my 100 FPE Co2 rifle prototype ( shotgun )

[Rob M]

PREFACE :
Its ugly , i know that.. But it was a test platform for the valve idea.. Im already planning a version 2 , and the real goal was to prove not only the valve , but the massive potential energy of big bore co2..I need to credit Gipetto with some inspiration from his old coax build also ..
  Well , theres a lot to cover and I was going to postpone this build thread.. The gun was not made for looks , but to test my hammerless piston valve.. NOW , I planned on making a  drawing in cad to explain the valve , but its actually pretty simple . ( although used in ZERO airguns im aware of )
 EXPLANATION - Where a hammer valve would normally be in a co2 gun is replaced by 1 moving part instead.. its a .500 diamter piston , seated in a .500 hole , which blocks the transfer port.. when the 40cc co2 chamber is filled , the piston is now under 180 lbs load on the face ( temperature dependent ) pushing rearward.. The b18 trigger is retaining the 180 lbs rearward force somewhat like a cocked nitro piston.. Upon pulling the trigger , the force slams the piston rearward exposing the transfer port and firing either the 178 grain roundball or shotshells which ive yet to make,. The barrel is 3 feet, 304 ss and smoothbore which for this design makes no difference.. the lenght was really key , and for the shotgun function or even the roundballs  , smoothbore is fine/..Im betting i can put the hurt on an armadillo at close range either way (-'


co2 cannon by murphyrobert9, on Flickr


cannon 2 by murphyrobert9, on Flickr

A lot of my conventional wisdom when it comes to tinkering and modding was out the window , so i painted myself into many a corner.. Initial testing had the rear charging handle made from cheap grade china screws.. Same went for the sear engagement nut.. Things were breaking and i began to really grasp the violence of the piston slamming home on each shot.. These things have almost all been changed to grade 8 , and are holding up fine.. ANother hurdle as seen in the pictures was retention of a springer based trigger .. there is really nothing to hold onto , or at least not enough to retain the 180 lbs repeatedly so im actually on design 5 with the trigger which is ugly , cumbersome and backwards , but i will say its VERY  reliable now.. Ill need to make the forward facing trigger in the coming days after modding the b18 trigger group blade.

cannon 3 by murphyrobert9, on Flickr


The gun is super light , and actually has 11 mm dovetails up top , but ive never even aimed at anything , just over the chrony..Fyi, 100 fpe is 504fps ( 178 grain) . my scale says 175grain so who knows.. With the smaller tank seen above , its easy to handle at 54 inches. With the 24 ounce co2 , its a bit off kilter, but the shot count is great..
On this design , i actually had to make a lot of stuff that wasted time , and i mean weeks. the next version will have a few things different.. One thing for sure , it will not have a china on/off valve to feed the 40cc chamber, it will simply have a forward bleed hole that opens and closes as I move the charging handle back and forth ( so charging handle forward will fill co2 , charging handle back will seal the tube from the tank )

IMG_5234 by murphyrobert9, on Flickr

SO , whats my takeaway? Well i learned alot ,proved that my valve design is actually quite reliable , and I hope i encouraged some folks to pursue their summer co2 ideas.. I know were in the pcp generation now and most things are turnkey , But it wasnt long ago the crosman discovery wasnt even on the market and many a tinkerer built unique guns on the older forums,/How much futher will I go with 1.0 variant , im not sure.. reverse the trigger blade, throw a red dot on it since i made the dovetails , and do a consecutive shot string off the large tank.. Thats about it , since it is time i could be designing 2.0 , a leaner and meaner version with more ergonomics and creature comforts.. Some peopel may read this and say " couldnt this be done with a balanced valve and qb 78 ?" .. Well the answer is no.. The amoutn of gas that needs to move is mind bending , and no current valves are specifically designed to dump these volumes.. Thats not a knock on advanced /balanced valves, its just saying theyre more geared for pcp funtions and 3-4x the pressure . ANyway , I hope you found this interesting , when i have any updates ( if there are any on this model ) ill update the thread


20180525_002716 by murphyrobert9, on Flickr


20180525_002602 by murphyrobert9, on Flickr

and yes, im abig fan of pumpers and co2 (-'

[Rob M]

To add a few more things I wanted to cover.. The trigger complication is really the weight on the piston.. Aside from a springer or a crossbow , not many trigger groups are designed to release 180 lbs.. A tig welder would have helped mounting the b18 trigger , but ive got this sorted for rev 2( whenever that happens)..Cost of operation is way lower than percieved, i have fired a few 12 gram co2s from this where the full cansister is dumper per shot ( 85 fpe )  , thasts pricey.. But the 24 ounce tank is 3.99 to fill at academy and offers over 50 shots.Hornady round ball is cheap also.the intial goal was 120 fpe , and i still believe i can get there. Ill wait and see if anyone asks questions i guess.

