measuring barrel flats

[chuck-ia]

I know you can measure a barrel flat with a caliper and get pretty close. But I like to make things complicated. On a 7/8" barrel, how do you mathamaticaly, (don't even know if I spelled that right) figure the barrel flat? flinch

 

[B.Habermehl]

Go borrow your kids algebra/trig book or the neighbor kid's. As for me I'll stick to the dial calipers and the educated eyeball :winking:

 

[MikeFromON]

Is 7/8" the diameter of the barrel from flat-to-flat or from corner-to-corner? I can tell you how to calculate the width of each flat, but I need to know this info first. I can give you some hints right now though:

1) Think of a circle. The octagon can be drawn inside a circle, with each corner touching the circumference thereof.

2) It's an octagon, so you know each flat 'represents' 1/8th of the circle, or 45 degrees.

3) Now that you know each flat is actually the base of a triangle that has an apex of 45 degs, and you have some idea of the radius of the circle (which is also the the height of the triangle) you can use basic high school trig to calculate the length of the base/flat.

 

[paulvallandigham]

Octagon barrels are measured across the flats, from one side to the opposite, not from the corners. There are traditionally 8 sides( an " Octagon" ) to the barrel, although you will run to the odd Hexagon shapped barrel once in awhile. Because the bore is round, and it is important how much metal is left between the bore, and the sides of the Octagon, that shortest distance is some portion of the narrowest measurement of the barrel, or " Across the flats". Most barrel makers want there to be at least as much metal between the bore and the flats as is 1/2 the bore diameter, just for safety. In target barrels, you can expect that to double. In slug gun barrels, you can expect to triple or be even bigger, depending on caliber. Remember, the slug guns shoot " slugs", which are very long, and very heavy conicals, and not patched round balls. The pressures developed in these gun barrels is high, and can burn out the best of nipples. That is why the men who shoot these bench guns usually have a fixture that " closes " the primer to the barrel, so that no pressure can escape through the flash hole, out the wrong end of the barrel! The typical fixture is designed to fire a small pistol primer, and not a percussion cap. You will still see Underhammer slug guns that fire percussion caps, but they are getting to be the rare sight these days.

 

[Vaino]

.875 divided by 2= .4375 x tangent of 22.5 degrees {.4142136}= .18122 x 2= .36244 flat. I just use a scale to ascertain the width of the bbl flat.....Fred

 

[Zonie]

And if you wanted the measurement over the corners of the flats of a 7/8 inch octagon barrel (from one side to the other), it would be equal to:
.875 divided by the cosine of 22.5 degrees {.923880}=.947093 :grin:

This number might even be useful for someone. It means that the barrel must be at least .947 diameter or larger if you want to have sharp corners where the flats meet.

zonie

 

[chuck-ia]

thanks guys. flinch

 

[roundball]

And if you wanted the measurement over the corners of the flats of a 7/8 inch octagon barrel (from one side to the other), it would be equal to:
.875 divided by the cosine of 22.5 degrees {.923880}=.947093 :grin:

This number might even be useful for someone. It means that the barrel must be at least .947 diameter or larger if you want to have sharp corners where the flats meet.

zonie

.....and Daniel & Davey are sayin': HUH????? :shocked2:
"We just make'um the same size as the spur on an old Tom turkey"

 

Wow, what an education I just got! All this time I thought you just measured the width of the flat surface. Cosigns and tangents? I should have paid more attention to Mr. Moore's Algebra II class.

 

[chuck-ia]

I know I can make simple things complicated sometimes. I just get to thinking too much, about how certain things are done, I am attempting to inlet a barrel in a blank is why I was wondering, my main tool for this job is a 1/4" chisel and a hammer, so I guess I really don't have to be that precise, but then again the more precise I can be on my measurements, maybe the less room for error? I have access to a milling machine but want to do it by hand, kinda hard to understand huh? Was over to the friends shop yesterday and he was messing with the milling machine, he showed me how he was going to cut the barrel channel in his stock. I think once he gets everything set up, and maybe practice on a scrap piece of wood, I think maybe a half hour and the barrel channel would be done. Machines like that are nice if a person knows how to use them and is comftorable around them. flinch

 

[fho]

Flinch: I use the Michinery's Handbook

R = radius of Circumscribed Circle
r = radius of Inscribed Circle
s = width of flat
R = 1.082 r
S = 0.828 r

Olie

 

[08Desert Trapper]

Octagon barrels are measured across the flats, from one side to the opposite, not from the corners. There are traditionally 8 sides( an " Octagon" ) to the barrel, although you will run to the odd Hexagon shapped barrel once in awhile. Because the bore is round, and it is important how much metal is left between the bore, and the sides of the Octagon, that shortest distance is some portion of the narrowest measurement of the barrel, or " Across the flats". Most barrel makers want there to be at least as much metal between the bore and the flats as is 1/2 the bore diameter, just for safety. In target barrels, you can expect that to double. In slug gun barrels, you can expect to triple or be even bigger, depending on caliber. Remember, the slug guns shoot " slugs", which are very long, and very heavy conicals, and not patched round balls. The pressures developed in these gun barrels is high, and can burn out the best of nipples. That is why the men who shoot these bench guns usually have a fixture that " closes " the primer to the barrel, so that no pressure can escape through the flash hole, out the wrong end of the barrel! The typical fixture is designed to fire a small pistol primer, and not a percussion cap. You will still see Underhammer slug guns that fire percussion caps, but they are getting to be the rare sight these days.

Just so I totally understand and it’s crystal clear I would take my tape measure and put the little hook do hickey on one side of the barrel cross the tape measure over the muzzle, the barrel, and look at the measurement. Once it’s at the flat of the barrel.

Just making sure God bless. Keep your powder dry.

Squatting duck of the OVPR clan