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Bore width / barrel length ratio


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#1 Redrocketman

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Posted 09 February 2017 - 12:29 PM

I have built minatue BP cannons for a while. My current gun, ' Lil Betsy ' is just insane. 12.5mm bore, 250mm barrel length with the breach reduced to 10mm increasing strength and I am fairly sure enhances the burn, tho I can't justify that theory. Run 67 grains, and thumps a single .50 call lead ball clean through a 300 page novel or three 3l plastic milk cartons full of water!! It's crazy!!! I don't know what I'm doing right, I load the powder, wad & ram down to the breach gently, insert fuse ( electric) before ramming HARD!!! Load the shot, wad that and ram that HARD again!! Fire it with my rocket launch controller. Recoil is so savage its split 3 wooden carriages in half as well as snapping twin recoil chains. Tried 3" H/D extension springs - one shot and there near a foot long now!!! Put a cement pool block behind it and it was reduced to rubble!!!!
So I have that tuned perfect.
I am now confident to take it up a notch and am moving onto my semi mini 28mm PAC gun.
What would the ideal barrel length be for a 28mm bore?? Will have a triple sleeved breach

#2 lloyd

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Posted 09 February 2017 - 12:46 PM

That's a 20-calibre bore.  I'm not sure what BP will do vs. NC powders in such guns, but a typical NC gun is about a 50-60-calibre bore.  (3"x58-calibre being a common military gauge)

 

If you're using BP, and if the 20-calibre bore is working well for you, stay with that formula... make the bore about 28*20, or about 560mm long.

 

LLoyd


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#3 Redrocketman

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Posted 09 February 2017 - 05:04 PM

I've gone 750mm with 100mm breech, the reason being even tho the powder is excellent, not too fast but extremely powerful so having the 100mm extra should give time for the shot to reach its maximum velocity given that it will be a very large charge burning at a much more constant rate than smokeless. I'm not 100% sure it just my theory in my head!! The reason my magnum rifles have a longer barrel than a non magnum of the same calibre

#4 Redrocketman

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Posted 09 February 2017 - 05:06 PM

Yes there using smokeless but it's cause of the volume of powder that's going to burn and the required time to get maximum pressure from that given charge over the length of the barrel. I'm going with the same idea

#5 dynomike1

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Posted 10 February 2017 - 07:35 AM

Splitting carriages? You are probably fixen to see some barrel splitting if you are lucky. I didnt see the barrel material, i hope it's not metal, but i am sure it is.


Edited by dynomike1, 10 February 2017 - 08:33 AM.

There are very few problems that cant be solved with explosives.

                             Explosives are a bang up job.


#6 Redrocketman

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Posted 14 February 2017 - 09:18 PM

What else wood!!! I've seen ice barrels too!!!
After I finished fabrication the barrel was send to a close friend, a very old ( no offence Reg!! ) but extremely respected and proficient gunsmith. And it passed. Loydd your estimations were near perfect, finalising with 585mm 28mm ID smoothbore barrel with 100mm sleeved breech. I haven't chronographed it yet as I'm working up to max chafe and projectile grain weight. Ist test shot was 140 gn & 12x 8mm shot. No issues
2nd test 250gn moderate rammed with 500gn single custom cast pill. Like it wasn't there. Barely move the gun ( mind u this one is full steel workshop fabricated)
3rd test soon, 260gns rammed REAL hard pushing a 1209.12 grain custom slug. Now we'll see how she goes. I'm 100% confident, or I wouldn't push the switch. Another point, all remote fired.
It is ( I haven't received the papers but will soon ) to 550 grains BP tight packed pushing up to but no more than a 1600 grain projectile. It is genuine high velocity. I won't load any more than 280 and the current 1200 pill. That's HEAPS!!!
I fabricated a proper gas porting muzzle brake and I believe it really helps also.
Put forward your opinion please!!!

#7 Redrocketman

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Posted 15 February 2017 - 10:12 AM

The 20 cal withstood rigorous testing today without an issue, firing a 215gn BP charge & 1209 gn solid no problem. Ran the final charge weight and projectile that I'll now use consistently at 250gn BP pushing either a single 800 gn solid or five 12mm round lead shot which I'm yet to weigh. It's amazing!! But the genuine power stunned me. The five ball shot from 15 meters punched clean through a 20l plastic dum of water, 6 1/2" pine boards and into the bank. Well disintegrated the last 3 boards, the 1st 3 perfect clean holes👍🏻👍🏻👍🏻😊😊😊




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