New project — Homebuild 45 cal
As the “regulars” here know, I build inline air guns.
The latest project is a .458 Cal. inline.
The most significent feature of this gun is a 44 inch long barrel.
I have been testing it for the last few days.
The first few shots were about 375 FPE.
Did a little valve work, this evening and got it up to 450 FPE.
Its shooting a 300 grain hardcast bullet @ 825 FPS for 453 FPE, on a 2700 PSI fill.
Not yet as powerful as Voltar’s or cyngus’s big bores, but I am just starting out.
I been studying high pressure air flow. There is somthing called “sonic choking”. If you try to flow lots of high pressure air, through a restriction, the airflow will will go supersonic. This supersonic shock wave will restrict the flow rate (and the valves performance). The only way around this is a bigger valve orifice or more pressure. So this evening , I opened the valve orfice from .25 inch to .3 inch. The “book” on airflow says that the orifice needs to be .35 inch to achieve the performance I am looking for.
No pictures yet.
Mark
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Viewing 9 replies - 1 through 9 (of 9 total)
You two are getting some good numbers….wow!!!. That info on the port holes being .35″ for maximum is awesome. You got me now wanting to open the port hole on my DS from .300″ to .35″ and install a longer barrel. Maybe a 28-30″ from my current 24″.
Thanx for posting that info. and good luck to you both.
Mark, I am most interested in your project.
thank you for the kind words 🙂
What is cool about theoretical is we can put a fudge factor in to correct and gain accuracy in predictions with the maths 🙂
I luv this stuff!
Walter….
Walter:
It was with your help that I got the long barrel. Thanks!
I built this spreadsheet, in microsoft Excel.
Its set up to calculate:
1 bullet transit time
2 average pressure in the barrel
3 average force on the bullet
4 FPE
you input bullet weight, barrel bore diameter, barrel length, and muzzel velocity.
If you study the various variables above, using the spreadsheet, two things become clear: to get more power — increase the bore size and to get more power — increase the barrel length.
Most people here know that you get more power when you go from a 12 inch barrel to a 24 inch barrel.
With the spreadsheet, Voltar, I input your 30 inch barrel, 405 grain bullet, and 720 FPS
Your results are:
466 FPE
1131 PSI average barrel pressure
186 pounds force (average) to accelerate the bullet
If you increase your barrel length by 10 inches and maintain the same average barrel pressure, you would make over 600 FPE and pick up another 100 FPS
My valve is not as good as yours — you are getting 1131 PSI barrel pressure and I am only getting 800 PSI average barrel pressure.
My results are only from the extra barrel length.
If we put your valve with my long barrel, it would produce 670 FPE.
I’ll get pictures of all this stuff — including the valve dimensions soon.
Thanks for the encouragement, Walter!!
Mark
This stuff is all theroretical, but it is based on the physics of accelerating a bullet down the barrel.
Very good results there!!! way to go!!!!!
with light bullets I am no where near your fpe and only when I went to the 425 grain soft lead did I get my 466fpe.
You are doing very well and I for one would love to see a sketch of your valve ideas.
You got me thinking of a purpose buit bigbore to try and stay in the race!
Walter….
Opened up the valve to .316 inch diameter.
Took another shot:
852 FPS, 300 grain, 3000 PSI, for 484 FPE
Tired, done for tonite.
Mark
Took another test shot:
840 FPS, 300 grains, 3000 PSI, for 470 FPE
I gotta open up the hole in the valve next.
Mark
Teflon:
This barrel is a little “loose”, ie the bore is a little on the big side.
So far I cant tell the difference between hard and soft bullets. I have both , in 300 grains. I also have a few 500 grain hard cast that I am hording till the gun is tuned up. Also have some sabos and 150 grain bullets — hoping to go well over supersonic with those.
The valve was producing low velocity at high pressures until I reduced the valve return spring length.
Right now, the valve is closing too soon, as evidenced by a “crisp” sound and low air useage. I think I need to lighten up on the valve return spring a little more.
I shouldnt say it at this point (only producing 450 FPE), but I am hoping for 700 FPE, mostly due to the extra long barrel.
Mark
Sounds very interesting indeed, Mark. Do you think that the hard cast bullets are robbing a little velocity due to friction? Would softer lead help in that regards? At 2700psi are you experiencing any valve lock or inhibiting factors? If not, do you think that you will aim for 3000psi or more?
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Been tuning on the valve.
Drilled it out to .323 inch orifice.
Now Its easy to get 880 FPS, 300 grain bullet, for 516 FPE.
A 150 grain sabo round, gets 950 FPS, for 300 FPE.
With an overfill to 3200 (not really an overfill — the tank is rated for 4500PSI), I get 903 FPS, 300 grain bullet, for 543 FPE
The muzzel blast is huge!!!
Each time I try to fire a 500 grain bullet, the crony screws up because of the muzzel blast. The recoil is MUCH bigger than the 300 grain bullets, so I know its cranking out big power. I only have 3 more 500 grain bullets.
This thing is gonna make a hell-of-a deer rifle! With the huge BC of these 300 grain hollow base rounds, it will be flat shooting from 15 to 75 yards.
Mark