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Wasted Velocity
#1
Wasted Velocity

Posted By: ML - Posts: 486

Posted At: (3/11/04 12:52 pm)

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Muzzle Velocity and Recoil Pulse



I spent last weekend in Daytona Beach, Florida, crewing at the AMA Superbike races there. In the paddock, I ran into Kevin Cameron, an old colleague and currently the technical editor for Cycle World magazine.



Mr. Cameron’s not a gun guy, but he is a polymath and a great student of the physical world. Conversations with him are always enlightening. He and I talked about a couple of things, one of which got me to thinking about the true bullet speed of high-velocity rifles.



If you stayed awake during Fr. Koch’s Physics 101 lectures—and I managed to on some occasions—you’ll remember the Sir Isaac Newton’s Third Law of Motion: namely, for each action there is an equal and opposite reaction. You intuitively know that as it applies to firearms: launch a heavier bullet, or launch it faster, and the rifle kicks harder. A violent kick is unpleasant, and contributes to flinching and inaccuracy on the part of the operator. But it does more, too: It reduces muzzle velocity. How? Consider this.



In the real world, we only care how fast the bullet flies compared to the target, the wind, or the ground. But the faster the rifle kicks back, the less speed is imparted to the bullet. Look at two extreme examples, both using a cartridge/bullet/powder/barrel-length combination which launches the bullet at a nice, round 3000 feet-per-second (fps) velocity at a constant altitude/atmospheric density. In Example A, we bolt the rifle down solidly to the Hoover Dam, a 5,500,000-ton edifice. The rifle cannot move. When we touch off the charge, we see a true bullet speed (velocity) of 3000 fps.



Now, Example B. Let’s say our bullet weighs 150 grains (we should also, theoretically, add some weight for the ejecta of unburned powder and the weight of the propelling gas, but we’ll ignore those here for the sake of simplicity). And let’s say our imaginary rifle also weight but 150 grains. When we touch off the charge, the bullet and the rifle will each move with the same velocity, but in opposite directions. Thus, we will see our bullet propelled at 1500 fps forward (just half of its original value relative to the ground/target), while our very light rifle is also propelled at 1500 fps backward.



In reality, of course, our rifles weigh much more than 150 grains. There being 7000 grains to the pound (at least this morning), a typical deer-hunting rifle with a telescopic sight, a sling and a magazine full of ammunition weight something more like 63,000 grains, or 420 times the amount of the bullet in this example. And factor in, too, some fraction of the weight made up by the shooter’s shoulder and upper body.



Still, though, the result is undeniable: as a rifle kicks you harder, it’s stealing velocity from the bullet, and this is part of the reason that an ammunition manufacturer’s published velocity data is often higher than what you will see from your chronograph. First, they often gather velocity from an action bolted solidly to a test fixture. Second, they often use long, 26-inch barrels (each additional inch of barrel length may boost velocity between 20 and 40 fps for a given typical rifle caliber). They’re not lying to you, but you’ll be hard-pressed to duplicate their results unless you duplicate their testing methods.



What does this mean in the real world? Not much. Precisely because the bullet weighs so much less than the rifle, increasing the powder charge or using a more powerful chambering almost always does increase velocity. Just remember, it just increases it at both ends.



Here’s another way to think of it. At Daytona, race fuel for the Superbikes costs about $15 per gallon. Seem steep? Consider this: About $12.30 of that was "wasted" money. How’s that? Because out of that $15 worth of gas, only $2.70 cents actually went towards propelling the racebike down the track. The other twelve-bucks-and-change (or 81.9 percent) went towards just heating up the atmosphere (waste heat through exhaust 45 percent, waste heat into the cooling system 30 percent, other internal losses—mostly friction—seven percent or so). Damn that Fr. Koch and his laws of physics!



Think of all that "wasted" gas as the equvilent of recoil. I’d sure like to have that money back. And I sure wish my .375 H&H didn’t kick so hard. But that’s the price of doing business in the physical world (thank you, Wallace Stevens). And, personally, I wouldn’t have it any other way.



