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Gunsmithing RE-profile/thread barrel hurts bore dia.?

TresMon

TresMon

Gunny Sergeant
Full Member
Minuteman
Dec 3, 2007
1,241
137
NW USA
#1
I have my own opinion but am keeping it to myself for now.
I got into a mildly heated debate over the subject with some local seasoned shooters.

So lets say I pose a standard question so the poll results will not be skewed.

"Can I take a given barrel as received from a premium barrel maker and alter the contour/profile or thread it at the muzzle and it not alter inner bore/rifling dimension??"
 
#2
Re: RE-profile/thread barrel hurts bore dia.?

Is the question whether re-profiling will alter the bore diameter in any possibly <span style="font-weight: bold">detectable</span> way, or whether it will alter the bore diameter in any <span style="font-weight: bold">meaningful</span> way?

I <span style="font-style: italic">think</span> that the answers would be different?
 
#4
Re: RE-profile/thread barrel hurts bore dia.?

Some of the things that come to my mind....although I'm certain there are other things I haven't even imagined.

I think that if you remove metal from the outside of the barrel either during the initial or subsequent re-profiling there is the chance that relief of stresses in the metal can cause some change in bore diameter that <span style="font-style: italic">might</span> be measurable.

Profiling can easily bend a barrel if not done right. That'd be bad.

Certainly fluting a barrel causes a bore shape change that needs to be lapped out.

I suppose that the question I was asking is whether a measurable (and I'm sure there are ways of measuring extremely small variations in diameter) would occur or whether that change would have any discernible effect on accuracy (The question that I though your poll was asking?)

How much does the bore change between out hunting on a 20 degree day and shooting on a nice sunny 80 degree day?
Is that measurable? Does it affect accuracy?

Since steel isn't the perfectly homogeneous, consistent, monolithic stuff that most imagine.

So then the answer <span style="font-style: italic">might</span> also depend on exactly which barrel was being re-profiled?
 
#6
Re: RE-profile/thread barrel hurts bore dia.?

i voted i dont know or care. i actually do care but i have no clue and the only way i could see results was to vote somthing and i didn't want to alter the actual results
 
#7
Re: RE-profile/thread barrel hurts bore dia.?

Heat and induced stress is the deal breaker.....period. Avoid both like the plaque and you'll be fine. If your going to re-profile a barrel blank or thread it at the muzzle it's fine.

When re-profiling I would employ the following

1. Sharp Tooling
2. Flood coolant
3. Light Cuts <.020" per pass

I just sent Krieger a barrel blank back to re-contour from a #18 to a #6 and then flute it. No problems, no worries. Came out 100%
 
#9
Re: RE-profile/thread barrel hurts bore dia.?


XP100:

Gary Schneider told me via phone he wanted to contour my barrel to at least extremely close to what the final profile will be. He says he does the profile work FIRST, then final bore lapping to dimension...
 
#10
Re: RE-profile/thread barrel hurts bore dia.?

There isnt any stress in cut rifled barrels, so correctly machining them to re-countour, flute, thread is OK.

Button rifled barrels can be a problem if they were not stress relieved properly. Hammer forged even worse.

Threading them for brakes, suppressors Etc shuld be fine if done properly. Fluting and Recountouring I would stay away from
 
#14
Re: RE-profile/thread barrel hurts bore dia.?

If I may just to clarify a couple things.

Whenever a tool is put to a piece of stock you are not inducing or causing stress. You are stress <span style="font-style: italic">relieving</span> the material.

This can be demonstrated a couple ways.

Take a piece of delrin plastic. Say a 4x4 plate. Drill a 1" hole in it and then slot the hole. I don't care how careful you are when you drill the hole or how light a cut you take. The moment you slot the opening the hole is going to close up a bit and the slot is going to get narrower than the kerf on the saw that cut it.

The stress was already in the material. All you did was relieve some of it when you machined on it. THAT is what caused the material to move around.


Next example. Take a piece of plate cold roll and face one side in a mill. Cut as heavy or as light as you want. If it's a half inch thick and you wizz off a 1/3rd of it, there's a high probability that when you pop this thing out of the vise it's going to bend like a banana. Again, the stress was already there and you relieved some of it.

