Is a barrel's resonant frequency static?

[Max Heat]

Does a barrel's resonant frequency change at all with variations in things like powder, bullets, temperature, & humidity? Or is it based purely on it's physical attributes, like length, diameter, taper, weight, etc? I'm also wondering if the reason for having taper is to help dampen the resonance.

I caught an episode of mythbusters several weeks ago, where they had a high-speed camera looking right down the muzzle of a sniper shooting a heavy barreled .338 lapua. You could see the barrel flexing up-and-down significantly when a round was fired. I would have never thought that such a heavy barrel would bend that far out of straightness,. But seeing IS believing. Photographically speaking, that shot only lasted maybe 2 or 3 seconds, but it caught my attention so much that it replays perfectly in my mind, every time I think about it.

 

[BillR]

Does a barrel's resonant frequency change at all with variations in things like powder, bullets, temperature, & humidity? Or is it based purely on it's physical attributes, like length, diameter, taper, weight, etc? I'm also wondering if the reason for having taper is to help dampen the resonance.

I caught an episode of mythbusters several weeks ago, where they had a high-speed camera looking right down the muzzle of a sniper shooting a heavy barreled .338 lapua. You could see the barrel flexing up-and-down significantly when a round was fired. I would have never thought that such a heavy barrel would bend that far out of straightness,. But seeing IS believing. Photographically speaking, that shot only lasted maybe 2 or 3 seconds, but it caught my attention so much that it replays perfectly in my mind, every time I think about it.

A few years ago on Benchrest Central Bill Calby who is a small bore gunsmith made the statement that he could stop a barrel from doing just that by the use of a what I call a harmonic balancer on the end of a barrel. This was a piece that went on the end of the barrel and was threaded so a weight could be added to it and by turning it in and out you could find a sweet spot that turn the barrel into what is basically called a hummer that shot great no matter what load just by adjusting the weight to match the load rather than adjusting the load to match the gun. Thats kind of a short sweet version of the whole thing. Bill was a bit short on his info and wanted someone else to develop it and said he didn't have the time and wanted someone else to build it and make it work. He got laughed at a lot but in the end a couple believers built some to try out and found that they worked to a degree but I about that time I got into long range shooting and kind of let my directions change and don't really know where that all went. They are used on small bore rifles and do work but any more than that I can't tell you. I think there are still some articles on Benchrest Central about how the barrel will whip and info on what is happening when you are changing a load to match the guns harmonics and finding what is called the sweet spot. It would of been nice is someone had more money than I had and the time to work on it because it is a pretty good idea that Bill C put foreword. It would of been a case of finding your fastest load and then tuning the weight to match that load. Just never found out if it worked or not.

 

[Nimrodmar10]

Sims Vibration, the folks that make the Limb Saver Recoil Pad, make a rubber barrel deresonator that slips on the barrel that does this. You move it up and down the barrel to find the sweet spot.
Browning/Winchester makes their "Boss" System. It's an adjustable muzzle brake that is tuned for the load you shoot. So I'm guessing that the barrel resonance changes anytime you change anything on the gun or load.

 

[Trickymissfit]

A few years ago on Benchrest Central Bill Calby who is a small bore gunsmith made the statement that he could stop a barrel from doing just that by the use of a what I call a harmonic balancer on the end of a barrel. This was a piece that went on the end of the barrel and was threaded so a weight could be added to it and by turning it in and out you could find a sweet spot that turn the barrel into what is basically called a hummer that shot great no matter what load just by adjusting the weight to match the load rather than adjusting the load to match the gun. Thats kind of a short sweet version of the whole thing. Bill was a bit short on his info and wanted someone else to develop it and said he didn't have the time and wanted someone else to build it and make it work. He got laughed at a lot but in the end a couple believers built some to try out and found that they worked to a degree but I about that time I got into long range shooting and kind of let my directions change and don't really know where that all went. They are used on small bore rifles and do work but any more than that I can't tell you. I think there are still some articles on Benchrest Central about how the barrel will whip and info on what is happening when you are changing a load to match the guns harmonics and finding what is called the sweet spot. It would of been nice is someone had more money than I had and the time to work on it because it is a pretty good idea that Bill C put foreword. It would of been a case of finding your fastest load and then tuning the weight to match that load. Just never found out if it worked or not.

