Confused about recoil.

[Echo9]

I was out recently firing a friend's Sig 556. I forget what type of muzzle brake it had, but it wasn't factory standard. And it worked amazingly well.

Compared to other rifles chambered for 5.56 of .223, it had almost no kick. It almost felt like a .22LR. So I'm confused now.

As I understand it, recoil is just the conservation of linear momentum. In other words, the mass of all ejecta x the velocity of all ejecta = the mass of the firearm x its rearward acceleration during the recoil impulse.

Doesn't the bullet constitute the overwhelming majority of the mass of the ejecta? How could redirecting gases reduce recoil so much? If I remember my physics, the mass of the escaping gas must be equal to the substance that sublimates into that gas -- the mass of the powder equals the mass of the gas it produces when ignited. How much could the gas possibly weigh? The same phenomenon exists with some other weapons I can think of off the top of my head. Not that I've fired one personally, but I'm told that some .50BMG rifles recoil less than a 12 gauge shotgun because of the muzzle brake.

Also: I learned in a THR discussion not too long ago that the recoil impulse begins the moment the bullet is pushed forward by the ignition of the powder, and the only reason that recoil appears to start only once the bullet has left the muzzle is that the bullet leaves the barrel rather quickly. Now I'm reading that the escaping gas pushes the gun rearward. That seems to make sense, but again -- shouldn't that rearward push by only slight, because the gases have so little mass?

Help me out here.

 

[Jon_Snow]

Perceived recoil and mechanical recoil are two very different things. Mechanical recoil is related to the momentum of the ejecta, but that expresses itself as a force over time. Perceived recoil is related to the peak force. If you spread the recoil force out over alonger period of time, you have the same amount of mechanical recoil with a lower perceived recoil. Then, because human perceptions are far from perfect, a lot of other factors come into play here as well, including how well the gun fits you, how loud it is, how much muzzle flash there is, etc.

How much could the gas possibly weigh?

It's not about how much it weighs, it's about how much momentum it has. The gas is moving very quickly; at least some portion of it must be going faster than the bullet. And the weight of the gas isn't trivial. If what you assumed is correct, for a typical 308 load, you have a ~160 grain bullet propelled by ~40 grains of powder that has been sublimated to gas. Keep in mind, bullets don't weigh that much either compared to the weight of the gun.

Now I'm reading that the escaping gas pushes the gun rearward. That seems to make sense, but again -- shouldn't that rearward push by only slight, because the gases have so little mass?

Recoil starts the moment the gases begin to expand. Escaping gases CONTINUE to cause recoil, but that's not where it starts. Where-ever you heard this, it's wrong.

 

[rcmodel]

Think of a rocket engine.
The volume and speed of the burned fuels exhaust gas is nowhere near proportional to the volume of fuel being squirted into the engine and burned.

The expanding super heated gas is what pushes the rocket through the air.
Same thing applies to a muzzle break, only it's push is designed to counteract the recoil.
SO, it pulls rather then pushes so to speak.

Also, the Sig 556 is a piston operated gun. It doesn't have a heavy buffer slamming back and forth in the stock like an AR-15 you may have shot.
Could be it wasn't just the muzzle brake making all the difference.

rc

 

[CZguy]

Same thing applies to a muzzle break, only it's push is designed to counteract the recoil.
SO, it pulls rather then pushes so to speak.

Much like thrust reverser's on a commercial jet.

 

[Echo9]

Yeah, the Sig is long stroke piston with the recoil spring around the piston itself, far from your shoulder. I thought that was why it had so little little recoil, but then I fired a different 556, which actually had a heavier hand guard but a different muzzle break, and it kicked a lot more. The one with barely any recoil was a 556 Classic, the other was a Classic Swat.

Ok, so when someone says "grains of powder..." Is that the same metric we use for bullet weight? Or are we talking about individual pieces of cordite?

 

[The Lone Haranguer]

Ok, so when someone says "grains of powder..." Is that the same metric we use for bullet weight? Or are we talking about individual pieces of cordite?

