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rsterne
Now that I have some shop time with no major projects on the go, I decided it was time to revisit the Hayabusa Mk.3 version, which is a .410 shotgun and .457 rifle…. Ever since the SS Valve came on the scene, I have been itching to make one for this big-bore, as it REALLY needs it…. The gun uses a 1/2 lb. hammer with 2″ of travel, and it takes over 28 lbs. of cocking force…. If there was ever a gun that needs a reduction in hammer strike and cocking force, this it it…. :
I pulled the gun completely apart, had a look at what I was dealing with, and started with the new valve internals…. I scaled up my SS valve to use a PEEK poppet with a 1/2″ big end and a 5/16″ small end…. The thimble is turned from a piece of 5/8″ 1144 Stressproof, and after drilling and then finishing the bores with end mills and sandpaper, I turned the outside down to 9/16″ at the back and 3/8″ at the front…. I turned the poppet from a piece of 1/2″ PEEK rod, drilled it for a 1/8″ stem and a 0.040″ bypass vent, and pressed it onto the valve stem with some Loctite 638 for good measure…. The last step was to turn the O-ring grooves, working gradually deeper until I had a nice slide fit and the valve spring would reset the poppet…. Here is what it looks like, along with the original parts….
The original valve was just a rear half, there is a long pedestal inside the reservoir for a spring seat…. not an elegant arrangement, but it did unshroud the poppet completely…. I will have to make a complete valve body for the SS Valve, because it has to mount the thimble…. The exhaust port is 3/8″ (0.375″) and the throat is 13/32″ (0.406″), and the original poppet OD was 0.453″…. At 3600 psi, the force holding the valve closed was 580 lbs…. With the new SS valve, it will only be 280 lbs., less than half…. I will use either 27/64″ (0.422″) or a full 7/16″ (0.437″) for the throat in the new valve, as either will work with the new larger (0.500″) poppet…. Next up is to figure out how to make a new valve body to mount everything inside….
Bob
rsterne
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I continued working on the valve yesterday and today…. I got the body bored out inside, with a shoulder for the thimble mounting wheel, and a groove for the retaining circlip…. Then I made the perforated wheel that mounts the thimble…. Here are those parts….
The total area of the eight 3/16″ holes is twice that of the 3/8″ transfer port…. The valve is bored out to nearly 7/8″, which gives an annular gap of 0.150″ around the thimble, so that area is three times the area of the transfer port…. You can see the 1/16″ vent hole in the thimble mounting screw…. As usual for my SS valves, there is a collar on that screw so that I can tighten it against the end of the thimble without having the thimble mounted rigidly to the wheel…. In fact, the hole is oversize, so that the thimble can align itself with the poppet to prevent any binding….
I also drilled and tapped the 10-32 mounting holes, machined the O-ring groove, and milled out the huge exhaust port…. I machined this a bit differently than I have done before, and I really like the way it turned out…. I did a 1/2″ flat for the transfer port, and then ran a 3/8″ ball end mill in until the center was just past the flat…. I then used a 5/16″ end mill, tilted on a 25 deg. angle towards the valve seat, and milled through to the throat…. I moved the mill sideways in the hole until the width of the port was the full 3/8″, and fore and aft until it just touched the front and back of the 3/8″ hole…. I then removed the valve from the milling attachment, and finished it with a spherical burr on my Dremel…. I recently purchased some adjustable hole gauges, and I set the diameter to 3/8″, and after grinding away any steps inside the port, I kept checking it and grinding the restrictions until the exhaust port was at least 3/8″ everywhere…. Here is a photo of the valve with the gauge in place, and the thimble mounted in the perforated wheel….
This photo shows how massive the exhaust port is, with the adjustable gauge beside the valve, for those of you that have never seen one….
The main tube had a bit of rust on it, so I polished it down and re-blued it with Van’s Cold Blue…. I then lapped the poppet to the seat until it appeared to seal, and then reassembled the main tube, using my “mini-tank” instead of the 500 cc bottle, in case there are any leaks…. I filled it to 3000 psi, and it appears to be holding…. I will leave it overnight before working on a new hammer and spring, because I think (hope) the old one will be way to much for this replacement SS valve….
