Project Fury 20mm BIGbore. 3100FPE!

[MJP]

So this is darkside, and I have been bitten by the bigbore bug. I have built couple of mediocre bore rifles in the past but this is my first real bigbore build. I have got today one of three barrels that have sat in storage greased and unused for decades. Just to be build in to something else that they originally where designed for. It starts with one picture, with one .457 290grain slug for comparison.


Marko

[Mod90]

 
I hope you're aware you're gonna need to use an scba tank as a reservoir & more likely than not that gun's only gonna give you one shot.


But oh what a shot it will be

[MJP]

Oh yeah. Lots of air is going to be used with a dump valve, air is the easiest consumable in this build, think about the size and weight of the lead... With powder the bullet weighed around 120grams...
I was planning on something like half of that.
I'll start with around 800cc air volume in the plenum and work from that to what ever volume is needed for speed. 

Edit, the barrel is 1100mm now that I cut the chamber off. Its a 20mm AA gun (anti air gun)
So when you see this Bob Sterne, would you be so kind and tell me some numbers on that air volume in 300bar?

[rintafile]

Accurate has mold which is .755 and 1320grains. It should be possible to get wider like .787 or .788 of course weight increases also 
http://www.accuratemolds.com/bullet_detail.php?bullet=75-1320-D.png

It is over two times bigger than mold which I have 683grains 

[MJP]

Makes me think about sabots and .50cal slugs... All that surface area and light bullet 

Marko

Edit, not that 600grains ain't that light to begin with...

[rintafile]

If You can make a sabot I got those .510 and 683 grains casted so that should be good start.    or 650 grains...

[rsterne]

No idea what you are considering for velocity.... but here are some idle thoughts to consider.... At 670 fps, you get 1 FPE per grain of bullet weight.... 60 grams is 926 grain, so let's call it 1000 FPE at 700 fps.... My rule of thumb for plenum size is 1 cc per FPE, but 800 cc will certainly work, just a bit less efficiency is all, hardly noticeable.... For reference, a roundball of that caliber is 731 gr.... and a 1 caliber long cylinder is 1096 gr....

A 20mm bore is 0.787".... With full bore-size porting, running on 3000 psi in a 24" barrel, the theoretical maximum FPE using my spreadsheet is 1459 FPE, if you are using a bullet of 730 gr. (ie a roundball).... That means it would be challenging to reach 950 fps with a roundball in a 24" barrel.... With your barrel length, a piece of cake, providing you had a large enough plenum to keep the pressure near 3000 psi during the shot....

I can calculate other possible outcomes, at different pressures, and also give you an idea of the bullet weight required, or possible, for a given velocity.... but more information is required....

Bob

[MJP]

Thanks Bob, that was quick. I have 4000cc tank that is the biggest plenum volume that I was thinking of using. It's a 300bar / 4500psi tank. And well bullet weight is not a problem as they can be turned in a lathe or a mold made pretty easily for the tooling size is no longer a problem so around 1300-1600grains maybe.
But at first I'll start with the 800cc to see how things work, mainly how to seal the chamber properly without O-rings flying out. Hafto use viton seals I think.

Marko
 Edit, and for speed, around 1000fps, is nice, but I will try to push it as high as possible just for the fun of it and to see how much can be had with 20mm air cannon err gun. But for shooting around 1000fps.

[rsterne]

Sorry, I didn't see the 300 bar number.... Carrying on, using 4350 psi, my spreadsheet says a maximum SD of 0.244 to achieve 950 fps in a 24" barrel.... It predicts a maximum, with bore-sized porting, of 2116 FPE with a 24" barrel.... If you use a 36" barrel (leaving some for the inefficiencies that will occur in any build), that goes up to 3174 FPE, and the SD increases to 0.365, or 1582 gr being the heaviest you could push at 950 fps.... So if the plenum was large enough, you should be able to push a 100 gram (1543 gr.) bullet at about 950 fps using 4350 psi in your barrel.... The problem is, that you need a plenum of about 3000 cc to keep the pressure up....

If you use 80% porting, you are down to about 2500 FPE, ie a 1250 gr. bullet at 950 fps, using a 2500 cc plenum.... Other combination are possible, of course.... To reach 1000 fps, your bullet weight (SD) will have to decrease....

