If you plan on dumping all the air with each shot, about 25-50% of the barrel volume is what to use....
the the transfer port would direct the energy into a parallel tube where the breech and barrel would be..The only time air is commonly held like this is in a drop block for a tank , but those don't hold volume, theyre a solid block with a tiny weep hole just to transfer air.
That is correct.... the MAC cartridges were much too small in volume to produce real power.... They were exactly a dump valve....
Would using using 50% of bore volume on a big bore produce enough power for hunting?
Single shot, dump valve, big-bore is possible.... but you will need over 1 CI (16 cc) of volume at STP to dump with each shot for each FPE as a minimum, and it could need twice that.... That means the volume will depend on the pressure used.... As a rough guide you might use 1 CI per FPE divided by the pressure in bar.... 100 FPE needs more than 1 CI (16 cc) at 100 bar.... With dump valves and high power, you could need 2 CI (32cc) for 100 FPE at 100 bar.... or 1 CI (16 cc) for 100 FPE at 200 bar....Again, that 1/2 the barrel volume for the reservoir is a good maximum to use, regardless of pressure....
Just for laughs, you might want to check out my alternative .410 bigbore on the Bob and Lloyd gate. This is a CO2/850 psi gun, with a QEV, piloted dump valve. I'm still experimenting with it, but my current reservoir size is about 18 cc, and I'm getting in the 470 to 500 fps with an 84 grain projectile, out of a 21" barrel. My calculation (been away from chemistry for 13 years) indicates that it is using about 2 grams of CO2 per shot, which translates to about 1000 cc of gas at atmospheric pressure. I'm planning on trying 850 psi HPA on it also.
There is a difference between a "dump valve" (which exhausts all the air in the reservoir) and a "dump shot", which is the term I use when the valve dwell is so long that the pellet leaves the muzzle while the valve is still open.... In either case, any air exiting the muzzle after the pellet has departed has NO chance to add to the energy of the pellet, so both are wasteful of the amount of air used.... Both situations will result in the maximum power being produced for that gun with that average pressure….
the 50% rule is because the pellet has long since left the barrel by the time the rest of the gas can act on it. even 33 % might be maxed out
The absolute maximum power in a PCP, without wasting air, would occur if you could get the valve to close at the exact moment the pellet left the barrel.... but that is still quite inefficient.... However, closing the valve earlier does not lose much power, and saves a lot of air.... If the valve closes when the pellet has only moved halfway to the muzzle, you will only lose about 3-5% of the velocity.... but use only half the air.... This means the efficiency (in FPE/CI) will be about 90% better....
youre gonna do a lot of pumping at 75 caliber and were back to how to get that amount of flow.A true dump valve would be ideal , but again , it will be all day pumping and not much shooting..were I to do a 75 caliber pumper ( personally ) id actually make a single stage pump long sidelever with a large piston , to achieve huge volume at low pressure ( like 800 psi -1000psi) and use a dump valve.
haha pumping is another matter...but if thats the case id be willing to pump it up but how does a true dump valve look like?? is it a plunger with seal?? pics would really save me..thanks
well , my idea would be an entirely different direction than you were heading .. also , with the larger caliber, a longer barrel would also be great.. and that's an issue also , who makes a 40 inch rifled barrel ?? I don't have a true dump valve diagram , im sure someone else will post one .. this all being said , a 75 caliber pumper would be awesome .. even with 10-12 cu inches swept volume in the pump , youd be look at dozens or possible 100 strokes per shot
Might have missed it, but what's your actual goal with this? Short range hunting, or just punching holes in "stuff"?Is pumping from zero after each shot acceptable to you? Muzzle loading or breach loading?.75" bore with a dump valve can do some real damage even at lower than normal pcp pressures, but will require a significant volume. I used a .76"x 36" barrel (~16cuin) and 10cuin reservoir. 300fpe is quite doable at only 600psi.Don't know if the animations will work for you, but worth a try;http://www.spudfiles.com/pneumatic-cannons/topic8157.htmlSome examples of dump valves that are activated by a mechanical striker;Al
I referenced my .410 experimental build in an earlier reply. Since then, I've got it running well. It uses a piston type QEV dump valve, and has a 21", .410 barrel. I have it set up to use CO2, or 850 psi HPA. Mine uses an 18 cc reservoir that translates to about 1000 cc at STP. I'm getting about 50 FPE on CO2, and about 60 FPE on 850 psi HPA. You might be able to scale your build from this information.