Author Topic: Minimum barrel wall thickness question  (Read 1710 times)

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Offline tacotime

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Minimum barrel wall thickness question
« on: March 06, 2012, 11:44:13 AM »
This is a question about the minimum barrel wall thickness that is safe in a centerfire rifle.  Since screw holes are routinely drilled into barrels to secure iron sights and reduces the metal thickness over the bore, obviously one could drill too deep, leaving a very thin barrel wall in that spot under the hole.
 
Does anyone know if there is an official spec for minimum barrel wall thickness?  Rifle in question is a .358 Win with some deep screw holes.  .358 is a fairly high pressure round.  Just noticed the depth.  Rifle has seen 42 rounds since new, so I can't say it's time tested.  Barrel is stainless.
 
Thanks.

Offline keith44

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Re: Minimum barrel wall thickness question
« Reply #1 on: March 06, 2012, 07:49:02 PM »
I've wondered this myself, but have never found a definitive answer.  Just gonna follow the discussion.
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Offline swifty22

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Re: Minimum barrel wall thickness question
« Reply #2 on: March 07, 2012, 09:23:53 AM »
Howdy, Was interested at one time in having a Mod. 94 Win. 30-30 (30 WCF) rebored to a .375" groove 38-55. The reboring gun maker (Clearwater Reboring) said that the minimum that they would have to have under screw holes, dovteail cuts ect. was .2" ( 2 hundred thou.) and my 94 wouldn't make it, we measured. My XP was originally a 35 Rem. and the front sight screw hole was bored clean through the barrel, I put a plug screw in it and everything seemed fine.-Muddy   

Offline Rangr44

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Re: Minimum barrel wall thickness question
« Reply #3 on: March 07, 2012, 09:37:14 AM »
The req'd bbl wall thickness differs, dropping the further forward/away from the front of the chamber the datum point is located - since the internal pressure generated by a fired cartridge is also dropping along the way.
 
Generally, at the muzzle of a 22" bbl, bbl wall thickness must be at least .121", making for a minimum muzzle OD of a .358"  rifle  .600". ( .121" + .121" = .358" = .600" )
 
Theaded holes in the length of the bbl generally should/usually have a screw tightened in them, re-inforcing the void - IOW, if the factory did it, fahgettaboutit.
 
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Offline tacotime

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Re: Minimum barrel wall thickness question
« Reply #4 on: March 07, 2012, 11:38:04 AM »
Yes but the factory occasionally makes mistakes or a drill press gets out of adjustment or whatever.
 
If I do my math right, there is only .016 and .02 of metal under two holes about 6 inches from the throat on this high pressure round, so I have asked the factory to confirm what their specifications are for these holes.  Waiting for reply.

Offline trotterlg

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Re: Minimum barrel wall thickness question
« Reply #5 on: March 07, 2012, 03:38:09 PM »
If you do the math you will see that the load on a screw size hole is very small.  An 1/8 inch screw hole at 25,000psi only has a load of about 300 pounds on it.  I have had shotguns with scope base holes drilled through into the chamber that shot just fine.  If you have a screw in the hole I don't think there is any problem at all.  Larry
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Offline tacotime

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Re: Minimum barrel wall thickness question
« Reply #6 on: March 08, 2012, 03:54:17 AM »
With the .358 closer to 48,000 I imagine over 500 lbs sitting on a piece of metal hardly thicker than a razor blade.  Seems like too much. 
 
The hole at .21 is 68% deeper than the .125 screw shaft going in it.  Seems drilled too deep to me.
 
It will be interesting to see what the manufacturer says about it.

Offline Flash

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Re: Minimum barrel wall thickness question
« Reply #7 on: March 08, 2012, 01:34:52 PM »
My smith won't drill past 80% of wall thickness for a #6 hole and won't drill larger than that on a barrel
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Offline tacotime

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Re: Minimum barrel wall thickness question
« Reply #8 on: March 09, 2012, 04:12:41 AM »
And that seems perfectly reasonable to me.
 
Still waiting for a response from the factory...

Offline tacotime

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Re: Minimum barrel wall thickness question
« Reply #9 on: April 23, 2012, 05:51:52 AM »
Ruger factory service never answered.

Offline SHOOTALL

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Re: Minimum barrel wall thickness question
« Reply #10 on: April 23, 2012, 05:58:18 AM »
I had a XP-100 that the smith drilled thru the bbl when installing the sights. ( the factory holes were so far off , one in one direction the other the other way) The smith replaced the gun but I had a 60 round match to shoot. I shot a couple hundred rounds for pratice and the match with no ill effect. It would seem the screw holding the sight would act like a pipe plug ? The gun was 7-BR loaded above publish data. I know of a Ruger M-77 in 243 that had sights installed and the same thing happened and it also shot fine.
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