Help. round barrel to octagon

[nekshot]

I am in a med freeze of my brain. Would size is the flat when using a 1 inch barrel and turning it into a octagon barrel? I realize a bad cut too deep and my project is toast!

[Texas by God]

Give me a minute to check something......

[Cap'n Morgan]

Width of flats .3826
Measurement over flats .9240

[Ballistics in Scotland]

You can find calculators online for most regular figures. This one gives area, which is useful for calculating the weight. You jut subtract the area of the bore cross-section πr2, and multiply by the barrel length. Then if the bore radius is in inches, multiply by 4.5 for ounces per cubic inch, and you won't be far out for the weight in ounces.

http://www.had2know.com/academics/octagon-measurement-calculator.html

It takes a very long milling machine to do this with perfect alignment in the middle. You can do it on a powerful and coarse bench grinding machine. preferably with a couple of pieces of angle iron and a couple of clamps or vices to keep the alignment. However you do it, remove a little from each side in turn, so as not to warp it with one-sided heating.

[Moleman-]

I have one of the HF drill/mill machines that had enough travel to mill the flats on the barrel for my 1891 mauser, but the table wasn't big enough. Ended up bolting a 3/4"x4" steel bar to the table, then bolted the indexer and tailstock to the bar. Took many passes with an index between each pass but the barrel turned out perfect. Thank goodness for power feed.

You'll also loose some of that 1" diameter as the bore will not be perfectly centered in the blank.

[Texas by God]

I came up with .375" across the flats. Cap Morgan is probably right I bet. My figure came from the circumference of a 1" rod (3") divided by 8 (octagon)=.375"
I could be and frequently am wrong so I assume no liability other than the ability to lie.
Best, Thomas.

[nekshot]

Thanks fellas. I knew this would be candy to somebody and I got tired of drawing and cutting out octagon shapes that were miss shaped. Now after the fog clears I will go forward with this project.

[nekshot]

Moleman I really like the look that barrel gives the rifle you have there.

[country gent]

I have only did one octagon barrel and I did it in a big milwalkee horizontal mill. Its what we had in the shop. It had a double 1" black oil coolant flow that kept every thing cool and lubricated. set it up in a dividing head and tail stock with a screw jack in the middle to support it.
Light cuts on opposite surfaces to keep heat and stress down is a big plus. Takes longer but the end results are better. I prefer to most of the contour before drill ream and rifling ideally.

[Moleman-]

Thanks nekshot. Saw the rifle in the local gun shop a while back for $100 with a barrel from a 6.5 carcano on it. Thought it odd that someone had put a new front sight, Fajen stock, rear peep sight on then decided to fit a frosted and pitted carcano barrel on it. The was one of the early carcano barrels with the hex just in front of the receiver that hadn't been all turned off. There was a Brescia arsenal stamp on what was left of one of the hex flats, the chamber end was bright as were the threads where as the rest was cold blued over deep pitting. The receiver is also well marked as Argentine, so wasn't one of the 1891's that were bought by the Italians. I have more money into the barrel blank than the rifle. I know it's not correct, but I really line those fancy German hunting rifles from the late 1890's and early 1900's with the octagon barrels. Chambered it in 44x1.8 with a reamer I made. My next octagon project will either be a 444x1.8" barrel for my 444 marlin or a 357Max barrel for my 336 so I can get back to using them for deer hunting in the Limited Firearms Zone here in MI.

[Ballistics in Scotland]

I came up with .375" across the flats. Cap Morgan is probably right I bet. My figure came from the circumference of a 1" rod (3") divided by 8 (octagon)=.375"
I could be and frequently am wrong so I assume no liability other than the ability to lie.
Best, Thomas.

The circumference of a circle one something in diameter is π , i.e. about 3.1416 somethings, and an eighth of that is 0.3927 of a something. Even that would be the vanished curved surface of each long sliver of metal you have milled away. The flat surfaces, and thus the flat of the barrel, would be 0.3827 somethings, whether they are inches, millimetres or miles.

In the days before online calculators, it was easy to cut nine paper squares. Fold four of them corner to corner to make right-angled triangles, and you can assemble them to make an octagon from which you can do the calculations.

[Bent Ramrod]

The calculations for generating polygons inside and outside of circles of a given diameter are in Machinery's Handbook.

I never tried to turn a full octagon barrel to round. I would turn two sections at the same taper, the larger diameter first one the usual third or so of the full barrel length. If the outside circle was, say, 1" diameter (0.5" radius), the start of the round part of the barrel would be 2 x 0.9239 x 0.5", minus, say, ten thousandths or so for luck.

This would generate a flat of 0.8284 x the 0.5" radius of the barrel when the octagon was finished. I did, of course, set the barrel up on the horizontal mill so the taper of the barrel was horizontal to the cutter, with jacks under it so no major sagging would occur during the cutting. I would turn the dividing head 1/8 of a turn and take off ten thousandths at a time, resetting the jacks as the stock was cut away. I'd measure the width of the flats with a caliper to tell me when I was getting close, and reduce the cuts accordingly. However, the job was done, as far as I was concerned, when the edges of the cuts touched. The extra ten thousandths or so reduction in the round barrel diameter allowed the flats to end octagonally, rather than smearing into the round part of the barrel.

Everybody in the Shop Class was afraid of the horizontal mill, or could see no real use for it, so I could hog it for weeks at a time. :mrgreen:

[Texas by God]

My brain just exploded.
And I drowned in the mathtub.
Best, Thomas.

[Bent Ramrod]

Yeah; way back when, all us College Prep types used to think that Shop Math was simple calculations for the Great Unwashed to do on their fingers. Anybody who actually tries using the stuff to make something real gets disabused of that quaint little notion PDQ.