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Double Rifles, Single Shots & Combinations >> Paradox and Bore Guns
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ironoxide
.224 member


Reged: 14/08/19
Posts: 28
Loc: Poland
8 bore pressures and barrel dimensions
      #331888 - 05/09/19 01:16 AM

I've recently acquired an old (pre 1877 judging by the proof marks) Belgian single shot 8 bore muzzleloader. The barrel measures true 8 bore (21.2mm - 0.830 inch), but the original calibre is listed as 20.4 mm ( 0.803 inch). So I assume that at some point someone honed, or bored the barrel out.

I'm planning to "proof test" it with a load only slightly exceeding the service load for 8 bore rifle, but before I do that I'm trying to establish if there is any possibility the barrel may be too weak. I don't want to blow the gun up. If it is too weak I would rather sleeve it to a smaller calibre.

The outside dimension of the barrel near the breech is 1.47 inch.The barrel is octagon transitioning to round. Using few available calculators online they give approximately from 13500 psi to 32000 psi as maximum pressure a tube with this barrel's wall thickness can survive depending on steel type. I used numbers for historic steel strengths I found online.

So now the question is what kind of pressures should I expect with various black powder 8 bore loads. Ideally I would like to shoot few 820 grain round balls, and 1 15/16ths ounce of shot at 1200-1300 fps.

Does anyone know design parameters of 8 bore barrels? Specially design pressures, barrel thickness used in modern replicas etc? I found various charts online, but they all go up to 10 bore only.

Also, I found out steels available in mid 19th century had ultimate strengths from 220MPa or 32000psi (for lowest grade) to 530MPa or 76870 psi (for best grade). Does anyone know what kind of steel was the most likely non-laminated barrel steel in Liege at the time?

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DarylS
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Reged: 10/08/05
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Re: 8 bore pressures and barrel dimensions [Re: ironoxide]
      #331892 - 05/09/19 02:02 AM

The "nominal" bore size for a true 8 bore is .835". A pure lead ball that size, weighs 875gr.

It was common to use one bore size smaller ball, which would be 9 bore, at .803" with a substantial patch. A 9 bore pure lead ball weighs 777.8gr.

The pressure for whatever charge you could handle, will be extremely low. I would estimate never over about 6,000 to 8,000 psi at 1,200fps to 1,300fps - for whatever ball diameter you choose.

I had a late 19th century SxS 12 bore, stamped with "Whitworth Fluid Steel" barrels. I assume it is hot cast, then bored. They were nitro proofed.

Keep up "posted", ironoxide.

For an .830" smooth bore, I would most likely use the 9 bore mould to allow a thick patch and lots of lube. You should be able to load a ball that size and .018" denim patch quite easily.

The bore might need a scrubbing with scotch-brite cloth and oil. This is an abrasive thick plastic conglomerate that really smooths minor problems out well.

I used it on a ladies' rifle barrel at the rendezvous last week and now she's back to shooting the whole course of fire without having to wipe the bore, not even once. With a decent patch, you wipe the bore as you load. Before, it had a 'rough' spot just about under the sights that prevented her from pushing the ball past after only 2 shots. This roughness happened due to poor cleaning practices - using commercial shitty products.

I made about 30 passes with the scotch-brite, lubed with WD40, full length of the bore and this smoothed it up.

--------------------
Daryl


"a gun without hammers is like a Spaniel without ears" King George V

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ironoxide
.224 member


Reged: 14/08/19
Posts: 28
Loc: Poland
Re: 8 bore pressures and barrel dimensions [Re: DarylS]
      #331966 - 06/09/19 11:30 PM

Thanks I'll probably initially try 0.820 balls only because I found them easier to buy.

I'm planning to do a proof like test on the barrel and then do a dye penetrant test just to be on the safe side. Following suggestion of another person on another forum I'm planning to use 10% over the service load and 30% over the shot load for this test.I initially planned to build a wooden contraption to hold the barrel and a caplock, but I'm leaning towards doing that test with the barrel in the gun with it suspended on two sets of V shaped pieces of thick rope. This should allow the gun to swing freely and lower the impact of recoil on the stock hopefully. I saw a test like this done in a youtube video after a custom big bore double was made.


However, I discovered another issue with the gun. The nipple is not original (not a surprise in a 150 year old gun), but the nipple hole seems to be taper threaded or the top of the thread has been worn to the point where the top of the hole has diameter of 0.342 ( 8.7mm ) and the bottom is 0.263 (6.7mm). Then the nipple has a metric M8x1 thread which grabs only by 2.5 turns at the bottom of the hole. Although I calculated that in the worst case scenario regarding steel quality every single turn of the thread can withstand 1100 punds of force (5KN) and even 10k psi of pressure produces only 750 pounds of force on the area of the nipple so it should be fine, I'm considering making a new nipple. Making that tapered thread on the lathe without a taper attachment will be quite difficult.