As for tooling. A properly designed machining tool will put the bulk of the heat generated by the cutting action into the chip being sloughed off. Tooling manufacturers spend millions of dollars a year in research to improve upon this. With carbide turning inserts the tool depends on the material going molten right at the insert. This is how it cuts. (ever wonder why carbide inserts really aren't all that sharp compared to HSS?)

If the tool is heating up the part its due to using the wrong tool for the application or the tool is just wore out. There are some exceptions however I can't think of any that would apply to barrel work exept one: Machining stellite liners for crew serve barrel chambers.

Depth of cut should only be limited by the horsepower of the machine, the rigidity of the setup, and the robustness of the tooling used. If the tool can successfully run .1" deep and produce a good finish then by all means do it! CNC's live in this world every day and they work with all kinds of materials that are held to much kinkier tolerances than what's found on gun barrels. TIME is the enemy of a machine shop. The longer it takes to less money you make. Cycle times are constantly being scrutinized.

As for your direction question/poll (to the OP)

I would be less concerned with distorting the bore due to machine work than I would the bore bell mouthing due to pressure over time.

To expand on this a little more. It's a sort of case by case issue. If I build a 300 RUM on a NBRSA Max heavy barrel finishing around 30+ inches I don't concern myself much with the bore bellmouthing at the crown nearly as much as I would/do with a light sporter contour. The length of the barrel combined with the heavy barrel contour will greatly reduce the likeliness of this happening.

However I build that same gun on a #3 sporter finishing at 20" and I put a brake on it I need to be really careful.

Why?

The longer the barrel the less pressure there is at the crown when the bullet leaves the bore. Less pressure means less force to push stuff around. So longer barrel means less pressure where's a shorter one is going to have more.

Barrel contour's mean there's more meat around the bore to resist the "inflation" force being placed upon it by the powder charge.

I build my own brakes from scratch precisely for this reason. If I have a light barrel that the customer wants a brake on, I use the largest thread OD I can because I want as much wall thickness as possible. Simply going off the OD of the thread isn't good enough. You have to factor the root diameter of the thread and the major ID of the bore. Get that wall thickness too thin and the crown is much more likely to grow like a church bell (hence- bell mouth) over time.

Now apply that to the real world. You have a gun that starts out shooting great and then progressively gets worse over it's lifetime. Your left scratching your head because you haven't changed anything. As far as you know it should still perform.

It can be quite frustrating

Rules to live by:

Buy barrels from recognized marquee manufacturers.
Use the heaviest contour you can for the application.
Have them fitted by someone with the experience and reputation for good work


Then go shoot and enjoy the gun.

Hope this helped.

C
 
#15
Re: RE-profile/thread barrel hurts bore dia.?


Thanks C.

When I went to get a M brake for my 300WM I did not like the thread sizes. Seemed to leave too little barrel wall. So I made my own brake for the same reason you said- to use the largest thread dia I could.
 
#16
Re: RE-profile/thread barrel hurts bore dia.?

if the steel is dead, and the work is supported correctly, you can do just about anything to any barrel, outside work, inside work, but it has to be dead or completely stress free. If the steel retains residual stress after the original profiling process, the bore will move no matter how light of cut you take..Anschutz used stress relief to their advantage, whether it was done by accident at first, but their target .22's are a parallel profile with a big parallel lump at the muzzle where the foresight attaches...you can feel right where the change in outside diameter is by pushing a tight patch thru, the muzzle chokes. They often shoot well until you either turn that down or cut it off. Hoop stress and longitudinal stress, those are the two culprits, hoop causing the dilation of bore diameter, couple with longitudinal you can start with a straight hole and end up with a pipe you can't see thru..BTDT.
 
#18
Re: RE-profile/thread barrel hurts bore dia.?

Mr. C. Dixon, You are the man, I could sit and read your input all day long, wish i had a stool in the corner of your shop to
watch you all day long. Retired Machinist and i know great work
when i see it. Thank You
 
#19
Re: RE-profile/thread barrel hurts bore dia.?

As long as we're discussing this, will cryogenic treatment reduce the adverse effects of reprofiling or fluting? What I have read states cryo will relieve stresses and be a good process if you were worried about affecting accuracy.
 
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