.224 build advice needed (in progress)

[darkcharisma]

Denis, the valves exhaust is .250. i plan to enlarge for 30 caliber when the TJ liner gets here. the barrel is currently .218x .245 oblong...i still have more room to work with.

Bob, i can tune above the cliff with the gauntlet but i will have to sort new springs and make an ssg for this one. right now its oem hammer and spring backed out to the max.

ahh...i just sat and thought about it. previously i filled the bottled to 4ksi but the output was at 2ksi. so when i up the reg to 2.9ksi. i think i lost overall pressure in the bottle. don't know how much exactly from 4ksi to top 85cc from 2ksi to 2.9ksi. but i will do a shot string again to see the shot counts. i thought it was weird...

yep just confirmed it was actually 3.5ksi to 2.3ksi for 5 shots.

[darkcharisma]

getting better but not by much. starting pressure 4000psi ending 150bar.

reg set at 2900psi shooting 39 grains.
8 shots...2-3 shots tight es. but declining afterwards.

the cliff was pushing these at 450fps. only a 1/4 turn and on is the valve.

i may have to set the reg lower...much lower than 2900psi. i should get more shots by working the reg around 2500-2600psi and port the barrel out for more fpe.

any other suggestions? i think my huben 22 has better shot counts haha

i recovered the 55 grains that was going 300fps. at that setting its very efficient haha.
this is clearly a deficit of the short 24 inch barrel and caliber size 224 vs 257 or 308

[Hobbyman2007]

There’s something definitely not adding up . With 500 cc you  should be doing way better than that . It must be some loud with that much air getting passed through the barrel . I doubt you’ll gain very much by lowering the reg pressure to say 2600 psi ( just shy of 950fps ) . I’m getting close to the same power from a 480 cc bottle for 36 shots for example using a cobra chamber on my rifle . Is it possible that the valve is hanging up open due to the big ports . Maybe try a smaller transfer port close to full porting in 22 cal . There’s also a chance the valve has a bad shuttle and once open is dumping all the air in the plenum , only to close when the regulator moves enough air to seat the valve .

[darkcharisma]

It sounded like a rimfire and it rings in your ears but not as long and no sonic crack. I have no idea what the valve is doing atm. It maybe dumping all 85cc per shot, that explains why the first 3 shots already brought the 500cc bottle down to 200bar and it keeps declining from there. ill open it up again and see if it's internals are assembled correctly. I thought i did but it could be me. I can make a smaller TP and see if it's any better.

[rsterne]

Here are typical curves for a Cothran valve, plotting velocity vs. hammer strike....



Note that the velocity is essentially constant, and as you reduce the hammer strike (in this case by increasing the SSG gap) the velocity suddenly drops drastically (which I call falling over the cliff)…. On the edge of the cliff, the velocity tends to become very unstable, and you can have shot-to-shot velocity swings of 50-200 fps, which is obviously useless.... At the bottom of the cliff, the gun barely "pops" (a 50% or more drop in velocity), or may nor fire at all.... and you get virtually no pressure drop on firing....

The best place to tune a Cothran valve is just above the cliff, but far enough up on the plateau that the velocity is stable.... In the above examples, that would be about 3 turns of gap on the black line, 5.5 turns of gap on the red line, and 6 turns of gap on the blue line.... Increasing the hammer strike (reducing the SSG gap) more than that will progressively use more and more air, and the effect can be very dramatic.... It is possible to reduce the volume of air used to 50% or more by tuning close to the cliff, rather than just bashing the valve open with brute force....

If you are tuning just above the cliff and the gun is using huge amounts of air (eg. dumping the 85 cc plenum), then you must have either a LOT of hammer bounce (reopening the valve after the bullet has left the muzzle), or something else going on.... Did you modify the Cothran valve in any way?.... eg. remove the metering rod from inside the valve stem, drill out anything, or change the way the balance piston is mounted in the front of the valve?.... The balance piston is supposed to be free-floating in the front housing, with virtually no axial (fore and aft) play, but able to move radially (sideways and up and down)…. That allows it to self-align with the moving poppet.... Maybe your poppet is sticking open until the plenum dumps for some reason?....

Bob

[darkcharisma]

i have not mod the valve in anyway. i opened it up once and assembled the metering rod with the machined smaller end towards the stem. which should be correct. ill make and ssg to eliminate bounce

[darkcharisma]

opened her up. nothing seemed out of the ordinary. lots of silicone oil i accidentally put in the valve and tube though. cleaned up and now shes shooting 10 shots from 4100psi to 180bars. reg still sits at 2800psi. got 39grains to top out at 1003fps. going to try again...10 shots is doable. want to see if i can get 15 or more.

=========
12 shots that averaged at 988fps. i wanna tune it down to 960fps for 80fpe.
 
the 55 grains were going 865fps...
no ssg yet. i probably need to go lighter on hammer weight.

[rsterne]

13 shots averaging about 990 fps with 39 gr. is 13 x 85= 1104 FPE total....

500 cc is 30.5 CI and the pressure drop is 110 bar, which is 3355 std. CI.... That works out to only (1104 / 3355) = 0.33 FPe/CI, which so me indicates that the valve is staying open until after the bullet has left the muzzle....

