Barrel Vibrations Hypothetical

[Tony C]

A friend posed the following scenario and I thought I'd pass it along to the experts:


A thought occurred to me the other day: What if one REALLY exaggerated the "bull" barrel! Take a cylinder of steel 30" long and 30" in diameter. Chamber it and rifle the barrel, taking care for a perfect crown. For all practical purposes, harmonic vibes would be out of the picture. If they occur, increase the diameter to 72". What would accuracy be like with this huge barrel?



Impractical, but any thoughts?

 

[gzig5]

It would be a bear to move from the car to the bench. Other than that, they should all go in the same hole, right?

 

[Vibe]

It would be a bear to move from the car to the bench. Other than that, they should all go in the same hole, right?

Nope
It's been done (more or less) - I think it was Hatcher that cast a barrel in a solid concrete wall. SD velocity spread resulted in vertical as I recall.

 

[Old Gunner]

Nope
It's been done (more or less) - I think it was Hatcher that cast a barrel in a solid concrete wall. SD velocity spread resulted in vertical as I recall.

Sounds right, with zero vibration they'd be no compensation factor.

 

[Tony C]

Sounds right, with zero vibration they'd be no compensation factor.

Old Gunner

Can you tell me a little more about compensation factor?

Thanks

 

[Old Gunner]

Old Gunner

Can you tell me a little more about compensation factor?

Thanks

I don't know a whole lot about it myself, but the general principle is that when a barrel is bedded properly and the load best suited is used the bullets at the lower end of the velocity spread will leave the muzzle on the up swing of the vibration, while those at the higher end of the spread leave at the lower end of the swing.
The difference is tiny but the result is that beyond a certain distance the tiny change in elevation allows the bullets to group better in the vertical plane.

This is a factor well researched in relation to the SMLE rifles with its slim barrel and complicated bedding procedure with spring loaded bands and plungers that tuned the barrel to make up for velocity deviations of the military ammo.
A rifle might show some vertical spread at 100 yards but print a tighter group at two hundred and even tighter at 300+ till vertical spread was equalized.

To some extent the same would apply to any barrel no matter how heavy, at least in principle.
Choosing a load that matches the vibration pattern and intensity of a specific barrel profile might be a matter of calculation, but it seems like something more likely to be sucessfull if done in the tried and true by guess and by golly method.

 

[Tony C]

Vibe and Old Gunner

By looking at your explanations, the problem in the hypothetical is not the barrel itself, but in the velocity spread encountered in the real world and how to compensate for it.

Thanks Again

 

[Boyd Allen]

A barrel's length and weight distribution are what cause its rate of "swing", determining changes on where it is pointing on a target at any given instant. This, combined with the bullet's velocity, determines where, in that cycle, the bullet will exit. If the bullets leave on the up slope of the "where it is pointing" curve, where it flattens out, just before the peak, faster bullets, that exit sooner, which will fall less on their way to the target, will be pointed slightly lower than slower bullets, that will drop more, thus reducing the vertical spread between their respective bullet holes. Varmint Al is the source on this. His work on this has been the key to understanding the subject.

 

[Old Gunner]

Vibe and Old Gunner

By looking at your explanations, the problem in the hypothetical is not the barrel itself, but in the velocity spread encountered in the real world and how to compensate for it.

Thanks Again

Yep but you mentioned hypothetical barrels not the impossible to acheive hypothetical loads without any measurable variation in velocity.

Without any variation in velocity or other factors a bull barrel would be un necessary anyway, even the hypothetical pipe cleaner barrels would print single hole groups since every shot would cause the barrel to react in exactly the same way. Heating of the barrel steel aside that is, and this could be gotten around by firing only when the barrel was at a constant fixed temperature.

A barrel actually lengthens very slightly when heated, it also expands very slightly in diameter, both the bore and external dimensions.
A barrel also lengthens ever so slightly under the pressure of firing.
A huge barrel like the one you describe would act as a heat sink, and its bulk would prevent any measurable effect from pressures.

The main reasons given for use of a bull barrel are its heat sink properties, and in the early days of barrel making the greater mass offset the effects of any wandering of the bore during manufacture. The Mauser Stepped Barrels were designed to compensate for these factors nearly as well as a bull barrel while being only a bit heavier than a tapered barrel.

 

[mks]

A barrel's length and weight distribution are what cause its rate of "swing", determining changes on where it is pointing on a target at any given instant. This, combined with the bullet's velocity, determines where, in that cycle, the bullet will exit. If the bullets leave on the up slope of the "where it is pointing" curve, where it flattens out, just before the peak, faster bullets, that exit sooner, which will fall less on their way to the target, will be pointed slightly lower than slower bullets, that will drop more, thus reducing the vertical spread between their respective bullet holes. Varmint Al is the source on this. His work on this has been the key to understanding the subject.

Boyd,
A very nice and concise explanation, that I would modify in only one small way. Near the peak, the "where it is pointing" curve may not change fast enough to fully compensate for different muzzle velocities/ barrel exit times. It is more likely that one wants to be in the middle of the upswing, where "where it is pointing" changes the fastest. Varmint Al's simulations show that even there, the upswing may not be fast enough to eliminate vertical from this source of dispersion.

Cheers,
Keith

 

[tim in tx]

all very good points and might add

if you are not worried about the barrel heat a thinner barrel actually can stay in tune for longer then a bull barrel , your actually using dispersion or flex from vibrations to correct a wider range of velocity variance hence a wider tune. a thick barrel will flex less so you would need to have level velocites which in it self widens the tune . others may not agree this and i will tottally respect there beliefs.but this was just my testing at home. mks is correct in that the middle of the upswing if it proves a constant tune ,however somtimes in along the upswing there will be inconsistancies ussually right in the middle of the upswing. i believe high frequency ripples which can basicly be seen on target as going out of tune a slight bit along a rise of temperature swing,so in boyds respect towards the top of the swing might a safer area of tune for a given barrel if the velocites are not level. hope this helps. tim in tx