Can anyone help please?

[sage 100]

Can anyone help me please - I have reason to answer the following question.

 

If an 8lb competition shotgun, 12 gauge, fires a 28 gram cartridge with the gun in a parallel position, what is the g force experienced when the trigger is pulled? The reason for this question is an accelerometer is being fitted to the gun as a form of switch to activate a procedure in my coaching program. The accelerometer is range rated, for example, Type 1 would be from 5-10 forces of G, Type 2 would be 10-15 and so on. I genuinely have no idea where to indicate we first start, so a learned answer would assist me greatly, save the expenditure of purchasing unwanted pieces of kit, so a starting point would be a great asset. I am fully aware of all the variables - angle of the gun, type of cartridge, resistance offered by various size shooters, climate and many others, thankfully the product is not that exacting that an exact number has to be achieved, but a good starting point would be splendid. I have spoken with two cartridge manufacturers, the proof house and a number of learned people from within the gun and cartridge industry and, surprisingly, I still have no answer.

 

Thanks chaps

[semiautolee]

still thinking dennis.... bare with me

[semiautolee]

no its out done me iv searched the net everywhere and no answers i think you will have to be the first person to put it to the test dennis

[wymberley]

still thinking dennis.... bare with me

Sage,

Beginning to worry about the company you keep and what goes on up north where you are. Surely, you're a bit too old for "I'll show you mine if you show me yours". :o

[semiautolee]

Sage,

Beginning to worry about the company you keep and what goes on up north where you are. Surely, you're a bit too old for "I'll show you mine if you show me yours". :o

 

 

thats for us to no

[Kes]

This a difficult one, as calculating the g force involves widely differing units, ft/sec, grams, lbs, and gravitational force.

Rather than calculate it I would hazard a guess at 3-5g. Racing drivers regularly experience up to 5g on cornering at high speed, close to the limit of normal endurance.

Less than 3g would not bruise your shoulder after 100 shots so, hence my guess. Be interesting to know what the average carts recoil works out at in an 8 lb gun!!!!

[semiautolee]

This a difficult one, as calculating the g force involves widely differing units, ft/sec, grams, lbs, and gravitational force.

Rather than calculate it I would hazard a guess at 3-5g. Racing drivers regularly experience up to 5g on cornering at high speed, close to the limit of normal endurance.

Less than 3g would not bruise your shoulder after 100 shots so, hence my guess. Be interesting to know what the average carts recoil works out at in an 8 lb gun!!!!

 

 

kes i think you could be quite close there.......i was thinking maybe 2-4g it is a hard one

[wymberley]

The only people that I know of that accelerate people with explosives are Martin Baker of ejection seat fame; based in Bucks. May be worth a chat.

Assuming the 8 lb lump has two barrels and one trigger, it's a pity that you can't utilise the second barrel "arming" (recoil), remove that barrel trigger mechanism so the barrel then "fires" immediately without the need to pull the trigger and somehow provide a signal to your equipment. Just a simple inertia switch in other words.

[sage 100]

The only people that I know of that accelerate people with explosives are Martin Baker of ejection seat fame; based in Bucks. May be worth a chat.

Assuming the 8 lb lump has two barrels and one trigger, it's a pity that you can't utilise the second barrel "arming" (recoil), remove that barrel trigger mechanism so the barrel then "fires" immediately without the need to pull the trigger and somehow provide a signal to your equipment. Just a simple inertia switch in other words.

Thanks for that I will look up Martin Got to find an answer ASAP.

[HW682]

I'll have a go at some basic calculations to get a rough starting point. I will show the working out so anyone can check the calculations (it is getting a bit late)and also anyone can change any of the assumptions made.

 

Imagine the gun suspended by thin wires, like those ball bearings in the executive desk toy, so that it free to move. Make the usual schoolboy assumptions of no friction, all parts are incompressible etc.

 

It should be fairly easy to calculate the force on the cartridge load. We then assume the same (equal magnitude but opposite direction) force on the gun and using F=ma we get a figure for acceleration of the gun.

 

mass of load= 0.028Kg, what is acceleration?

 

Assume it accelerates uniformly to 1350 fps in the first 18 inches of the barrel.

 

From v2 = u2 + 2as, re-arrange to get a= (v2 - u2)/2.s

 

putting in u=o m/s; s= 0.457m; v=411 m/s gives a=(411*411)/2.0.457 = 185,166 m/s2

 

Force on cartridge load, F = m.a = 0.028 * 185,166 = 5185N

 

 

 

Assuming the same force is applied to the gun, it will accelerate backwards at:

a= F/m = 5185N/3.6Kg = 1440 m/s2.

 

Dividing by g=9.8m/s2 gives acceleration of 147g.

 

This is for the gun suspended as described. Any friction of the load up the barrels will reduce this, also compression or moving of gun components will reduce the acceleration of the whole gun backwards (semi-autos in particular have many moving parts which "soak up the recoil").

 

To take this further is going to quickly lose accuracy, but for an example lets assume that the gun is mounted to the shooter and also has to accelerate the part of the shooters body.