that would be Bill Calfee, and he lives about 85 miles due south of me. Hope he made it thru those tornados down there. Folks laughed at him, and they also had their lunch ate by him. The trick is to find the node in the barrel, and build off that point. I built several tuners a few years back, and they seriously do work. Basicly a tuner simply changes the point at which the sonic wave exits the barrel. We put together a Cooper 22 rimfire that had the factory barrel with a 52D chamber. Out of the box it was a solid 3/8th" gun at fifty yards. With a new chamber it was shoot high twos and very low threes. With the tuner it shot groups in the very low twos and a few in the high ones. Best groups were around .170". His Saturday night bench rest shoot is a lot of fun with a great bunch of guys. But you'd best have your stuff together cause all these guys can shoot the eyes out of a snake at fifty yards
gary

 

[Trickymissfit]

Sims Vibration, the folks that make the Limb Saver Recoil Pad, make a rubber barrel deresonator that slips on the barrel that does this. You move it up and down the barrel to find the sweet spot.
Browning/Winchester makes their "Boss" System. It's an adjustable muzzle brake that is tuned for the load you shoot. So I'm guessing that the barrel resonance changes anytime you change anything on the gun or load.

it does change a little bit. I remember the difference between Greentag and Federal Gold Medal was roughly six clicks on the tuner. There was a few guys fooling around with tuners on centerfire rifles over in the St. Louis area, but how well they worked I don't know. I'f talk to Time Precision to see what they have to say.
gary

 

[REDHEAD]

I was understanding it as the timing of the bullet to leave the barrel. The same is done by adjusting your load -faster/slower for the bullet to exit the barrel at the zero point of the sine wave.

 

[Max Heat]

Thanks for the info. So the load WILL affect the "tune" of the barrel. The "flashbacks" that I am seeing (in my mind) of that .338 barrel, show that it was quivering vertically, having 2 "crosspoints", each one being about 1/3 of the way from each end of the barrel. As the center flexed up, the ends flexed down, and visa-versa.

Ever wonder what that "fat spot" is, roughly close to the center of a (M1) battle tank's main gun barrel. Well, you know what is has got to be, right. But in addition to being able to be adjusted up or down the length of the barrel, it looks like it can also be adjusted radially, around the circumference of the barrel, as there is an off-center bulge on it. I'm sure that the amount of weight in it can also be fine-tuned. Since they run smooth-bore to achieve the maximum the velocity possible, apparently it needs to be tuned to the hilt, to maximize it's accuracy.

Correct me if I'm wrong, but I believe that EVERY barrel "walks" it's fired rounds in a particular direction, as it gets heated up. I'm guessing that might be what the off-center weight is supposed to compensate for. But I've never seen anything like that on a rifle barrel.

 

[Trickymissfit]

I was understanding it as the timing of the bullet to leave the barrel. The same is done by adjusting your load -faster/slower for the bullet to exit the barrel at the zero point of the sine wave.

techinally your right, but that's also just part of it. The barrel is moving in two directions at the sametime as the bullet is traveling thru the bore. Then after it there is a like reaction doing the opposite behind the bullet. Also there is a shock wave in front of the bullet, and another behind it. If you ran all this thru a mechanical CAD program that allows you to input the variables; it would scare you to death!
gary

 

[Trickymissfit]

Thanks for the info. So the load WILL affect the "tune" of the barrel. The "flashbacks" that I am seeing (in my mind) of that .338 barrel, show that it was quivering vertically, having 2 "crosspoints", each one being about 1/3 of the way from each end of the barrel. As the center flexed up, the ends flexed down, and visa-versa.

Ever wonder what that "fat spot" is, roughly close to the center of a (M1) battle tank's main gun barrel. Well, you know what is has got to be, right. But in addition to being able to be adjusted up or down the length of the barrel, it looks like it can also be adjusted radially, around the circumference of the barrel, as there is an off-center bulge on it. I'm sure that the amount of weight in it can also be fine-tuned. Since they run smooth-bore to achieve the maximum the velocity possible, apparently it needs to be tuned to the hilt, to maximize it's accuracy.