I definitely try the scotch-brite polishing method.

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tinkerModerator
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Reged: 12/03/05
Posts: 4835
Loc: Nevada
Re: 8 bore pressures and barrel dimensions [Re: ironoxide]
      #331967 - 06/09/19 11:57 PM

You should consider contacting Tom Armburst
He's done a great deal of work on the study of internal ballistics relative to pressure.

--------------------
--Self-Appointed Colonel, DRSS--



"It IS a dangerous game, and so named for a reason, and you can't play from the keyboard. " --Some Old Texan...

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DarylS
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Reged: 10/08/05
Posts: 27005
Loc: Beautiful British Columbia, Ca...
Re: 8 bore pressures and barrel dimensions [Re: ironoxide]
      #331968 - 06/09/19 11:58 PM

I would not shoot the gun until I repaired the nipple seat and installed a new nipple.
Larger hole at the top, tiny hole at the bottom.
The gun test you refer to was Colin Stoltz and Son on the 2-bore double he built.
Cap lock rifles that have a drum sticking out the side of the breech, sometimes spit the drum out the side when the threads become loose from over use.
I would not trust 2 1/2 threads

--------------------
Daryl


"a gun without hammers is like a Spaniel without ears" King George V

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tinkerModerator
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Reged: 12/03/05
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Re: 8 bore pressures and barrel dimensions [Re: DarylS]
      #331977 - 07/09/19 04:18 AM

For the taper on that thread, you can cut it to proper size with your lathe, then accomplish the taper with a thread file.
It's not difficult to get it right

--------------------
--Self-Appointed Colonel, DRSS--



"It IS a dangerous game, and so named for a reason, and you can't play from the keyboard. " --Some Old Texan...

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ironoxide
.224 member


Reged: 14/08/19
Posts: 28
Loc: Poland
Re: 8 bore pressures and barrel dimensions [Re: tinker]
      #332111 - 09/09/19 09:30 PM

Yesterday I cut various threads on the lathe attempting to get something that would fit the existing nipple seat. It turns out that what I took for a tapered thread is basically M9x1 thread covering 60% of the length, then M8x1 covering the rest. M9 thread is worn on the top more than on the bottom so the whole thing looks like a tapered thread.

I suspect the original thread was M9x1 and the bottom 30% of the nipple seat was not threaded. Then someone came and threaded that hole, then made a nipple that is held by only threads at the very bottom of the seat.

A normal M9x1 thread is held properly despite the wear visible. I'm considering making a sleeve that would screw into that M9 thread, with a hole threaded with M6x1 into which a standard musket cap nipple could be screwed it. When making that sleeve I'll probably try to make its lower portion go into the M8 thread too so it is held very securely.

I also measured the bore diameter properly this time and it measures 20.7mm or 0.815 so half a millimetre too tight for an 8 bore. I have mixed feelings regarding this. On one side it is good that the measurement is closer to the original calibre stamp of 20.4mm as it means the barrel is probably close to its original strength. On the other side it is a bit of a disappointment that I can't think of the gun as "real 8 bore".

Edit: It turns out metric musket nipples have M8x1 or M8x1.25 threads which is a bit too big to sleeve in M9x1. A tiny revolver nipple would look silly on this gun. Probably making a nipple that fits into the existing M9 thread will be easier.

Edited by ironoxide (09/09/19 11:17 PM)

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ironoxide
.224 member


Reged: 14/08/19
Posts: 28
Loc: Poland
Re: 8 bore pressures and barrel dimensions [Re: ironoxide]
      #354374 - 16/06/21 10:07 PM

Some time has passed and today was the first time I had a chance to properly shoot the gun so I thought it is a good time for an update :-)

Over the time I rust-blued the barrel and discovered it is made of twist steel. Remade the nipple for a better fit. I fixed the wood (Light colored inserts were done previously but someone else. I would've matched the wood). I remade a couple screws so they look properly now. I "reproofed" it with 250grain load of powder and 2 ounces of shot.

I ordered a round ball mold for the gun. Nominal size is 0.805, but it casts 0.802. As the gun is actually 0.803 it shoots a naked ball pretty well. (with 135 grains of 1.5fg).

So I would like to ask. Is there any record of those guns being used with a naked ball originally?

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