Bob

[MJP]

Thanks again Bob. Those are some serious numbers, and very much attention will be put in to the frame stiffness, some serious forces are at play.
This will be my most difficult build in every aspect.

Marko

[Pellethuntr]

Very interesting build, I am tagging along for the ride.

[Matt15]

Very interesting build, I am tagging along for the ride.

+1!!

[rsterne]

Just a quick idea of how plenum volume affect the pressure, and hence the velocity.... Your barrel is 1100mm = 43.3", so the volume is 21 CI.... If the plenum is 2500 cc, that is 152 CI.... If the pressure in the plenum is 300 bar (4350 psi), once you open the valve and the bullet reaches the muzzle, the air has expanded to occupy 152 + 21 = 173 CI of volume.... That will drop the pressure to (152/173) x 4350 = 3822 psi at the instant of pellet exit.... The smaller you make the plenum, the more the pressure drops....

Something else to think about.... If you use a dump valve, you will be using 152 CI x 300 bar = 45,600 CI of air per shot to produce just 2500 FPE, an efficiency of 0.055 FPE/CI.... If you close the valve just as the bullet leaves the muzzle, you get exactly the same power, but you only use 21 CI of air at 3822 psi, which works out to 5568 CI, or 0.45 FPE/CI.... If you close the valve when the bullet is half way down the barrel, you will lose about 30 fps (3%), but use half the air again, and the pressure remaining in the tank will be 4067 psi, so now the efficiency is up to 0.79 FPE/CI.... I give you these numbers to encourage you to used a conventional valve, rather than the extremely wasteful dump valve....

Bob

[MJP]

Something to consider indeed, the trouble is opening the valve big enough for big power. I will definetly use air assisted valve, small pilot valve opening the main. But then the closing is a big question, will it close and how quick.
Valve seating on 20mm hafto be around 23mm and the force keeping it shut is over 1200kg!
Knocking that open with conventional hammer is out of the question.
Drawing and working different ideas for sealing the chamber and so on.

Marko

[rsterne]

Perhaps you can use a port reading barrel pressure, and close the valve after it drops to "zero" (ie when the pellet exits).... Even saving some of the air is better than nothing....

Bob

[MJP]

One thing I have been thinkin is a gas piston like most semiautomatics use, barrel port around 2/3 way from chamber, and the pressure behind the bullet slams the valve shut before the bullet exits the muzzle.

Marko

[rsterne]

Good idea, if you can get it to function properly.... By the time the bullet is 2/3 of the way to the muzzle it has about 98% of the maximum possible FPE.... The time lag to close the valve might still have it open when the bullet exits....

Bob

[MJP]

Some work done here and there, first part picture of tapping the breech for the madsen barrel. Thread is M30x1.5


Turning the madsen barrel.


Marko

[rsterne]

I would think that a balanced valve would be the way to go, particularly if you want to use a dump valve.... Something configured like the Cothran Force Reduction valve, but simply vent the balance chamber in the head of the poppet to atmosphere.... The balance chamber would reduce the force required to open the valve, and once the poppet cracked off the seat the balancing force on the valve would actually slam it open.... It would stay there until the reservoir is empty, at which time a small spring could close it.... Something like this conceptual drawing....



The chamber on the right is vented to atmosphere.... there could be a small spring in there to close the valve after the pressure drops (or a much larger one to close it sooner?).... The opening force can be very small, as the air pressure only acts on the DIFFERENCE in area between the poppet seat and the balance chamber.... As soon as the valve cracks, the only significant force remaining is the balance piston, and that assists the opening of the valve.... The Cothran valve works on this principle (except the balance piston chamber is inside out, the piston is on the valve and the cylinder is part of the poppet), but the stem is drilled to vent that chamber into the exhaust port area, providing a closing force, and there is a "leaky check valve" inside the stem to slow down the airflow and prevent hammer bounce.... Neither are needed for a dump valve....

Bob

[MJP]

Thanks Bob, that's a good concept aswell. But I'm going to use pilot valve to open stemless main valve.
There's going to be adjustable pressure feedback from tp to close the valve again.
Trial fitted the breech blank to the barrel. And the barrel is ready, man I hate turning barrels. Everything else is worth just material cost but barrels cost really something, try to find virgin AA barrel see what they cost. 
That actual one was cheap but to replace it if you mess up on the lathe..



Marko