A slingshot hammer may not have enough power to knock open a bigbore valve. I would suggest starting with one piece hammer.
this is a fabulous idea and I myself built a large volume low pressure big bore ( co2 ) .. Its easier in one sense , your fighting the poppet pressure less , harder in another sense,you need serious volume per shot and a long barrel.. I chose co2 because its store in a liquid state and even a small 9 ounce tank holds tremendous usable volume. As far as the tubes go, you have a huge range of materials for 800 psi , and tube sizes.. Thicker wall and deeper threads are always better, and the wall thickness is dictated by the tube diameter, so youd have to be far more specific for anyone to offer suggestions.For the resivour tube , youre gonna want a ton of volume ..
for the pump tube , you could hijack a 2100 or 760 assembly.. save a few hundred in machining a brand new design
they can be pumped to 2500 psi no issue.. SO pumping to 1000 psi even on a large volume should be ok
simple hammer designs will not flow the gas VOLUME needed .. Especially not when it comes to bigbore.. Not too many people have built big bore low pressure guns , not a lot to go on.. heres mine to if youre bored https://www.gatewaytoairguns.org/GTA/index.php?topic=143832.0
Something to keep in mind about tube wall thickness.... It is proportional to the OD to keep the same MSWP.... so as you increase the diameter, you need a thicker wall.... and the weight goes up pretty fast.... I would build the gun strong enough for CO2, which can reach 1900 psi at 120*F.... You will need a big valve for a big bore.... and that means a big hammer and spring.... Your saving grace will be the low pressure....Bob
Quote from: Rob M on August 13, 2018, 12:52:21 PMthey can be pumped to 2500 psi no issue.. SO pumping to 1000 psi even on a large volume should be ok2500 psi!!how is that even possible without the pump being a multi-staged pump?? is it due to the long lever arm ??Quote from: Rob M on August 13, 2018, 12:52:21 PMsimple hammer designs will not flow the gas VOLUME needed .. Especially not when it comes to bigbore.. Not too many people have built big bore low pressure guns , not a lot to go on.. heres mine to if youre bored https://www.gatewaytoairguns.org/GTA/index.php?topic=143832.0so modern knock-open valve will be not so good at flowing the gas volume NEEDED for this job?? hmmm maybe full bore porting and long valve open time would do that??Quote from: rsterne on August 13, 2018, 08:57:12 PMSomething to keep in mind about tube wall thickness.... It is proportional to the OD to keep the same MSWP.... so as you increase the diameter, you need a thicker wall.... and the weight goes up pretty fast.... I would build the gun strong enough for CO2, which can reach 1900 psi at 120*F.... You will need a big valve for a big bore.... and that means a big hammer and spring.... Your saving grace will be the low pressure....BobMSWP... sorry.. but what is it...?? never heard of it before.anyways you mean as the reservoir tube I.D. increases thicker wall is required.. I'll bear that in mind.and one more sily question to ask.. does more hammer energy keep the valve opened longer?? or is there some kind of bar of minimum energy required to open up the valves, and more hammer energy doesn;t affect the valve/airflow anymore? thanks.
single stage pumps ,with a rigid setup front to back , depending on piston size ( in this case .625 ) can easily produce 2500 psi.. All the more reason to use a proven design so you don't have to learn all the math before starting. ( although learning all the math might be good too )
But a regular hammer valve from a stock airgun would be the worst starting point in terms of flow.. theyre designed to be efficient .HAMMER STRENGTH.. well its a combo of 2 factors.. One being dwell , or duration.. Other being distance opened.. there are 1 million combos here, but in low pressure high volume applications , I find it best to start with a combo that should really blast the valve open.. then retard backwards as neded to find efficiency.. Bob and I both actually built 9mm 12 gram co2 guns.. based on different guns.. We did have similar findings and one I recall was medium weight hammer, which I thought would be impossible to get flow with. Bobs was about 16 % more efficient , so hes the expert there.
MSWP = maximum safe working pressure.... ie the pressure the gun is designed to use, day in and day out, for thousands of cycles, and still have a high safety margin.... Typically the burst pressure is about 3.5 times the MSWP.... so if you want to use 800 psi for your fill pressure, the tube should be safe for at least 2800 psi.... CO2 tanks, although they normally run at 850 psi at room temperature, have a MSWP of 1800 psi because CO2 can reach that on a hot day.... They are therefore designed for at least a 6300 psi burst pressure, and in fact most are rated at 500 bar (7250 psi)…. which is a 4:1 safety margin....If you want to build a .45 cal, then you should be looking at port of at least 0.34", and for a .50 cal at least 0.375".... The valve throat will have to be larger than that to allow for the loss of area due to the valve stem.... Let's say that is 1/8", then the throat for a .50 cal should be at least 0.406", which means the Poppet OD will have to be about 1/2".... That will require 157 lbf. just to crack it off the seat at 800 psi.... The heavier the hammer, the longer the stroke, and the stiffer the spring, the easier it will be for the hammer to open the valve to allow for maximum airflow.... The best way is to overbuild the hammer system so that you can find where the maximum performance is, and then back it off a bit to increase the efficiency.... so you are not blowing air (or CO2) out the muzzle after the pellet has already departed.... You are building something uncommon, so it is pretty hard to even make a wild guess what you might need for a hammer....
The pump to fill your beast, however, is another matter.... If you are thinking about a simple single stage pump, I suggest you forget it.... Let's say you make your pump 1/2" diameter (that would take 157 lbf. at 800 psi, remember), with a 16" stroke.... That means it can move 3 CI of air per stroke.... You mentioned the reservoir will be about 870 ml (cc), which is 53 CI.... Filling that to 800 psi (55 bar) requires (53 x 55) = 2915 CI of air.... That means it will take you (2915 / 3) = 972 strokes (assuming 100% pumping efficiency) to fill the reservoir.... You can count on more like 1200 strokes, and at full pressure, those last strokes will require 157 lbf…. All I can say is, better you than me....If you want to pursue this idea, I would strongly recommend you either use CO2, or figure out some other way to fill it, either from a tank, or at least a 3-stage commercial PCP hand pump.... That might fill it in about 200-250 easy pumps....
I'd love to work on a single pump side lever (SSP) rifle in .22", delivering something like 600fps with an 18gr JSB. Seems about ideal for squirrels. Unfortunately I lack the engineering how-to so can't begin to guess how practical such a thing might be, and don't want to just start cutting tubes and parts and winging it only to find that it delivers something closer to 300fps with an 11gr Hobby pellet, which is essentially useless unless one is plinking in the basement. Still, if I did run across some numbers which seemed to indicate it was practical if using a very long lever - say 20" long - it seems an aluminum I beam with a 2mm thick carbon fiber tube around it, filled with JB Weld steel filled epoxy, might just do the trick for the lever. Keeping it light weight while being plenty stiff to run all the pressure up in one stroke. If keeping the whole works sort of skeletonized, just a CF tube for the 'wire' stock and a lightweight scope, overall weight could maybe be kept below 8 pounds. Be a heck of a fun field gun. No pump to worry about, enough power for small game or decent holdover out to 50 metres, not even very noisy. A Redman or FX .22" barrel liner covered with CF tubing would help keep weight down as well.