Is there a general chart for minimum needed octagon barrel thickness at a dovetail?

[Larry (Omaha)]

Is there a chart or rule of thumb for approximate barrel thickness needed after a dovetail is cut? That is a broad statement, so let me narrow it down some. A parallel barrel is straight forward as it has consistent strength in it's length. A swamp however, varies in strength in it's length. Ballistic pressure is also a factor of where a dovetail is placed. Now, with all that rambling, is there a rule of thumb one can use (we are talking average not specifics) to determine how much metal should be left for a given caliber for say 10" forward of the breech? (underlugs/barrel loops are not in this discussion)
Flintlocklar

 

[EC121]

Because pressure is measured in pounds per square inch, the pressure drops quickly as the pressured area increases the farther you go up the barrel. .1" should be plenty that far up the barrel. Dovetails only need to be about .040". My Caster rifle only has about .050" under the back sight(12" up the barrel) where he made the dovetail way too deep. The other point is that the thin spot is tangental to the bore so the thin spot is a very small fraction of a square inch that may even fall on the land area instead of the groove depth that I used for my calculations.. That would add another .014" to the measurement.

 

[plmeek]

(I apparently was typing this at the same time that EC121 was writing his reply, but he beat me. We basically are saying the same thing.)

Not that I know of.

The rule of thumb for barrel thickness at the breech or powder chamber is about 0.2". A 15/16" barrel in .54 caliber has a wall thickness of (0.9375 - 0.54) / 2 = 0.199". A 1" barrel in .58 caliber has a wall thickness of (1.0 - 0.58) / 2 = 0.21". A 1-1/16" barrel in .62 caliber has a wall thickness of (1.0625 - 0.62) / 2 = 0.22".

But the barrel doesn't need to be that thick further down because the pressure in the bore falls off as the ball travels down the bore. That's why swamped barrels and tapered barrels are practical.

The pressure curve depends on a lot of variables such as powder type and granulation, powder volume, projectile, caliber, bore roughness, etc. Too many variables to make a meaningful chart.

Another way of looking at this is just how deep does one need to cut a dovetail to hold whatever is placed in the dovetail?

For some reason you wanted to exclude underlugs in this discussion, so you must be asking about dovetails for sights. What does it take to hold a sight on a barrel? Not much. Most rear sights for muzzleloaders come with dovetails that are much deeper than needed. If you are placing a sight on a barrel where the wall may be approaching 0.1" thick, make the dovetail less than 0.050" deep.

Even though you didn't want to talk about them, the same applies to underlugs for barrel pins and keys. The underlugs don't have a lot of force exerted on them. The breech and tang/tang bolt are absorbing most of the recoil. They pretty much only need to be strong enough to hold the forestock on the barrel. The master gunmaker that I'm taking lessons from has instructed me that standard dovetails of 0.060-0.050" deep are all that is needed and can be as shallow as 0.030" in the waist of a swamped barrel.

A common problem of deep dovetails in thin wall sections is that the process of cutting the dovetail and installing the lug or whatever can sometimes dent or push in the thin wall under the dovetail making a restriction in the barrel or tight spot in the barrel.

There is a thread on this forum where a guy had a jag stuck in the bore of a large caliber smoothbore a few inches into the bore. My initial thought was that the barrel has a restriction possibly caused when by dent or a deformed dovetail.

Jag and Rod stuck in barrel

 

[Larry (Omaha)]

For some reason you wanted to exclude underlugs in this discussion, so you must be asking about dovetails for sights.
[/QUOTE]
Thanks Phil, I excluded underlugs because I like to solder them if the barrel is swamped. I found that if I use a jewelers saw I can do a dovetail removing about .020 and it is pretty solid. I do not like overkill on my dovetails. Just my preference.
Flintlocklar

 

[M. De Land]

As a rule I would never go under .100 thickness between a groove bottom and the middle of a dove tail on a barrel flat on the breech end, especially with 12L14 steel used in a lot of muzzle loading barrels. 12L14 is not considered to be gun barrel quality steel by metallurgists but black powder low pressure allows it's use when it is hot rolled in production! Green Mountain uses 1134 and that has much better shock load strength.
At the muzzle you can safely get away with less steel under the dove tail center because of the pressure reduction from increase expansion ratio (volume) .

 

[powderhombre]

I realize this is an old thread but it speaks to a subject of my interest. I have had 2 TC .50 cal, 15/16" barrels 3/8" dovetailed at the original sight placement location so that the buckhorn sight would cover the screw holes that the ugly TC factory sight mounted. Both gunsmiths who performed this request never batted an eye or admonished me for doing so. My not so educated opinion was that although metal was being removed weakening the barrel structure, once the rear sight was installed the metal was being replaced by the sight. Sort of a patch. Long story short, when I short started a TC conical that Blew the nipple out of the barrel and full cocked the hammer The sight showed no sign of damage. I sent the barrel to Thompson Center for inspection and they replaced it along with a very stern warning against doing anything of that sort in the future. Which I did again anyway. I haven't had any issues. So my question is am I wrong in my patch theory?

 

[plmeek]

My not so educated opinion was that although metal was being removed weakening the barrel structure, once the rear sight was installed the metal was being replaced by the sight. Sort of a patch.

The "patch" as you call it does not restore the barrel structure to its original integrity. The sight in the dovetail is only as strong as the shear strength of the wedge of barrel material holding the sight in place.

This might be enough strength to withstand the pressure from normal loads and shooting, but it reduces your margin of error if one makes a mistake in the loading procedure.

You are fortunate that the nipple was the weakest point when you "short started a TC conical".

As you can see in the photo below, the nipple was not the weakest point on this gun. The dovetail was!

So my question is am I wrong in my patch theory?

The photo above suggests, yes, you are wrong.

 

[jerrywh]

I see some things in that photo that make me suspect that the sight dove tail was not the cause of the failure. I would love to examine that barrel. Don't see any black powder fouling for one thing. Too clean. Split lengthwise like a seam maybe. In 60 years of building muzzleloaders, I have only seen 2 barrels fail. One was the loaders fault, The other was bad steel tubing. so called seamless.

 

[rich pierce]

Powder hombre, I’m not sure you stated how deep those dovetails were. You said they were 3/8” dovetails but that is width unless one is building a dovetailed wooden box.

 

[powderhombre]

Yes I was very lucky. Lucky that nipple didn't take out some face or eye. The current dovetail is 0.106 deep.