Correct me if I'm wrong, but I believe that EVERY barrel "walks" it's fired rounds in a particular direction, as it gets heated up. I'm guessing that might be what the off-center weight is supposed to compensate for. But I've never seen anything like that on a rifle barrel.

the barrel on an M1 tank (120mm) is a smooth bore, and it still moves. Better yet look around for a video of a 175mm gun (M107) shooting a zone three charge out there about twenty miles. That barrel moves all over the place, and even has a distinct "wiggle as the shock wave travels thru the barrel. Barrels on a typical howitzer are pretty stable, and a typical 8" howitzer was known to hit garbage can lids at 9 miles; one round after another.
gary

 

[Max Heat]

That's some pretty **** incredible accuracy to consistently hit a garbage can lid at 9 miles, without having any GPS or inertial guidance! And howitzers aren't even direct-fire weapons, right? But they do have the benefit of rifling, to keep the projectile on it's intended flight path.

I'm trying to remember what the velocity of the fin-stabilized kinetic anti-armor rounds (tungsten darts) that the M1 fires is. I think it's something like 5,800fps, which is close to the theoretical limit for chemically based propulsion in a gun.

I was wondering, is zone# related to the # of bags of powder behind the projectile, on the 175mm gun?

BTW, did anybody else catch that episode of mythbusters? They were testing the myth that if one can see the muzflash of a sniper's bullet, they could dodge it. It WAS busted, because a human would have to be about 500 yards away to be able to dodge it, but the flash cannot even be seen by a human at 200 yards. And that is without a flash supressor or muzzel brake.

The sniper's shots were VERY impressive though, but he did have the benefit of a spotter (not sure if they were soldiers or marines though). At a distance of 1.200 yards, the shot was perfect in elevation, where he needed to use 30 feet of holdover, and windage was only off by about 3 to 4 inches (to the right).

 

[Trickymissfit]

That's some pretty **** incredible accuracy to consistently hit a garbage can lid at 9 miles, without having any GPS or inertial guidance! And howitzers aren't even direct-fire weapons, right? But they do have the benefit of rifling, to keep the projectile on it's intended flight path.

I'm trying to remember what the velocity of the fin-stabilized kinetic anti-armor rounds (tungsten darts) that the M1 fires is. I think it's something like 5,800fps, which is close to the theoretical limit for chemically based propulsion in a gun.

I was wondering, is zone# related to the # of bags of powder behind the projectile, on the 175mm gun?

BTW, did anybody else catch that episode of mythbusters? They were testing the myth that if one can see the muzflash of a sniper's bullet, they could dodge it. It WAS busted, because a human would have to be about 500 yards away to be able to dodge it, but the flash cannot even be seen by a human at 200 yards. And that is without a flash supressor or muzzel brake.

The sniper's shots were VERY impressive though, but he did have the benefit of a spotter (not sure if they were soldiers or marines though). At a distance of 1.200 yards, the shot was perfect in elevation, where he needed to use 30 feet of holdover, and windage was only off by about 3 to 4 inches (to the right).

** there are two sabot rounds fired in the M1 tank. One is more or less a practice round with about 25% less velocity, the other is about 5800 fps. The German Leopard tank uses the same round, but has a different barrel for a little more velocity. High velocity rounds are not new, and have been around for almost fifty years. The first true high velocity anti tank round was the 75mm PAK 40, and this later migrated into the PZKW IVF2 in N. Africa. (the older MK. IV also was a 75mm bore, but far less powerfull). Later that same basic bore was kept on the Panther, but they used a more powerfull verson. Then an even more powerfull one was developed. There was an 88mm antitank round that had a 4600 fps , and was really hard on just about anything in that time period (PAK 43). These guns would evolve to use tungstin cored ammo, and could crack the hull on any track on this planet.

** 175 guns are kinda odd in their ways. They do not call out powder charges like the others, but there are three charges anyway. They are zone one thru zone three. A zone three charge is about a foot in diameter, and five feet seven inches long!! The round looks just like a scaled up Hornaday 7mm AMAX bullet, and is pretty much proportioned just like one. In otherwords a very high B/C. The round weighs 147 lb. and travels faster than the 53 grain bullet fired in an M16 (3300fps or 3600fps I can't remember). The barrel is 32 feet long and weighs 19,000 lb.! Has 1.5" of droop in it when level with the earth. Barrel life is about 80 rounds or less. The round does not contain TNT, but uses Comp B. Comp. B is much more powerfull. Lots of folks have been killed while shooting a 175 gun from breech explosions and seperations of the breech from the barrel.