[stalwart]

Following, with great interest!

 

[Doug Wall]

Sounds interesting. Do you use a on-off valve to charge it for each shot?

[StevenG]

I have the same sorta question, is this a dump valve? How does the piston get reset for each shot?

Very cool though.

[Rob M]

Steven and Doug; The side charging handle just racks the piston back over the sear , which seals off the main tube again.. ( piston in forward position ) Its effortless really since there is no resistance at that point.. Yes, for now its fed with 1/4 turn on the small on/off .. SO I can get a shot about 1 every 10 seconds .. cock the gun , load a ball , add gas , pull the trigger..
in the next version , the charging handle will do both things, load gas and reset the piston for each shot.. SO it will be a very conventional loading process. Eric , thanks for looking , Im watching some of your threads also ( 9mm xisco

[grimeszee]

Following

[Gippeto]

Pretty cool project Rob, not many folks bucking trends and messing with blow open stuff these days.

Something that might be of interest are the mechanisms used in some of the airsoft (40mm) grenades...lock up is similar to a quick connect...would look hard at these as the design base if I were to take another kick at this cat...JMO. Reduced requirement for opening force also solves the trigger issues.





Regards,

Al

[Rob M]

heres the piston , I made this rough drawing just now so you can see how the charging handle , sear nut , and piston are all 1 piece


20180525_091106 by murphyrobert9, on Flickr

[Rob M]

Thanks AL.. I see that valve design takes none of the gas pressure directly , man the paintball community has some brilliant designs..! ID need a very large version of that I assume to flow the 40ccs needed almost instantly.. And the coax got me thinking about this build years ago , as mentioned in the preface (-'
I think when my new design has a .394 piston ( .500 was overkill in several senses ) , and works the weep gas system all at once , id actually like the current design more.. we will see. rev 2 would feed gas from the rear also

[Gippeto]

A great many fantastic ideas out there to build upon for sure. Would take some re-arranging and changes to have breech loading, but it's do-able.

40cc isn't really that much...co-ax was dumping 10cuin....wonder what Markos 20mm uses eh? 

I assume that's an o-ring groove on the piston and that is doing all the sealing?

If you add an orifice/sealing lip to the chamber and a sealing face to the piston, you can reduce the "held" force without having significant effect on opening force...once cracked, full area would be exposed to pressure.

Some other thoughts;

Keep an eye on piston travel as it contributes to dead volume and reduces available pressure...typical D/4 rule applies....I usually use D/2.

Personal preference is to have the barrel choke the flow...porting gets progressively smaller if needed and the bore is the smallest diameter.

Have you done any modeling with ggdt? It works quite well for blow open valves if you feed it accurate information.

Al

[Rob M]

as far as breech loading , I assume you mean " gas filling from the breech"// the gun is ammo breech loading with a conventional breech. ( like a crosman)
piston travel is always the same , its just barely exposing the .342 TP and bottoms out hard right there. Yes , 40 cc is quite moderate , I may go larger and  because of the tube wall in the main tube , I can almost double that.


"ersonal preference is to have the barrel choke the flow...porting gets progressively smaller if needed and the bore is the smallest diameter."

I like that idea (-'
just googled the halls gas gun design page , very interesting .. My friend / acquaintance  a mechanical engineer and familiar with ags estimated 588fps would be the outer limit with my current design , so im still a long ways off ideal

and no , I haven't modeled anything , this was proto 1 , the second one will likely have way more design planned out before turning on the lathe/ mill./

When you mentioned the orfice, I think you were referring to balanced piston which is something I debated.. Something like a QEV.. otherwise id need a diagram..

there are a lot of ways to get the gas flow needed, I just went with the least moving parts , and what seemed interesting at the time.. Id love to get the piston force down to almost nothing.. a lot of considerations.

[Gippeto]

Meant breech loading for the projectile...as opposed to muzzle loading.

I believe I know to whom you are referring.

IMO, it would be useful to spend some time with ggdt before cutting metal for #2. Just too easy to alter variables and see the results.

Something like a qev...but not. Function would be similar to what's referred to as a chamber sealing piston valve, where upon initiating flow (cracking) a larger surface area is exposed to pressure with a subsequent increase in force. There is a valve that utilizes the principle for pressure relief, but google and I are not agreeing on terminology atm. Will try to draw something up later for clarification, but in the mean time;

https://www.spudfiles.com/spud_wiki/index.php?title=Piston_valve#Chamber_sealing_piston_valve

Al

[Rob M]

Meant breech loading for the projectile...as opposed to muzzle loading.