--ML



11.iii.4



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Re: Wasted Velocity

Posted By: Clifton Clowers - Registered User

Posts: 33

Posted At: (3/11/04 4:35 pm)

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ML,



Is the action-reaction of a bullet and rifle literally instantaneous? That is, does the rifle begin to move backward at the exact same instant the bullet begins to move forward? Everything I've ever read about internal ballistics and mechanics leads me to believe that it does -- or that it does at 300 km/sec (speed of light). If so, why is it that the barrel does not apparently move enough to affect accuracy by the time the bullet leaves the barrel? To wit the recoil of, say, a .458 Lott in a 9 pound Model 70 is rather severe. Why doesn't the barrel rise affect the path of the bullet before it leaves the muzzle? Is the barrel rise not sufficient to affect the barrel? It sure doesn't feel that way.



Tango mike,



Clifton



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Re: Wasted Velocity

Posted By: Birdog - Posts: 4096

Posted At: (3/12/04 6:54 am)

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ML,



So what effect does a recoil compensater have, compared to bullet velocity?



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Re: Wasted Velocity

Posted By: Cody - Posts: 121

Posted At: (3/12/04 1:30 pm)

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"Why doesn't the barrel rise affect the path of the bullet before it leaves the muzzle? Is the barrel rise not sufficient to affect the barrel? It sure doesn't feel that way."



I'd guess that the force, in the form of expanding gasses within the chamber, must overcome inertia of both the rifle and the bullet.



So a rifle with 420 times the mass of the bullet will accellerate much more slowly than the bullet itself. I'd bet that by the time the bullet has accellerated and gained enough velocity to leave the barrel, the gun itself is just overcoming inertia (its tendancy to stay at rest) and beginning to move significantly.

Edited by: Cody at: 3/12/04 1:32 pm



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A Couple of Answers

Posted By: ML - Posts: 488

Posted At: (3/12/04 1:30 pm)

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Two excellent questions. And, luckily, we’re in possession of a couple of answers today. (Whether those answers are of equal quality, we shall soon see.) First, the issue of how fast all this occurs.



Yes, it is instantaneous. Recoil starts the moment the cartridge ignites. Remember, though, as another Forum member points out, that the rifle, weighing so many hundred times more than the bullet, is also in possession of much, much more inertial mass, and consequently is much slower to accelerate rearward. In addition, once set into motion, it continues to accelerate (recoil) rearward long after the bullet has left the muzzle.



Recoil certainly affects the bullet’s flight through the barrel and accuracy, although the physiological effects here far outweigh the purely physical—by that I mean since a human being is part of the equation, flinching or pulling the shot generally far outweigh the effects of recoil per se on the firearm.



So far, this is just a bunch of theory, but we can see it handily in the real world. Keep making the bullet heavier and the firearm lighter, and the practical physical effects of recoil begin to reveal themselves. Here’s a fine example:



What do you call a two and a half-pound rifle with a short barrel firing a 300-grain bullet? A large-caliber handgun. Let’s take a revolver chambered for the .44 Remington Magnum or the .45 (Long) Colt and fire a light bullet through it, and note the point of impact. Now, let’s fire a heavier bullet through the same handgun. Most logic leads us to believe that the heavier bullet will strike lower than the light bullet, as it travels more slowly and consequently should drop more as gravity acts upon if for a longer time. Yet invariably we see heavier bullets fired out of large-caliber handguns strike higher at typical handgun ranges (say, 25 yards). Why?



Recoil. Since the heavier bullet travels through the barrel more slowly (technically, it has a longer barrel-residence time), the forces of recoil act upon it to a greater degree while it is still in the barrel, compared with the lighter (faster) bullet. To exaggerate the case, the lighter (faster) bullet has already left the barrel by the time the handgun’s muzzle begins to rise appreciably, while the heavier (slower) bullet is still in some part of the barrel, and the rising muzzle changes the barrel’s angle, sending the bullet on a flight path which results in a higher point of impact. As an additional proof of the recoil-not-gravity phenomena here, take that same large-caliber handgun, rotate it 90 degrees (clockwise or anti-clockwise, your choice) and repeat the light bullet/heavy bullet experiment. You’ll find that in every case, the heavy bullet prints above the light bullet’s point of impact (the word "above" as used here meaning above the orientation of the original sight plane), even though gravity or air resistance did not rotate 90 degrees with you.