** prior to the advent of the M110, the most accurate piece of arty in the world was the 8" towed howitzer. When the military did away with it, they replaced it with the M110 SPG. The towed gun was even more accurate due to it better three point platform. The SPG is well known all over the world for it's built in accuracey. The idea of a three foot diameter hit at nine miles has been proven many times over. Most of those tests were done with the tracks setting on hard packed clay and the spade dug in deep. A looser wet soil would open up the groups a little bit, but the howitzer would settle in by the third or fourth round. A 155mm towed howitzer is good for about a forty foot circle at 8.5 miles if everything is near perfect, and have seen howitzers hold a 25 foot circle using contact lots of powder and projectiles (in otherwords one crater). Towed guns are almost always more accurate due to their better three point suspension (although a typical SPG uses the same idea). Now I've not been around the newer M198 series or the M777, so can't say alot about them. The 198 looks far better than the 777.
gary

 

[Catfur]

The "bulge" on the barrel of a tank cannon is called a fume extractor, it's purpose is to prevent the toxic combustion gasses from pushing out of the breech after stub base (M256 rounds are combustible case, but the base with the primer is metal) is ejected. The extractor is hollow, and there are gas ports in the barrel that point backwards towards the hollow of the extractor. When the round is fired, and the projectile passes the extractor, gas passes through the ports and pressurizes the interior of the extractor. When the round exits the muzzle and the pressure in the barrel drops, the high residual gas pressure in the extractor generates a puff of gas which is directed towards the muzzle, generating an airflow through the barrel towards the muzzle and venting the fumes. The extractor cannot be adjusted either radially or up and down the barrel, the large nut is there to allow it to be removed and cleaned out. Some older Soviet tanks had the fume extractor located at the end of the barrel, but all modern designs I am aware of have the extractor located in the middleish. BTW, I'm pretty familiar with this as I was a tank crewman in the Army, and have cleaned plenty of these things (every time you fire it).

M256 Rhinemetall tank cannons do not have powder bags, they are utilize a round much like a rifle or pistol round, only the case walls are combustible (nitrate impregnated cardboard), the case base is metal. The practice rounds for sabots are cone stabilized, and are actually lighter (made from steel, real penetrators are made from depleted uranium)and a little faster than warshots (real rounds) but won't fly as far, and will destabilize when the round goes subsonic.

 

[Trickymissfit]

The "bulge" on the barrel of a tank cannon is called a fume extractor, it's purpose is to prevent the toxic combustion gasses from pushing out of the breech after stub base (M256 rounds are combustible case, but the base with the primer is metal) is ejected. The extractor is hollow, and there are gas ports in the barrel that point backwards towards the hollow of the extractor. When the round is fired, and the projectile passes the extractor, gas passes through the ports and pressurizes the interior of the extractor. When the round exits the muzzle and the pressure in the barrel drops, the high residual gas pressure in the extractor generates a puff of gas which is directed towards the muzzle, generating an airflow through the barrel towards the muzzle and venting the fumes. The extractor cannot be adjusted either radially or up and down the barrel, the large nut is there to allow it to be removed and cleaned out. Some older Soviet tanks had the fume extractor located at the end of the barrel, but all modern designs I am aware of have the extractor located in the middleish. BTW, I'm pretty familiar with this as I was a tank crewman in the Army, and have cleaned plenty of these things (every time you fire it).

M256 Rhinemetall tank cannons do not have powder bags, they are utilize a round much like a rifle or pistol round, only the case walls are combustible (nitrate impregnated cardboard), the case base is metal. The practice rounds for sabots are cone stabilized, and are actually lighter (made from steel, real penetrators are made from depleted uranium)and a little faster than warshots (real rounds) but won't fly as far, and will destabilize when the round goes subsonic.

ammunition shot in most tanks is known as "fixed ammunition", and is primarly designed for direct fire (line of sight). There has been some experimentation in the past with tanks shooting indirect fire (last I heard was with the M60), but the real issue with that is that tanks do not use a perfect plain to shoot off of (three point suspension). That's why we have self propelled guns. Just a completely different application of a round being shot. SPG's as a rule do not need super high velocity rounds like a tank does, but they must have a very precise form of location (and readjusting) fire because they usually can't see the target. Direct fire from an artillary piece is not very good setup, and one usually prays he never has to use that method (I have several times).