If the gun + shooter was 80lb, then as a first approximation the acceleration would be 1/10 of original so 14.7g.

 

(Obviously most shooters weigh considerably more than 74lb, but then again the gun doesn't have to move all the body backwards, only the top bit. Also the human body is anything but incompressible. And it would depend how tightly it was held etc). This is the bit that is guess work.

 

 

Having said all that, if you are only using the sensor as a switch, then consider using the smallest range you mentioned and it will give a full scale output when the gun fires.

 

 

Hope this helps

[sage 100]

I'll have a go at some basic calculations to get a rough starting point. I will show the working out so anyone can check the calculations (it is getting a bit late)and also anyone can change any of the assumptions made.

 

Imagine the gun suspended by thin wires, like those ball bearings in the executive desk toy, so that it free to move. Make the usual schoolboy assumptions of no friction, all parts are incompressible etc.

 

It should be fairly easy to calculate the force on the cartridge load. We then assume the same (equal magnitude but opposite direction) force on the gun and using F=ma we get a figure for acceleration of the gun.

 

mass of load= 0.028Kg, what is acceleration?

 

Assume it accelerates uniformly to 1350 fps in the first 18 inches of the barrel.

 

From v2 = u2 + 2as, re-arrange to get a= (v2 - u2)/2.s

 

putting in u=o m/s; s= 0.457m; v=411 m/s gives a=(411*411)/2.0.457 = 185,166 m/s2

 

Force on cartridge load, F = m.a = 0.028 * 185,166 = 5185N

 

 

 

Assuming the same force is applied to the gun, it will accelerate backwards at:

a= F/m = 5185N/3.6Kg = 1440 m/s2.

 

Dividing by g=9.8m/s2 gives acceleration of 147g.

 

This is for the gun suspended as described. Any friction of the load up the barrels will reduce this, also compression or moving of gun components will reduce the acceleration of the whole gun backwards (semi-autos in particular have many moving parts which "soak up the recoil").

 

To take this further is going to quickly lose accuracy, but for an example lets assume that the gun is mounted to the shooter and also has to accelerate the part of the shooters body.

If the gun + shooter was 80lb, then as a first approximation the acceleration would be 1/10 of original so 14.7g.

 

(Obviously most shooters weigh considerably more than 74lb, but then again the gun doesn't have to move all the body backwards, only the top bit. Also the human body is anything but incompressible. And it would depend how tightly it was held etc). This is the bit that is guess work.

 

 

Having said all that, if you are only using the sensor as a switch, then consider using the smallest range you mentioned and it will give a full scale output when the gun fires.

 

 

Hope this helps

THANK YOU SO MUCH.

Thank you for your time and effort. will PM you

[HW682]

THANK YOU SO MUCH.

Thank you for your time and effort. will PM you

 

No problem. The unrestrained gun problem is fairly straightforward (although the calcs could do with a check thru by a fresh pair of eyes.). How the acceleration of the gun changes once mounted by the shooter is harder to work out- at least for me.

 

Hope it gets you started at least.

[Spara Dritto]

HW682... wow.... Good effort. I take it your job lies somewhere is mechanical engineering or mathematics?

Edited May 9, 2011 by Beretta Italy

[Kes]

Whilst not precisely analagous, this ref does suggest 147 may be slightly high. Admittedly the max tolerance levels could be exceeded by a very short period force but these tests do test G force.

 

"The record for peak experimental horizontal g-force tolerance is held by acceleration pioneer John Stapp, in a series of rocket sled deceleration experiments culminating in a late 1954 test in which he was stopped in a little over a second from a land speed of Mach 0.9. He survived a peak "eyeballs-out" force of 46.2 times the force of gravity, and more than 25 g for 1.1 sec, proving that the human body is capable of this. Stapp lived another 45 years to age 89, but suffered lifelong damage to his vision from this last test.

 

Watch that recoil !!!

Cheers

[HW682]

HW682... wow.... Good effort. I take it your job lies somewhere is mechanical engineering or mathematics?

Graduated in Applied Physics / Electronics many moons ago. My job does involve Mechanical & Electrical Engineering. From what I remember, the simplified problem is about typical of A level Applied maths questions.

 

Whilst not precisely analagous, this ref does suggest 147 may be slightly high. Admittedly the max tolerance levels could be exceeded by a very short period force but these tests do test G force.

 

"The record for peak experimental horizontal g-force tolerance is held by acceleration pioneer John Stapp, in a series of rocket sled deceleration experiments culminating in a late 1954 test in which he was stopped in a little over a second from a land speed of Mach 0.9. He survived a peak "eyeballs-out" force of 46.2 times the force of gravity, and more than 25 g for 1.1 sec, proving that the human body is capable of this. Stapp lived another 45 years to age 89, but suffered lifelong damage to his vision from this last test.

 

Watch that recoil !!!

Cheers

 

Don't forget the 147g ball park figure was for just a gun on it's own. A couple of posts seem to have been thinking more of the acceleration that the human body can withstand, but I believe the question was about what g force the gun might experience.