While on occasion it can refer to an individual grain or granule of powder, it is most commonly a weight measurement of the total amount of powder, or the weight of the bullet. And the grain is not metric; it is an old English measurement, 1/7000th of a pound. The metric unit of measurement would be grams. Finally, cordite - popular at one time in British cartridges but not used for many years - is not granular in composition; it more resembles rope-like strands.

 

[Echo9]

Oh, I know it's not metric, I was just using the noun form of "metric" the same way you'd use the word "measurement."

Ok, so 40 grains is the same weight as a 40 grain bullet? Because that would make more sense to me as far as expanding gas producing significant recoil.

 

[Walkalong]

The Remington VTR in .223 has a built in brake, and the recoil is mild compared to most bolt action .223's.

 

[Supertac45]

I've shot a lot of diffent .223's and I never fired one that had anything that I would consider recoil.

 

[Freedom_fighter_in_IL]

Im with you Supertac45, I didn't even know that the .223 HAD recoil

 

[Shadow 7D]

easy way to fix this, shoot 5 rounds out of a mosin, then the sig, won't notice recoil from the sig.

 

[Freedom_fighter_in_IL]

Even better way Shadow, shoot 2 rounds from my Ruger#1 chambered in 450ne That will cure whatever ails ya in the lessor .223

 

[Drail]

Recoil from a gas operated 5.56 poodle shooter? Are you serious? I've used staple guns with more recoil than that.

 

[Echo9]

Hey guys, I really appreciate your input, but we were actually discussing the physics of recoil and the roles played by various components of the equation.

If you have something to contribute to that end, that would be great. Otherwise, I don't really know what the impetus was for the five posts before this one. The title of this thread is not "My Sig 556 hurts my shoulder. Help?"

 

[russ69]

One formula that I remember is to use 1/2 of the powder weight as part of the calculation. I don't think this is 100% accurate but it's not a bad rule of thumb. I think the correct answer would have the velocity and the charge weight as part of the equation.

Thanx, Russ

 

[Echo9]

One formula that I remember is to use 1/2 of the powder weight as part of the calculation. I don't think this is 100% accurate but it's not a bad rule of thumb. I think the correct answer would have the velocity and the charge weight as part of the equation.

Thanx, Russ

That's interesting, I wonder why use 1/2? Maybe because a portion of the gas escapes in a lateral direction, not really adding to the recoil?

 

[CZguy]

Finally, cordite - popular at one time in British cartridges but not used for many years - is not granular in composition; it more resembles rope-like strands.

It looks like angle hair pasta to me.

 

[jfdavis58]

Aside from totally improper terminology the problem here is a lack of knowledge of physics.

Within statistical variance one 223 round produces as much recoil as any other similar 223 round. The total force over time-an integration problem (within our ability to account for all factors and make a proper measurement exactly equals the total recoil (another integration).

The perceived recoil depends on all that other stuff: weight of rifle, presence of a muzzle break, type of rifle action--even the weight and muscle resistance of the shooter!

The movement of the action and the effectiveness of the brake depend on the pressure of the gasses from the combustion of the powder. Recoil force play a minuscule part.

Powder burns-an exothermic reaction of some vigor. Water sublimates and typically leaves rings of precipitated salts in the process-think snow melt even though the ambient temperature doesn't rise above freezing.

Grains are grains in this application; a unit of weight related to mass through the acceleration of gravity. Typically a constant for earth-side approximation which are usually sufficient.

And from my standpoint most AR platforms seem to have a reverse recoil; i.e. they pull forward more strongly than they push backward. While this is not actually happening the instinctive need to brace for recoil often leaves a shooter having little resistance to the immediate forward jerk caused by the bolt returning to battery--there is a perception that the rifle wants to jump forward in the minds of many new to the device.

 

[rcmodel]

I wonder why use 1/2?

Because half of the powder gas is pushing against the bolt, and the other half is pushing against the bullet. The half pushing against the bolt cancels out half the weight of the total gas generated.

rc