For some reason, your valve is remaining open MUCH too long.... Have you tried reducing the hammer strike until the velocity drops suddenly below 500 fps and then increasing it slightly from there until you are back close to 900?....

Bob

[darkcharisma]

yes i have tried from 300fps to 950fps by 1/2 turn. 1000fps is 1 turn from 300fps. right now i am trying to find the plateau then will tune down later. i think at 950fps which is just above the cliff will also yield but only a couple more shots...0.33 fpe sounds horrible. the regulator nno longer holds...the bleed hole keeps hissing air when i increase pressure from 0 bar to onwards. it was sitting at 4000psi and reg at 2900psi when it suddenly lets air out via the breathing hole...i think it is blown. so test on halt

any idea how and why the reg does that? it wont seal anymore when i increase pressure.

i am also thinking 77.6grams hammer weight is too much and it bounces?

[Hobbyman2007]

That hammer is some heavy for what you’re using . I’d be trying close to 40 grams . Can’t help with the regulator . Never had that happen . If your valve is working well and you’ve tuned it to just above the knee and stop dumping air , one I’d start looking for reduced hammer weight and second , a weaker spring . These valves open wayyyyyyyy easier than one would think .

[rsterne]

A half turn between 300 and 1000 sounds like you have a heavy hammer strike.... That could be hammer weight, but is more likely too strong a spring.... As Denis said, the Cothran valve is very easy to open.... An SSG should eliminate any hammer bounce problem, particularly if you are only 1/2 turn from the cliff..... Sounds like you have some other problem....

Bob

[darkcharisma]

i have a fixed 2800psi regulator i can still keep running tests. but the rti reg needs to be looked at.

ya, i am thinking about a slingshot hammer with less weight. that way i can keep the oem  spring and oem tension adjuster.

who said building airguns was as easy as 1,2,3 right?

[Hobbyman2007]

I’m finding it strange that even with the regulator you have a declining shot string . I would think you would get at least 5 shots on regulation . Is it at all possible that your hammer is hanging up ? Either on the cocking pin or on the sear ?

[darkcharisma]

i have travel marks left by the hammer on the bottom inside the tube. if that tell you anything.

i noticed that too. but these slugs are not weighted and sorted so i thought it was variation of pushing heavier slugs or something like that. but it does look like a very fast declining string

[rsterne]

The velocity with a Cothran valve decreases with pressure.... If your regulator is typical, as the pressure in the tank decreases, the output pressure drops slightly.... This will cause a slight decline to the velocity, even if the regulator is working properly.... That 13 fps drop you are seeing in the first 10 shots could be due to that effect....

Bob

[darkcharisma]

that sounds like your describing what the reg was doing and its true. i keep watching the reg after every shot, and it seems to decline ever so slightly even when the bottle still has well over 200 bars. this reg was creeping at first but now the creeping is gone after only over 50 shots or so. such a nice reg. need to send it to RTI...but i wanna open it so bad...

ill make a slingshot with interchangable 25-35-45 grams and work on the efficiency this weekend.

TJ .308 31 inches long just came in...i think the cothran deserves .250 bore and up.

[mackeral5]

I experienced my Cothran(s) staying open way too long (in spite of a light hammer and free flight) and wasting a ton of air---when using porting of .200 or smaller.  I sleeved the stem to create a restriction and this resulted in the valve closing sooner.  It has been debated why this occurred and I can't tell you the 100% accurate science behind it, but it did indeed work. 

I would be interested to see if this worked for you as well.  I used icemaker tubing as a temporary sleeve, it lasted long enough to test.  now I have brass tubing epoxied in place.  But, now that you have a 31" .308 barrel your .224 journey may hit a detour...

[darkcharisma]

Not a detour Mike.  think of all the calibers as contenders for the platform. whatever gives the best power, efficiency, and accuracy gets to be mounted. but i am partial to the .224. the .30 or .172 is optional, they can always go to another marauder as i have purchased another breech and tube from crosman.

i can probably try your sleeving method, seems easy enough but with a disassembly. ill show and tell how the slingshot hammer works soon

[darkcharisma]

Dont mind me guys, i am just documenting my findings. and what do ya know? this thing shows promise.

i tried drilling the stem hole bigger, went with .04 but the cheap drill broke so filed it away and re-drilled to 1.5mm.

the 55 grains were going 860fps for 23 shots from 4000psi to 2600psi. thats way better than before for shot counts and at the same FPE. rough numbers from chinese regulator but its something

the 39 grains were going slower at 977fps compared to 1009fps. i didnt do a shot string on the 39 grains.

aired up and shot at 10 yards just to find out if its gun range ready.

3 shots(all i had) 55 grains sized .223, same hole
5 shots, 39 grains sized .224, 0.6 inches
5 shots, 39 grains sized .223, 0.54 inches.

well now, i still dont have an ssg. stock spring and maybe 10 grains lighter hammer since i reamed the ID from 0.500 to 0.530 for shorter spring but that spring was too short.

so i gotta cast some more 55 grains, 47 grains, and 39 grains to go to the range and find out what it does at 55 and 100 yards.

[rkr]

That looks very much like the first test of my .224. I ended up adding a barrel band to make it accurate. With that thin barrel cf sleeve might also be a good idea.