The result is far less pronounced with a rifle, because the bullets are (almost always) much faster and lighter, not only in terms of absolute weight but in the more important ratio of bullet weight to firearm/upper torso mass. Yet the handgun example clearly shows that recoil begins far before the bullet leaves the muzzle, and we can readily see that it continues long after the bullet clears the barrel.



Which leads us right into our second question for the morning, concerning muzzle brakes.



Virtually all muzzle brakes function by re-directing the combustion gasses backwards and/or up, counteracting a typical rifle’s tendency to recoil backwards and upwards (the gas trying to push the firearm forwards and down). Now, some small amount of gas (air) is pushed ahead of the bullet as it travels down the barrel, but the lion’s share—by far—of the muzzle brake’s effectiveness comes from the much greater, faster-moving volume of gas which is pushing the bullet forward. So here, really no anti-recoil effect takes place until the bullet has cleared the muzzle (or, more properly, the brake). Yet we just stated that recoil starts at the moment of ignition, so how can a muzzle brake reduce recoil, which they most certainly do?



Ah, remember that we said that recoil lasts long after the bullet has left the muzzle? That’s exactly the portion of recoil that the muzzle brake attempts to reduce. That great mass of rifle and shooter, once set into motion, wants to remain in motion, and the muzzle brake attempts (and succeeds in some fashion) to impart an equal-but-opposite reaction.



A couple of other observations at this point: First, while some muzzle brakes do indeed reduce the felt recoil pulse, they invariably increase perceived muzzle blast by necessarily redirecting it towards the shooter, which may cause flinching problems of its own, especially for shooters who are not using hearing protection.



Second, muzzle brakes influence barrel harmonics. This is neither bad nor good in itself. A tunable muzzle brake like Browning’s BOSS may let the shooter fine-tune barrel harmonics to a particular load. They are also "infinitely maladjustable" as the late Gordon Jennings used to remind me—meaning that you’re probably more likely than not to degrade your rifle’s inherent accuracy with them. For real-world proof here, look to the world of the fussy target shooter. Invariably, rifles used for absolute pinpoint accuracy (at least in any chambering less than .50 BMG) do not use a muzzle brake; rather, they feature simple crowned "conventional" barrels.



And a final thought. Much of this recoil-pulse/lost velocity issue, while interesting, is only a small bit of the large equation of shooting—an interesting meditation for my morning commute, but nothing to get hung up upon. It is a truth, but only one part of the much greater truth effecting your shooting.



Still, thanks for asking, and I hope I’ve answered your questions adequately.



Yankee Victor Whiskey,



--ML



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Re: A Couple of Answers

Posted By: Clifton Clowers - Registered User

Posts: 35

Posted At: (3/12/04 6:45 pm)

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"...it continues to accelerate (recoil) rearward long after the bullet has left the muzzle." Hmmmm. I don't think so, not after the mass of the bullet and the gases have left the barrel. But the rest of the discussion... hmmmm... and hmmmmmmm. One must ponder for a while. Thou art a scholar of no insignificant magnitude, Dr. ML.



Quebec Echco Delta,



C. Clowers,

HMS Wolverton Mountain,

Commanding



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Recoil clarification

Posted By: ML - Posts: 489

Posted At: (3/12/04 7:25 pm)

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(Quote): "...it continues to accelerate (recoil) rearward long after the bullet has left the muzzle." Hmmmm. I don't think so, not after the mass of the bullet and the gases have left the barrel.



