Bolt hole depth

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Alange

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Question for Zonie ! I installed a johnson peep sight on my longrifle. It’s 50 cal. Green mountain barrel, 7/8” . I think that I might have the holes too deep. There is 0.75 of metal to the bore ,and they are on right in front of the breech plug. I have shot the rifle with a 65 gr. chargeof 2f and no problem.. Do you think it is ok for a charge of 75-80 gr. or should I consider getting the barrel shortened and retrenched?.
 

Larry (Omaha)

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Question for Zonie ! I installed a johnson peep sight on my longrifle. It’s 50 cal. Green mountain barrel, 7/8” . I think that I might have the holes too deep. There is 0.75 of metal to the bore ,and they are on right in front of the breech plug. I have shot the rifle with a 65 gr. chargeof 2f and no problem.. Do you think it is ok for a charge of 75-80 gr. or should I consider getting the barrel shortened and retrenched?.
I am not Zonnie obviously! "There is .75 to the bore????" I think you have that wrong! No disrespect intended.
 

Alange

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Yea your right. That’s 0.075 to the bore. And the rifle is a flintlock..thank you for corecting me..
 

SDSmlf

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General ‘rule of thumb’ that I follow for minimum barrel thickness, particularly at the breech end, is .150”, although some recommend a bit more. I doubt you have 7/8” barrel with a 50 cal bore, but if you do, screw depth should be no more than .027” assuming .010” deep rifling, for that .150” minimum thickness. Not a lot of threads. If you have 15/16” barrel, which is more likely, you could go .058” deep with your holes and have that .150” minimum thickness. Just my opinion. I’m sure others will follow with their opinions.
 

plmeek

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A 7/8" barrel has a diameter of 0.875". Subtract the bore size from that you have 0.875" - 0.50 = 0.375". Now subtract twice the groove depth and divide by two to get barrel wall thickness, (0.375" - 2(.012)) = 0.1755" wall thickness. If you accurately measure the depth of the holes you drilled at 0.07", then under the screws you have 0.1755" - 0.07" = 0.106" rounded to the nearest thousandth of an inch.

That's not a lot of metal right at the critical point where the pressure is at its maximum. You have absolutely no margin for error in case you double charge or double load your rifle! I certainly wouldn't want to be anywhere near you when you shoot that rifle!

I assume your Johnson Peep Sight is set up like this.

 

Alange

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yea thats how I have mine mounted ,maybe a 1/2 “ rearward from that one. If I can find someone locally that can rebreach it Iwould shorten the barrel by 2 ” .
 

Zonie

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Since I was asked, I think the remaining wall between the bottom of the hole and the bore would be totally safe to shoot.
The .106 amount of material that plmeek calculated is being supported by a lot of solid steel all around it so IMO the gun should be safe with powder loads of up to 100 grains if you are shooting patched roundballs.

By the way, I agree with plmeeks number.
(.875-.500)/2 = .1875
.1875 - .012 deep rifling groove depth = .1755
.1755 - .070 hole depth = .1055 wall thickness.
 

Alange

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If my screws are bottoming out in the holes does that make a difference?. Thanks to all that replied I won’t be going over 85-90 gr. with ff and maybe 65-70 fff. It’s does touching holes at 50 yards with 60 gr.of 2f very Happy! I have Put about a dozen shots of 60 gr through it since I mounted the sight.. I live in Sask. Can. and not many black powder gunsmiths around.
 
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You can find formulas online for calculating the bursting strength of a tube of certain diameters and bore size. They will give you a wall thickness. Treat the barrel as a round tube.
 

bubba.50

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Flinchlocks have a hole all the way through the barrel and people been shootin’ them for centuries.
 

SDSmlf

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Since I was asked, I think the remaining wall between the bottom of the hole and the bore would be totally safe to shoot.
The .106 amount of material that plmeek calculated is being supported by a lot of solid steel all around it so IMO the gun should be safe with powder loads of up to 100 grains if you are shooting patched roundballs.

By the way, I agree with plmeeks number.
(.875-.500)/2 = .1875
.1875 - .012 deep rifling groove depth = .1755
.1755 - .070 hole depth = .1055 wall thickness.
If a flat bottom drill/end mill was used, using depth measuring end of a caliper, or measuring a how deep a screw goes into the hole to bottom out will give a close approximation of the hole depth. If a regular drill with a 118° point was used, you are possibly a bit deeper. Assuming a 6-48 tap was used, a #31 (.120” dia) tap drill would be appropriate. Simple math says that the point of the the drill would extend .036” beyond the drill diameter, potentially reducing the wall thickness from the point of the drilled diameter to the bore (aka wall thickness) to .069”. Getting close to not enough material in my opinion. And I know guys like Bobby Hoyt get nervous when the walls between screw hole bottoms and the bore are much less than .150”.
 

Col. Batguano

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It's a small screw and a small hole. I think you will be fine, but if you are nervous, cut in a 118 degree angle to the screw tip and size it similarly to the way we time a breech plug so that it totally bottoms all the way down. Then you'll have full metal support.

Anecdotally, we have less metal than that around the opening of our vent liners, and they don't blow out and create a burr around the rim do they?
 

Alange

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Thanks everyone.. I can relax a little bit know . I will be sure to take it easy on this rifle. Our white tails can get pretty big up here,I live 30 miles away from where the Hanson buck was harvested, I am sure 70 gr load and a prb will do the job at 60-80 yards..
 

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