Bob
I made an SSG assembly yesterday, using two 2.5″ long x 0.48″ diam. springs made from 0.051″ wire…. They have a rate of 10 lb/in, but when you stack two inline that drops in half, to just 5 lb/in…. I had to make a spacer that runs on the guide rod to keep the ends aligned, and after the springs were set, they lost 0.1″, but the spacer replaced that, so the total spring length was 5.0″…. I set the preload to only 1/2″ (2.5 lbs.) and assembled the gun, with the adjuster backed off all the way, so the 2″ hammer stroke was reduced to only about 1.25″…. The resulting roar brought my wife out to the shop to see if I was OK, and I just explained to her that the machine gun she heard was an example of runaway hammer bounce…. 😮
Step two was to make an MDS hammer to the same dimensions as the steel one, which reduced the weight from 165 gr (237 gr. with the original guide and cocking handle) to just 24gr. plus the cocking handle, so the hammer dropped to 48 gr. or just 20% of its original weight…. At zero gap the gun fired with an anemic “pop”, so I knew I was going to have to end up some place in the middle…. So much for yesterday…. :
…. The really exciting part is that the cocking force is a fraction of what it used to be…. This SS Valve REALLY works…. 
This morning I machined away as much of the original steel hammer as I could, and at the same time moved the cocking notch back 1/2″ to reduce the hammer stroke to 1.5″…. The hammer was down to 82 gr. (half what it started out) plus the cocking handle, for a total of 106 gr…. The gun was beginning to show that it had a chance of being tuned, and I obtained velocity readings just under 820 fps with the 336 gr. Lee FN bullet, which works out to 500 FPE, using a 3600 psi fill of the main tube and mini-tank, which is about 145 cc (less than 0.3 cc/FPE)…. That is a pretty anemic plenum volume for a 500 FPE gun, but shows I am likely on track to match the 550 FPE I had before, when I have the 500 cc bottle in place, with 640 cc available to keep the pressure up during the shot cycle….
I ran into a problem when I dropped the action into the stock, the end of the SSG, while cocking, hit the cheekpiece, and wouldn’t allow me to cock the gun…. I pulled it out and shortened the guide rod, increasing the preload to 1″, and moved the sear notch back on the hammer another 1/4″, so I now only had 1.25″ of hammer stroke…. This allowed the end of the guide to JUST clear the stock…. Here are the new parts at this stage….
With this set of parts, I was still getting machine-gunning with 1/4″ of gap in the SSG, really loud inefficient shots with 3/8″ of gap, and workable shots with 1/2″ of gap…. The velocity was around 815 fps, with stellar efficiency of about 1.35 FPE/CI…. pretty remarkable at just under 500 FPE…. With the 216 gr. Lee bullet the gun was shooting just over 950 fps at that tune…. Now remember that I had 1″ of preload on the spring, and with 1/2″ of gap that means the spring was only being compressed a total of 1.75″ when fully cocked…. That works out to about 9 lbs. of force, with a 3/4″ effective hammer stroke on a 106 gr. hammer…. Yet I was getting similar power (other than the undersized plenum, which was costing me about 10% in FPE) to what I had originally with a 2″ stroke on a 237 gr. hammer that took 28 lbs. to cock it…. WOW !!! you can really see what an SS valve can do to reduce the cocking force, even on a big-bore….
I wasn’t happy with the 1/2″ of gap in the SSG, so I decided to try the MDS hammer again, with the full 2″ of stroke and 1″ of preload in the SSG…. With the gap set to 1/4″ I got the gun to fire properly, but the SSG rod was hitting the stock again…. So, I moved the cocking notch back 1/2″ (1.5″ stroke) and increased the preload to 1.25″, and I got just one “proper” shot, with 1/2 turn of gap on the SSG…. I am learning that this particular SS Valve is acting like a Cothran valve, it is either working or not, with no bell curve…. at least that is how it appears right now…. Then things went south…. The poppet failed, the stem started sliding back in the PEEK, so my testing is over for today, and I have to rebuild the valve….