As I said, if it had to also accelerate part of the shooter it would come down - so 80lb total weight (ie 10x gun alone) would then bring g force down to 1/10 ie 14.7g

The human body is "squidgy" though, and only some of it will move with gun - so I really don't know what the gun would recoil like when mounted.

 

(The gun might accelerate quite high, but that doesn't mean the entire shooter including brain and major organs etc are.)

Edited May 10, 2011 by HW682

[Spara Dritto]

I wasn't far off, good effort with that answer!

[Rem223]

Assuming the same force is applied to the gun, it will accelerate backwards at:

a= F/m = 5185N/3.6Kg = 1440 m/s2.

There are a lot of recoil calculators available for firearms which calculate the velocity of the recoiling firearm. For the sort of figures quoted, say 1300fps with 28grm of shot, the ballpark figure for the recoiling gun would be 13ftlbs, a velocity of around 3ms^-1. If a shotgun generating around 13ftlbs of recoil generates over 14g accelertion using your figures then how much acceleration would a 300win mag which pushes over 20ftlbs of recoil develop? Sorry but I dont buy it. Momentum is conserved mv = mv but your are assuming the kinetic energy is the same for the gun as for the shot. If it was I think we would all give up shooting pretty quickly.

[HW682]

There are a lot of recoil calculators available for firearms which calculate the velocity of the recoiling firearm. For the sort of figures quoted, say 1300fps with 28grm of shot, the ballpark figure for the recoiling gun would be 13ftlbs, a velocity of around 3ms^-1. If a shotgun generating around 13ftlbs of recoil generates over 14g accelertion using your figures then how much acceleration would a 300win mag which pushes over 20ftlbs of recoil develop? Sorry but I dont buy it. Momentum is conserved mv = mv but your are assuming the kinetic energy is the same for the gun as for the shot. If it was I think we would all give up shooting pretty quickly.

 

Well, you have given a lot of numbers there - but none actually answer the OPs question

Note that you are the only one to mention an energy figure (13 ftlbs).

I was using Newtons 2nd and 3rd laws.

I didn't have any special knowledge of internal balistics, but just broke the problem down into a simple model.

 

As the above numbers were posted without any explanation of the calculations used, or assumptions made, I thought I would have a quick search of the net to make sense of them.

 

From this website link the above figures would be typical for the final velocity of the gun. Imagine it is mounted on say a skateboard (with no friction in the wheels etc). When fired, the shot accelerates up the barrel and at the same time the gun accelerates backwards. Once the shot leaves the barrel, there is no longer any force pushing the gun backwards - so it would then stop accelerating and continue at a steady speed backwards (in reality it would gradually slow down due to friction).

The 3m/s might be typical of this final speed of the gun. The kinetic energy given is simply calculated by multiplying the weight of the gun with the square of the speed ( 1/2 mV2). This is what the recoil calculator gives.

 

What the OP needs to know is how quickly the gun will accelerate when going from standstill to moving backwards at say 3m/s. The final speed is of no interest.

 

In my original model I assumed that the acceleration of the shot was constant. In reality, it isn't and shows a definite spike, like this:

 

 

 

Googling shows that many people have looked at this already and it is known that the peak acceleration is proportional to the peak chamber pressure.

A well known company in Scotland, called Border Barrels, have a patent registered to measure chamber pressure based on the g force during recoil.

 

See this link. They suggest using a sensor ranged 50g to 1000g or 50g to 600g.

 

As a sanity check, I wanted to see if these figures were backed up anywhere else (just because someone files a patent doesn't automatically mean that they are right).

 

This link has a good article on the subject and gives a typical figure of 60,000 to 80,000g for the shot charge. If the gun is approx 100 times the mass of the shot, this implies the gun recoils in the range 600g to 800g.

Well, seems to tie in so far.

 

Another website tries to calculate the recoil g of various guns as an aid to deciding whether NV can be safely mounted (they suggest a limit of 400g).

For your info, a few figures are pasted below (for a 4kg gun):

22LR 132g

22-250 357g308win 667g

again, these all roughly in the range suggested by the patent.

 

One last mention on the web is http://www.chuckhawks.com/leupold_tour.htm which mentions scopes being designed to withstand 300g.

 

It doesn't really matter whether any of these figures are super accurate or not. The OP just wanted an idea of the g force involved to be able to select an appropriate sensor.

 

My very basic late night model suggested 140g for a shotgun.

Border Barrels expect 50g to 1000g for different guns and other weblinks all give figures in the range 100s of g. Therefore, I am reasonably happy with that

 

As already discussed with the OP, if he is only wanting to detect when the gun is fired - rather than actually measure g, then I would go for a low range accelerometer that wouldn't be damaged by going over range up to a few 100 g.

 

Again, I am more than happy for anyone to make a correction or suggest an improvement to the model, but just saying "ooh, that sounds like like a big number so I don't believe it" doesn't really help the OP does it?