Reduced recoil and FPE question?

[BondoBob]

I'm curious how energy ft lbs effects felt recoil among other factors but shot from the same gun. I would have thought lower ft lbs means lower recoil. But I notice the lower recoil hornady critical defense lite 38 actually has more ft lbs muzzle energy than the regular 38. How can this be?

The lite is 90gr, 1200 fps 288 fpe. The regular critical defense is 110gr, 1010 fps, 249 fpe, but has more recoil supposedly.

I'm asking because I am considering making some carry rounds. So far I've only been making plinking rounds. I'm wondering if there's a formula or if it simply means lighter bullets going faster have less recoil than heavier bullets moving slower.

Don't really know why, but I am a bit sensitive to big recoil.

 

[Unclenick]

By Newton's laws of motion, momentum is imparted equally and oppositely with the velocity being in inverse proportion to mass (mass of the ejecta (bullet and powder) and the mass of the gun) right up to the moment the bullet base is about to pass the muzzle crown. But energy is proportional to the square of that velocity, so it is not equal and opposite. Also, the powder charge's produced muzzle pressure has an effect. When the bullet clears the muzzle the gas accelerates rapidly and goes around it, causing "rocket effect" that helps propel the gun rearward. If a given peak pressure is reached with a smaller charge of faster powder, the rocket effect is less than a larger, lower pressure charge of slower powder with higher muzzle pressure, even when the bullet velocity produced is the same.

SAAMI has a document you can download for calculating recoil energy.

 

[nhyrum]

Ke=1/2 mv^2. Ke is kinetic energy, m is mass,v is Velocity. Measured in joules, use meters per second and kilograms. Velocity has more impact on kinetic energy than mass.

F=MA. F is force, m is mass, a is acceleration

like Uncle Nick stated, newton's 3rd law is part of what you're fighting. If you send a bullet down the bore with 1000 lb-ft energy, there's also 1300 lb-ft coming back through the bolt face. Springs and mass will absorb the energy to make the felt recoil be... Not the full force. Shoot an airweight 44 mag, and a big ol dirty Harry with the same loads, the air weight will absorb less energy, so your hand has to do the rest. So really... No, there's no way to increase muzzle energy and NOT increase recoil energy, given the same firearm.

There is what I'll call, the air bag principal. The air bag works by simply keeping your acceleration (deceleration) to a manageable level, by increasing the time from full speed to stopped. By only a fraction of a second. The net forces don't change, just the time. So, 1300lb-ft delivered over .0001 second will feel harsher or "snappier" than say 1300 lb-ft delivered over, say, .001 second. Same energy, but it's the acceleration that changes. Keep mass the same but change the acceleration, and the force changes proportionally. Slow down the acceleration, force goes down as well.

Sorry for the physics lesson. It's far from comprehensive, as my college physics classes were some time ago. But you can see that choosing powders on the slower side, IN THEORY, can help reduce felt recoil, simply by delivering the same amount of energy over a longer period of time.

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[74A95]

But you can see that choosing powders on the slower side, IN THEORY, can help reduce felt recoil, simply by delivering the same amount of energy over a longer period of time.

In theory, perhaps, but in practical terms, the slower powder uses more gunpowder weight to push the same bullet to that same speed, and this extra powder weight increases the recoil. Thus slow powders, because they require more weight, produce more recoil force than fast powders.

I don't know if most people, including myself, could feel any benefit from using the faster powder. It's a tough experiment to conduct because you'd need to make sure that both powders produce the same velocity. It's hard to match speeds with two powders.

 

[nhyrum]

In theory, perhaps, but in practical terms, the slower powder uses more gunpowder weight to push the same bullet to that same speed, and this extra powder weight increases the recoil. Thus slow powders, because they require more weight, produce more recoil force than fast powders.



I don't know if most people, including myself, could feel any benefit from using the faster powder. It's a tough experiment to conduct because you'd need to make sure that both powders produce the same velocity. It's hard to match speeds with two powders.

Agreed, I did overlook that. But yeah... I don't think many will feel it. Kinda line the "butt dunno"

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[BondoBob]

This is all a little over my head. I wasn't a good student when it came to physics. So energy ft lbs has nothing to do with it?

It's still vexing me that the higher energy load has less recoil in the same gun.

 

[Nick_C_S]

From years of chronographing, I know if I load the same bullet to the same velocity (same gun), but with different propellants, the round with the slower propellant will have more recoil. I call this "thrust recoil" - as in thrust from a rocket. I doubt if that is a formal term. But it means something to me.

