barrel thickness at muzzle

T

twoshots

CGN frequent flyer
Rating - 100%
97   0   0
Location
ON
Hey guys,


I had a 375hh brno factory hammer forged barrel shaved to octagon ahead of the stock to improve the balance. It came back a bit thinner than I expected but does balance really nicely now.

The barrel remains full factory bull thickness out to the end of the stock and then matches the orig. taper in octagon form out to the original front sight where it returns to factory round and factory diameter.

At the thinnest point the wall thickness is .070" at about 24" from the bolt face on a 25.5 inch barrel.

5 factory 300 grain federal soft points were fired at room temperature with no issues witnessed.

Does anyone feel this could develope into an issue over many rds or that I should avoid mono solids in it? I have read elsewhere that .120 is the magic minimum but there are factory doubles out there thinner I believe. I'm not really worried just fielding for opinions.
 
Last edited:
Wow, 65 views and no opinions? Is my post confusing? It's kind of difficult to describe but I can elaborate if necessary...
 
On my vernier 0.70 at the muzzle looks like there's still enough meat to be solid.

There are however a great many intangibles at work here. How was the barrel machined ? Were the flats cut in 2 passes with little to no coolant or were they cut in 6 passes and flood cooled to minimize heat as much as possible. Was this done by an experienced gun smith who's done it hundreds of times or a general machinist who's never cut a barrel before ?

If it was done by a smith you're best off asking them. I would hope they did QC on the barrel before they sent it back. They should be able to tell you what user feedback has been like with different bullet types. If you're concerned that the muzzle has been cut to far on the thin side then I'd stay away from solids.

hope this helps a bit.
 
twoshots said:
Wow, 65 views and no opinions? Is my post confusing? It's kind of difficult to describe but I can elaborate if necessary...
Click to expand...

I don't believe your post is confusing. It's probably more a matter of folks not knowing the answer given the octogon aspect.

So for what it's worth, all the research I've done on the subject tells me that a minimum wall thickness at the muzzle is .100". If the barrel was still in the round and had minimum thickness near the muzzle of only 0.07", I'd say it would be marginal.

Recently I had a barrel blank with a .366" bore turned down to .600". This left me with a thickness of .117"per side. This was completely acceptable to the 'smith doing the work.
 
The smith was not worried and I did point out that I felt it might be too thin. He has built several thin barrel double rifles and also done octagon conversions on brno single barrel so it should be ok.

I understand it's difficult to answer diffinitively. I'm just looking for opinions here and will take all with a grain of salt. I will avoid mono solids in it and hope for the best. The thinnest spot is not at the muzzle but about 1.5" before the muzzle. The barrel flares back out to .150 wall at the muzzle. The octagon shape keeps it quite rigid and accuracy is not my concern. I'm worried about a bulge or rupture at the weakest link. I don't think it would injure the shooter that far out but might hurt another hunter/bystander.

I think the pressure should be pretty low at 24" and probably worry too much...
 
I is probably fine. The bullet by that point is moving so quickly, and there's enjough space behind it, that the pressure isn't nearly as much as in the chamber. Keep and eye on it but it should be fine.

0.07 is probably thinner than I would have gone but I'm not basing that on any theory or empirical evidence.

How thick is it from the inside wall of the bore to the vertex of the octagon? (Thickest point)
 
I'm of the school that it's a bit thin according to my gut feelings. But that's just not all that scientific.

What is needed is to know the pressure in the bore as the bullet passes along that portion and just begins to leave the muzzle. With the pressure known one can calculate the tensile load in the metal through the thin area of the flats. I know that there are formulas out there for tensile load in a pipe under pressure. There may even be online calculators for this. Then one needs to know the metal used in the barrel and find the tensile yield point. If the tensile loads in the metal across the narrowest portion are well below the tensile yield point of the metal then all is well. If it's close then over time and due to complications from being an octagonal shape with a round bore you may get metal fatigue and eventual failure. If it's slightly above the yield point then the thinner portions of the web are stretching a little with every shot taken and eventually you'll get cracking and failure.

The other issue would be whipping of the barrel and poor accuracy. But from the sounds of it the modified barrel is shooting just fine. So now the question is about the pressure and metal values.

The problem is that the peak pressure doesn't mean anything. By the time the bullet passes into the modified forward section the pressure has dropped way below the peak pressure given in the loading data. It would take a pretty wild internal ballistics calculator programme to give you this sort of data. But there's so many things like that out there that it may not be as hard as I think to get a graph of the pressure vs bullet position along the bore. And that would put you into a position to use the tensile load in pipe wall formula to find the load across the thin portion. And then learn what the tensile limit is for the barrel steel and you'll have an answer.

Confused yet?.... :d

The "rule of thumb" of the wall needing to be .120 or .1 or whatever is based on what folks know works from the past. It also likely represents a very generous safety factor and the reality that the barrels are that thick or thicker as much to avoid accidental bending during handling and slight mishaps as for holding in the pressure. So I would not put much stock in any rule of thumb for indicating that you've got a ruined barrel or not. Just make it a point not to trip and fall while carrying the gun though.
 
So the average feeling is that it's a bit thin but probably ok for lower volume shooting of non-monolithic bullets and should be treated carefully to avoid a bend or dent similar to a nice shotgun.

This is pretty much how I feel and was looking for confirmation. I'll keep a close eye on it until it's proven in higher temperatures. Thanks for the input so far, I appreciate it.
 
###XX said:
I is probably fine. The bullet by that point is moving so quickly, and there's enjough space behind it, that the pressure isn't nearly as much as in the chamber. Keep and eye on it but it should be fine.

0.07 is probably thinner than I would have gone but I'm not basing that on any theory or empirical evidence.

How thick is it from the inside wall of the bore to the vertex of the octagon? (Thickest point)[/QUOT


OK, I took some more measurements. The vertex at the thinnest point measures .550 so about .090 wall thickness from bore to points.

The Octagon taper starts at 12" from the bolt face and and measures .660 on the flats after the plunge cuts so there is about .150 taper in the OD over the remainder of the barrel.
Click to expand...
 
You must log in or register to reply here.
Back
Top Bottom