Physic's Formula Question

[Deviant]

Skippy,

Thanks for the advice, but I'm looking to make a firing mechanism that is alot easier to reload in the field. Keep in mind that this will be used in a wargamer simulation.


carlnpa,

In response to taking apart an existing gun. I have owned most of the popular and not so popular models. I'm looking to improve on the short comings I see in them.

Most current airsoft guns are electric which takes away from the gun in my eyes. Most of the classics are gas and work on the o-ring pressure build up, however parts are next to impossible to get.

If I have to make parts, I might as make something new and better.

The bolt action guns, aps, vsr etc... all have bolts that are misplaced in regards to where the magazine would be on the real steel. Also the pull length/effort always seemed a little excessive which always bugged me. I had a KS model APS that had a really light pull, with extremely reliable velocity. This is one of the main reasons that I believe a gun can be made within the specs I'm looking for. And I'd guess that the weight pull on the KS Gun was around 5lbs. Of course I'm never hooked a scale up to see.

Which is to make small version of a bolt action similar to the sv-99. Since I'm decreasing the bolt size, I need to know whether it's feasible or not. If I cut the bolt volume in half, do I need to double the spring power. Etc.

((I don't have one of the guns currently to measure, hence all the speculation))

I've already begun working on a gas gun that should be very interesting, however alot of people shy away from gas do to it's unreliable nature in cold weather. *shrugs*


OCNC

Your equations give me hope, however I'm not following how to apply them 100%

Hopefully you can correct me.

Let's say I start with a 24" barrel which I want to shoot a bb at 500fps..

Now, if I follow your formula.

V^2=2(Ao)(S)

V = velocity which is 500fps

s = barrel length? 24" converted to 2 feet.

so it should be 500^2=2(Ao)2

Which if I do my algebra right.

Should be 250000/2 = 2(Ao) that then reduces to 62500 = Ao

But should it be ft/sec^2? That doesn't seem right.. I don't even think my rifles fire at that speed? Should something else be applied here? Maybe taking the square root or something? ((tosses idea in the air))

I believe my .270 is only around 3000+/- fps, at least it's muzzle velocity. Which isn't really the acceleration.

I guess I'm just having a little bit of trouble wrapping my brain around those numbers. I know that from a compressed gas standpoint. Most airsoft guns that fire 500fps, do so at around 80 psi. And the time for the bb to leave the barrel is definately something small like that.

As for the flow rate, I don't think that will be a problem. 3mm hole in the air nozzle is a pretty good for the low volume of the piston chamber.

I'm going to need to double up on the advil before this problem is solved. (lol)

[OCNC]

Originally Posted by Deviant I believe my .270 is only around 3000+/- fps, at least it's muzzle velocity. Which isn't really the acceleration. (lol)

I think that's the key point. Velocity is not the same as acceleration. Acceleration is the rate of change of velocity and velocity is the rate of change of distance. Velocity is measured as ft per second per second which is written ft/sec^2. If you use the example of a baseball pitcher and plug in the corresponding numbers you might be more trusting of the formula. For an 80 mph pitch (117 ft/sec muzzle velocity) and a barrel length of 6 ft (the apporximate distance the ball moves in the pitchers hand from the time he cocks his arm until he releases the ball as he steps off the rubber) I get an acceleration of 1140 ft/sec^2. The time it takes the pitcher to throw the ball (time in the barrel) is about 1/10 sec. The average force on the ball while "in the barrel" is about 11.5 lbs. based on a 5 oz. baseball weight ( which has a mass of .0098 slugs). By the formula given the acceleration is proportional to the velocity squared and inversely proportional to the barrel length. The higher the velocity and the shorter the barrel the greater the acceleration will have to be. The velocity term, being squared, dominates. In terms of the acceleration it really doesn't matter what it is that you're trying to motivate. If you wanted to get an elephant up to 500 fps in a distance of 2 ft you'd have to use the same acceleration of 62500 fps. The force involved however would be much different.

Chris

[id 10 t]

Hi Devian,
The formulas you need to solve your problem are here: http://www.atnet.it/lista/easne/mce263-3.html
I would set up spreed sheet and make a graph of velocity vs. length of barrel.
Additional formulas you will need are F=ma and v^2=2ax.
Colin

[Deviant]

Colin,

That's an awesome website, and almost exactly what I'm looking at doing. Cool find.

F=ma, Force = Mass x acceleration?

v^2=2ax, Velocity^2 = 2 x acceleration x X?

What is X?

Thanks

[id 10 t]

x is a variable and is the distance the pellet travels down the barrel.
From F=ma solve for a: a=F/m
plug into v^2=2ax: v^2=(2Fx/m)

Using the formula for force from the link you will have a relationship of velocity and distance the pellet travels down the barrel.
When you make your graph you will also be able to optimize the length of the barrel.

[trebby]

Right away it is apparent that all of the energy initially present in the system is in that compression spring behind the pistion. The energy in the cocked spring must be at least as large as the kinetic energy of the ball as it leaves the barrel, but in reality it must be considerably greater due to all the losses in the system.

If you want to only require 5 pounds of force to cock the spring, and a travel of 4 inches (a guess on the travel here), the spring can hold no more than 2.25 joules of energy. So, even with a perfect system you won't quite reach your goal of 500 ft/s. If we can use an 8 lbf spring, then the energy in the spring can be as great as 4.5 joules. If you can reduce the energy lost as kinetic energy of the piston, and optimize the barrel length so that a minimium of energy stored in the compressed air behind the ball is lost, then you might be able to get the muzzle velocity you are after. It would be tough though.

[Deviant]

Just a follow up to everyone involved.

I bit the bullet and started building the system last night.

I figured it would be cheaper to by the aluminum than advil.

I'll post more as I get a working or not so working product.

[Deviant]

Started on the piston tonight, still a little bit to go on it.

Definately looks small.

[Deviant]

Solid model for the part

Since the photo is kinda sucky.

[skippy]

Deviant, three cheers for taking "Nike's" advice (Just do it!) then see how it works and change/modify as required.

As I've never seen inside one of these guns I don't recognise the pieces. In your original photo (pg2) it seems like the cylinder is the long item at top (centre & right) and the piston is at top left. The piston appears to be about 1/4" long with a small spigot to locate the coil spring. Your picture above is nothing like the page 2 photo unless I'm not understanding it correctly? If this is the piston above then it's going to weigh substantially more than the other type which will negatively effect the performance of the gun but as I say, maybe I'm not understanding it.

EDIT Now that I've looked some more it seems that the bit to the right encases the coil spring and the slots are to hold the spring retainer and the piece at right is what you actually grab hold of the recoil the spring. Am I on track?

Anyway good luck with it.

[Deviant]

pg2, the smaller black part is the piston I've started making. It slides inside the larger chrome cylinder.

My piston is about half the size of the one on pg2. Also, a slightly different design.