I cannot tell you what's in the works for both arty and antitank, but I can say that the day of the sabot is numbered. They have a round right now that will go thru 4" of Chobham armor like a punch press at 4K yards effortlessly. Generated recoil is a very small fraction, and breech life will be longer than the life of the barrel. Also the chance of a fire inside the turret is reduced to close to zero. You also will never ever need to clean a bore vac again! (I well remember cleaning that 300lb. chuck of iron on an M109)

Just a side note:
anti personell has almost always been with a 50 caliber machine gun or a beehive round when folks are closer than 300 yards. Kind of a given fact of life. But there was one odd ball exception to this rule. The antipersonell round for a 175 gun was to simply lower the barrel to a minus three degree (somewhere in that area anyway). Load a zone three charge without the round, and pull the lanyard. The ball of fire and blast will make anything have a second thoughts. I like beehive rounds
gary

 

[Bart B]

Some myths are mentioned in this thread. I'll try to dispell a few.

A barrel whips, vibrates, wiggles at one frequency based on its weight, caliber and profile. Doesn't matter if pipsqueak loads or elephant busters are used, the barrel vibrates at the same frequency for every shot. Most centerfire rifle barrels have a resonant frequency at 30 to 100 Hz (cycles per second). That's the frequency that causes the greatest angular displacement at the muzzle's bore axis. Harmonics of this fundamental are 2, 3, 4, 5, 6, 7 and 8 (and more) times higher in frequency, but they wiggle the barrel much less in amplitude and are, for all practical purposes, negligible.

A weight on the muzzle changes the frequency a barrel vibrates at 'cause the barrel is now longer and heavier. These are popular on smallbore rimfire rifles 'cause they do work. But not needed if your particular lot of .22 long rifle ammo shoots no worse than 3/10ths MOA at 50 yards and 7/10ths at 100; that's as good as today's ammo and barrels produce.

That basic frequency doesn not change as the barrel heats up in most shooting events. The barrel will droop from softening from high temperatures when it does. If it's that hot, the cartridge will fire as soon as its chamberd; probably before the bolt's closed.

Properly heat treated and stress relieved barrels fit to receivers whose face has been squared up with the chamber axis do not change point of impact as they heat up. Too many folks testing handloads in match rifles using machine rests shooting 1 shot every 30 to 40 seconds putting 20 to 40 bullets inside 1/2 MOA at 600 yards kind of prove this. Arsenals testing 30-06 and 7.62 NATO match ammo shoot a couple hundred shots per test group in as many minutes or less. Ever seen a 200-shot group of bullet holes about 6 inches in diameter that was fired at 600 yards? High power long range competitors often put 25 to 30 rounds into a 1000 yard target during a 30 minute time limit for the match. They don't have a zero change; none whatsoever. Such events are shot with heavy, stiff "bull" barrels on some rifles and shorter, flimsy servie rifle barrels in others. They all shoot to the same point of aim as they heat up so darned much you'll burn your pinkie if you touch one; especially the service rifle ones. They get so hot a round left in the chamber for more than 15 seconds will heat the powder up enough to shoot the bullet out faster and it'll strike high.

Why do so many factory rifles walk bullet impact as they heat up? Their receiver face ain't square with the chamber axis and as the barrel heats up, extra pressure at the high point caused the barrel to whip in a different direction. The hotter it gets the more the barrel and receiver steel expand putting more pressure at that point making bullet go even further away from point of aim. And if the barrel's not properly stress relieved, that just add more impact error as it makes the barrel bend.

To summarize, rifle barrels are like steel strings on a musical instrument; guitar, violin, mandolin, banjo or whatever. For each one without a finger pressing it against a fret, plucking it makes one note; it vibrates at one frequeny. Doesn't matter if a fly barely plucks it with its left middle leg or an elephant swats it with its trunk. One will make a barely audible sound and the other very loud. The note's the same but the volume's different because it's whipping at different amounts. Press that string against a fret and its effective length is now shorter and it vibrates at a higher frequency.