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Discussion Starter #1
First let me just say ballistics is not my strong suit.

I am looking for a mathematical equation to solve the following problem.. How much barrel length could you have before the drag from the rifling actually lowered the muzzle velocity?

My question pertains to .270 Winchester with factory loads.
 

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First let me just say ballistics is not my strong suit.

I am looking for a mathematical equation to solve the following problem.. How much barrel length could you have before the drag from the rifling actually lowered the muzzle velocity?

My question pertains to .270 Winchester with factory loads.

This is a good question for Uncle Nick. I'm sure he"ll have an answer.
My favorite rifle is a 270 Win with a 25" custom barrel. That is my "beanfield rifle" and has great accuracy. I would have gone with a longer barrel, but then it becomes difficult to move around in a hunting blind.
There's a trade off to everything.
 

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I think your question is pretty much impossible to answer. You have too many variables: Bullet weight, powder burn rate, powder charge, and you even have to factor in the fact that no two barrels shoot the same because of minute bore differences even being turned out by machine.
 

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Ultimately, it's going to be one very very long barrel.

Might find more data on muzzle pressure, which would be the force opposing barrel friction.

If it takes 15-20" of barrel for the tiny 2gr. charge in a .22LR to start showing friction being worth more than barrel pressure, then the length needed for a case burning 35X as much powder is going to end up being mighty long.

A little help...remember reading some estimates by HAtcher, but don't have the books now to run that down.
 

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Geoffry Kolbe uses 19" as typical for .22 LR. That bullet has such a tiny powder charge that it just runs out of pressure adequate to overcome friction much beyond that length.

QuickLOAD has a barrel friction calculator that defaults to the start pressure value, which is actually low when the pressure is near its peak, and high near the muzzle of most guns, but probably isn't an unreasonable average for normal length. If you run the calculator you find that high power rifle cases, like the .270, hold so much powder and make so much gas that pressure at the base of the bullet maintains enough force to overcome friction for a long distance. You typically end up with break-even (no acceleration but no velocity loss) at numbers close to 50 inches of barrel. Nothing is commercially available that will get you to that length with the .270.

In QuickLOAD I put a 130 grain Hornady SST over a compressed load of IMR4831 in the .270 that generates 60,000 psi. I set the long barrel friction up and gave it a 100 inch barrel. I can see velocity peaks at 3479.8 fps at 51.5 inches. That is dragged down to 3452.4 fps at 70 inches (so the loss is quite slow past the peak). At 100 inches of barrel it drops to 3312.7 fps. Still respectable.

Worst case should be a lower pressure charge of fast powder under a heavy bullet, so I ran 32 grains of IMR4198 under the 160 grain Nosler Partition. That produces about 41,000 psi peak pressure. That load's velocity peaks at 2458.5 fps at 36.5 inches of barrel length in the program.

You'll notice that Palma match rifles often have 32" barrels. Broomstick rifles, they are called. The reason for that length is their 155 grain .308 loads are still gaining velocity even that far along, and they need all they can get to keep going fast to 1000 yards. And that's probably about as long as you could order a barrel in from most makers. I'm sure if there were any real advantage to 36" over 32" they'd be using those, but I expect there is not. And you have to worry about barrel whip getting worse and worse.

But as far as velocity alone is concerned, if you stay under 36" in the .270, it looks to me like you'll be velocity ahead with any normal load and a lot of not-so-normal loads.
 

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I seem to recall reading something that said barrels in the mid 30" length start showing diminished returns. I'll see if I can find it.
 

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Discussion Starter #7
Thanks Uncle Nick, thats what i needed to know. I built a .270 with a 26.75 inch barrel for a friend, he wanted something bigger than anyone else had, i don't have a chronograph and a couple of bench rest types at the range i went to, to do barrel break in, were telling me the barrel was so long i was hurting velocity, as a novice to the math behind ballistics i didn't know if they were right.
 

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They were right in principle, but wrong in scale. The 32" Palma barrels are the best example to offer up to them for comparison. The main problem with long barrels is they tend to be more picky in their load preference because they are whippy if you don't make them thick, and if you make them thick, they are heavy. Heavy is fine for Palma, where everything is fired from prone position, but is not so nifty for snap shooting in the woods.
 

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Discussion Starter #9
They were fright in principle, but wrong in scale. The 32" Palma barrels are the best example to offer up to them for comparison. The main problem with long barrels is they tend to be more picky in their load preference because they are whippy if you don't make them thick, and if you make them thick, they are heavy. Heavy is fine for Palma, where everything is fired from prone position, but is not so nifty for snap shooting in the woods.
LOL whippy wont be a problem here, the barrel is no contour.

Thanks for the info and help.
 
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