Most correct insofar as the word accelerate, and I retract that. But the mass of the firearm, once set in motion, continues in motion. Again, we can best see this exaggerated demonstration with a handgun. Observe the severe muzzle flip--certainly you've seen photographs or experienced it yourself. Often, the muzzle ends up pointing skyward at a 45-degree angle, or even as much as straight up. Yet clearly, the bullet has been launched on a (fundamentally) horizontal path--clear evidence to even a dolt like me that the recoil cycle endures long after the bullet has left the firearm.



Sharp observation, though. Thanks for the correction and keeping me (and everyone else here) honest.



--ML



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Re: A Couple of Answers

Posted By: Birdog - Posts: 4108

Posted At: (3/13/04 11:53 am)

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Thankjs for the excellent replay, but you bring up another question, What is a crowned barrel? How does it affect accuracy?



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Muzzle Crowns

Posted By: ML - Posts: 490

Posted At: (3/15/04 10:46 am)

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Muzzle Crowns



To answer Birdog's question, muzzle’s crown is the shape applied to the junction of the bore/barrel terminus. Now, that’s a bit of a mouthful, but it’s something you can easily observe for yourself with a rifle, a handgun, and many shotguns.



In theory, a firearm’s bore should be absolutely centered in the barrel, and absolutely perpendicular to the end of the barrel. In reality, that is often not the case, and, even if it were, the slightest ding or nick here could damage this critical bore/muzzle interface. Remember, this interface is the very last contact the bullet has with the weapon as it’s launched on its trajectory, and any instability or irregularity imparted here will act to sabotage the bullet’s true flight all the way to the target.



This being the real world, we must do something to protect this interface. And the solution that has evolved over time has proven quite practical and elegant: simply countersink this junction below the very end of the barrel.



Sometimes this is an angular countersink, a chamfer really, that breaks the sharp, 90-degree bore/end-of-barrel junction. Note that this will also move the true bore itself just slightly below the end of the barrel—we’re talking about 0.040 of an inch or so, enough to be easily seen with the naked eye or felt with a fingertip, but a subtlety likely to go unseen unless you’re looking for it to be sure.



On handguns—especially revolvers—a rounded crown is often used. Here, the "countersink" curves from the bore outwards, and then curves back towards the barrel’s exterior. Again, this serves to protect the bore’s terminus by providing a beefy "ring" of barrel material around it.



While you’re looking at your firearm’s muzzle (unloaded first, please!) here’s a little note of historic trivia. Oven the past couple of decades, I’ve examined the bores of quite a few Mosin-Nagant and Mauser rifles used in the Second World War. Many of them have had their bores counterbored back as much as an inch or more. We’re not talking about a subtle crown here—we’re talking about running a reamer down the bore big enough to completely remove all the rifling and open the bore up significantly. Why?



First, consider this: One of the reasons we clean firearms from the breech is because we lazy humans have a nasty habit of dragging grit-laden cleaning rods up against the muzzle end of the rifling, ovaling-out the bore at the muzzle.



Second, many of these arms saw hard service under field conditions for many years. Any soldier soon learns that dirt, snow, and moisture which gets into the barrel soon ruins the bore and attracts unkind words from your sergeant. So both soviet and German troops often plugged their Mosin-Nagant or Mauser Kar98k muzzles with small bits of fabric. Unfortunately, this fabric wicked and held moisture, even in the hot Russian summers.



Between the plugged-muzzle-induced rust and the out-of-round condition caused by prolonged poor in-field cleaning, many of these rifles were in trouble by 1945. Simply trimming the barrel back an inch or two and recrowning would have solved the problem, but since all of these weapons utilized iron sights, were set up to take a bayonet, and were in fact military arms (were everything must be uniform), the Russians used this deep counterbore to clean them up and maintain serviceability. (Every Kar98k I’ve seen counterbored in this fashion has been an arm captured by the Russians and evidencing other signs of soviet-arsenal overhaul.)



A quick little trip through Google turns up this website with a couple of good muzzle-crown photos which will no doubt help clarify my murky prose:



riflestocks.tripod.com/targetcrown.html



Hope that clarifies the waters a little,



--ML
woodsdrummer.com



"In the school of the woods there is no graduation day" Horace Kephart


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