 

[44 AMP]

So energy ft lbs has nothing to do with it?

Ft./lb energy has something to do with it, but its not a direct linear straight line effect.

Ft/lb energy is a straight math formula where velocity plays the biggest part (vel squared) and its essentially meaningless other than as a "standard" for comparison.

Example, you can load a .22-250 and a .45-70 to exactly the same ft/lb energy.
ON paper, the energy is identical. RECOIL is NOT.
And even though the energy is the same, one is greatly superior shooting buffalo over the other...

And, to further complicate the issue, there is the actual energy of the recoil (mass of gun and speed) and there is what you FEEL. Felt recoil is subjective, and the same number of ft/lbs energy will feel different to different people due to the shape of what they are holding, and how they hold it.

Ft/lbs is a system for comparing things using a standard reference. It clearly shows that a larger number is more than a smaller one. After that, things get "tricksy" Higher ME means more energy, BUT what you FEEL can be something else again. Lots of other factors come into play so its not a straight 1 for 1 thing.

 

[Unclenick]

So energy ft lbs has nothing to do with it?

Energy has everything to do with it. It just isn't the simple equal and opposite that momentum is. But at the velocity of a recoiling gun, ft-lbs of recoil energy produces a good representation of felt recoil. If there were no powder mass involved (pretend the bullet is driven electromagnetically for the sake of taking powder out of it for the moment). If the gun weighed 300 times what the bullet weighs, in order to have momentum that was equal and oppositely directed from the gun, the bullet has to be going 300 times faster than the gun recoils. So let's give the bullet a velocity of 300 and the gun a velocity of 1. 1 squared is 1. 300 squared is 90,000. So the bullet will be going 300 times faster but the muzzle energy will be 90,000 times greater than the gun's recoil energy. This is the main reason the target on the receiving end of the bullet is damaged more than the shooter is. You can add-in greater area of distribution of the rifle butt plate as compared to the bullet nose, but that's a smaller factor than the energy difference.

Why is gun recoiling energy a good predictor of felt recoil? I like to give the example of an 5.6 ounce 100 mph baseball vs. a 2000 lb small car in neutral that is just barely rolling along a flat at a very sedate one quarter of an inch per second (about 1/6 of a mph). Both have the same momentum (mass times velocity). Now you put your hand out to stop the baseball and your other hand out to stop the car. Which one hurts more to do? The baseball's kinetic energy is about 117 ft-lbs. The car's kinetic energy is about 0.013 ft-lbs. That's an 8800:1 difference and that's about what it feels like because if you stop both of them over the same distance (say, a foot) you spend 8800 times more time stopping the car than you do the baseball, so it takes 8800 times less force to stop the car. And that's what you feel. The amount of force involved in taking the energy out of something.

So, when your gun has 900 times less energy than your bullet, it takes 900 times less force to stop it over the same distance. You don't actually do that, of course. You actually stop the bullet over a shorter distance (the recoil distance) than the target stops the bullet over (a deer thickness, for example), so you don't get the full 900 times less impact than the target, but it is significantly less.

Rocket effect can be very significant, especially in overbore cartridges where the powder charge weight approaches that of the bullet. This is because propellant gasses are less dense than lead or copper so when the bullet uncorks the muzzle, these lighter gases have the same pressure behind them that the bullet did at the muzzle, but that accelerates them to a higher velocity than the bullet, which is why shadow graphs of guns firing show a sphere of expanding gas out ahead of the bullet for a foot or so. Of course, it looses velocity fast as it works to push air out of the way, so that by the time it gets into that size range, the bullet is moving faster than the gas pressure front and catches back up and goes on to penetrate the front of the gas bubble and move on forward. But while that gas is at its highest speed near the muzzle, it creates a lot of thrust to the rear as it vents from the muzzle and that accelerates the gun further toward the shooter's shoulder. For some overbore cartridges like the 22-06, the recoil energy due to muzzle blast actually exceeds the recoil energy due to accelerating the bullet. What a muzzle brake does is vent much of that gas sideways so it doesn't contribute to recoil. This is why muzzle brakes do nothing to reduce recoil of cat sneeze loads, but reduce the recoil of full-house loads significantly. the former have very little left by way of muzzle pressure when they exit, but the latter have a lot of it, so the brake makes a very noticeable improvement there.

The v² relationship of velocity to kinetic energy also explains why pulling a rifle stock hard into your shoulder reduces felt recoil. The action adds your mass to the mass of the gun, and thus the highest possible recoil energy cannot be reached because your body mass is in the way of the gun achieving its maximum possible recoil velocity.