we have recoil at one end of the firearm and the momentum of the projectile at the other, the effect of the projectile on game is not as simple as the amount of momentum nor is trajectory @Puffin. In my experience a sub sonic projectile with a given momentum will not kill as quickly as a high velocity smaller projectile with the same momentum despite laws of physics

I shoot a lot of animals with 30 cal sub and projectile choice makes a huge difference in quickness of kills for example the Lehigh Defence controlled fracturing with its petals sharing off in multiple directions resulting in more damage and had a few instance bang flops with them on a hilar shot. Shot a red the other day and only went 15m on a broad side lung shot, they work a lot quicker than a standard mushroom design projectiles.

Muzzle energy generally gives a pretty good indication of overall recoil. So if we take two identical rifles, but different calibres - the one with more muzzle energy should have more recoil.
Now that's out of the way, the cartridges ability to rapidly accelerate the rifle (recoil) can be reduced by making the rifle heavier, and/or with the use of a muzzle brake.

Now we have arrived at a base amount of recoil that is directly related to the muzzle energy and firearm weight, (with potentially a muzzle brake thrown in as a curve ball) - how that recoil is transferred to the shooter can be manipulated via stock design, fancy recoil pads and other technologies.

https://www.jbmballistics.com/cgi-bin/jbmrecoil-5.1.cgi

I have spent a while playing around with this calculator, which has powder weight as one of the fields you enter.

For example
7Lb Rifle, 165gr projectile, 2800fps MV, 41gr charge = Recoil Velocity:13.5 ft/s Recoil Energy:19.9 ft-lbs Recoil Impulse:2.9 lb-s

If you somehow managed to achieve the same MV with a 30gr charge (with all else being the same) your numbers would be: Recoil Velocity:12.5 ft/s Recoil Energy:16.9 ft-lbs Recoil Impulse:2.7 lb-s

Not an insignificant difference.