I believe I know to whom you are referring.

IMO, it would be useful to spend some time with ggdt before cutting metal for #2. Just too easy to alter variables and see the results.

Something like a qev...but not. Function would be similar to what's referred to as a chamber sealing piston valve, where upon initiating flow (cracking) a larger surface area is exposed to pressure with a subsequent increase in force. There is a valve that utilizes the principle for pressure relief, but google and I are not agreeing on terminology atm. Will try to draw something up later for clarification, but in the mean time;

https://www.spudfiles.com/spud_wiki/index.php?title=Piston_valve#Chamber_sealing_piston_valve

Al

brilliant design ! and quite simple.that might be employed on version 2..
Ok , yes in pic 2 , you can see the normal breech where I load the ball. might need to take another pic of that.. No muzzle loading , I have little patience for that lol.
I look forward to the diagram , and took note of the pilot diagram in the first pics on that page..  Keeping things simple is always beneficial , so the fewer moving parts, and the fewer parts with seals is always nice.. Having the combo of dump valve and self filling is the key.. My gen 2 idea was basically a rod attached to the face of the piston.. this rod linkage would lead to a forward piston all the way down the tube.. and that piston would control the bleed on of co2 making the charging operation from the charging handal all 1 operation..

YES,you do know who im talking about , I have some confidence in his math and estimations .(-'


there is actually a well documented 6mm bb gun on youtube that uses a similar design to what you mentioned.. the barrel is sealed by a rear piston , while loaded all is static and the force keeps the barrel sealed.. when there is a release from the rear , the piston slams back exposing the barrel to all the ambient pressure.. No mechanical attachments involved, extremely simple.

[Rob M]

just did some shooting , I will say its very consistent,502, 498, 504 fps.. obviously getting the exact same dose of co2 eacha nd every time goes a long way.. need to refill the 24 ounce and increase volume to 60 cc.


20180525_120407 by murphyrobert9, on Flickr

[rsterne]

If I understood correctly, you are using a .50 bore and 36" barrel, which works out to 7 CI (116 cc).... Having a dump chamber up to half that volume (eg. 60 cc) will show decent gains.... in this case Lloyd's spreadsheet predicts about 10%, or about 550 fps, going from 40 cc to 60 cc.... Past that and you will have to use a lot more CO2 for small gains.... With expanding 60 cc of CO2 into the 116 cc barrel, and assuming 850 psi starting pressure, you will still have a residual muzzle pressure of 290 psi, meaning 34% of the CO2 is being "wasted" each shot.... If you doubled the chamber again, to 116 cc (ie barrel volume), you might get to 600 fps.... but the muzzle pressure would be 425 psi, and 50% of the CO2 would be exiting after the ball leaves the barrel.... Just something to think about....

Looks like a fun design, and I'm sure you are learning a ton from it.... Well done !!!

Bob

[Rob M]

If I understood correctly, you are using a .50 bore and 36" barrel, which works out to 7 CI (116 cc).... Having a dump chamber up to half that volume (eg. 60 cc) will show decent gains.... past that and you will have to use a lot more CO2 for small gains.... With expanding 60 cc of CO2 into the 116 cc barrel, and assuming 850 psi starting pressure, you will still have a residual muzzle pressure of 290 cc, meaning 34% of the CO2 is being "wasted" each shot.... Just something to think about....

Looks like a fun design, and I'm sure you are learning a ton from it.... Well done !!!

Bob

thanks Bob.. I did notice diminishing returns when I went from 24 cc to 40 .. I think I gained 50 fps , for whatever that is ( 80 ish % ) .. So to max out close to my theoretical limit of 588 fps , 60cc will be my last attempt before starting V2.. Based on those findings , I should know the exact plenum size for v2 among other things.

[rsterne]

Made a small edit/addition to the post above....

Bob

[Rob M]

that 600  correlates well to my 588 fps I was told by an engineer.. SO if we graphed this , im really all downhill after 50 cc , or atleast that's my quasi educated  guess right now.. Ill go to 60 and the reason is , its no more work than going to 50cc.

[rsterne]

Dump valves up to 50% of bore volume are not unusual, although smaller ones are much more efficient.... Once you need more power than you can get with a dump valve of reasonable volume, you are better off to use a strike-open valve (and a much larger plenum) that closes before the bullet gets to the muzzle.... After all, any gas exiting the valve/chamber after that is completely wasted....

Bob