Bob
Well, it has been an interesting few days…. I rebuilt the SS valve with a 3/8″ small end on the poppet instead of 5/16″…. I bored out the thimble, and made a new PEEK poppet…. I used a 0.040″ vent parallel to the stem, and still the 0.062″ vent at the front…. The poppet looks the same as before, just a larger front end….
I tested it with the 107 gr. hammer, which has 1.25″ of stroke, and it popped, but that is about all…. Now remember, that with the 5/16″ front end on the SS valve, I needed to have 12 turns of gap with this setup…. 😮 …. That shows how much the diameter of the balance chamber changes the required hammer strike…. I changed the 0.051″ wire spring to a 0.055″, and eventually to a 0.059″, and with 1.25″ of preload and zero gap on the SSG I managed to get into the 700’s with a 336 gr. bullet…. The valve was stable and showed a pretty linear response to SSG gap, so I knew it could be made to work, but I needed more hammer strike….
I made a new hammer with 1.5″ of stroke, that weighed 140 gr. with the cocking handle…. With the two 0.059″ springs, set at 1″ of preload, I hit 810 fps with just the plenum and tiny tank, a total volume of 155 cc (less than 1/3 cc per FPE)…. As the gun was finally working properly, I stripped it down and installed the 500 cc, 250 bar tank, topped up my SCBA tank, and filled the gun…. Yay, no leaks….
I now hit 842 fps (529 FPE), which was close to the highest I have ever had, and faster than I knew I could get with any kind of efficiency and without a Korean Cliff for a shot string…. It is pretty interesting that the extra volume gained me over 30 fps and 40 FPE…. That really shows how much a small plenum hurts performance…. I backed out the SSG gap a turn at a time, and at 4 turns out I got three shots of 827, 820 and 810 fps with the 336 gr. Lee FN, which works out to an average of 819 fps (501 FPE), using 500 psi of air from a 3600 psi fill…. The total volume of air in this gun is 640 cc (bottle plus plenum), and the ports between them are larger than the exhaust port, so it all can feed the valve…. That works out to 1.12 FPE/CI…. which is pretty incredible for a 500 FPE gun….
I dropped back to the 0.055″ springs, but couldn’t get over 800 fps, so I settled on the two 0.059″ wire springs, but reduced the preload to 0.8″…. That required 2 turns less gap to get exactly the same velocity as before…. I am running out of bullets to shoot full strings, but here are the results, tuned like this…. Basically, the first 3 shots are right at the top of the bell curve, and I couldn’t be happier….
336 gr. Lee FN…. 3 shots using 500 psi total…. 824, 819 and 814, average = 819 fps (501 FPE) at an ES of just 10 fps (1.2%) at 1.12 FPE/CI….
216 gr. Lee RN…. I tried just one shot, it was 965 fps (447 FPE)….I had one other modification I needed to make…. When I built this gun, 4 years or so ago, the bolt slot ended with a thin web at the back of the receiver…. A couple of years ago, when chatting with Lloyd, he mentioned that the gun fired accidently with the bolt open could break through such an arrangement, because the bolt, being accelerated backwards, acts like a hammer, and can exert incredible force if it stops suddenly…. I made up a circle of 1/4″ aluminum, and retained it on the back of the receiver with three SHCSs tapped into the back…. I also added an O-ring at the back of the bolt channel, to provide a bit of a cushion for the bolt, should the gun ever be fired with the bolt not locked down…. I feel confident that this accident waiting to happen (and one I missed the potential of) is now a non-issue…. Here is a photo of the Action showing the new rear bolt stop and the new SSG….
Although I don’t know what a full string would be, tuned the way it is, three shots at 500 FPE within a 10 fps ES will certainly suffice for any deer hunting…. My son has applied for a Limited Entry Permit for our area, and I hope that someday he will have a chance to give this PCP the ultimate test…. I checked the trajectory using ChairGun, and sighted at 50 yards, the POI is within 1″ out to 60 yards…. or I can sight it in at 64 yards and have a 4″ KZ out to 75 yards…. I think the 50 yd. zero makes the most sense….
Here is a summary of the hammer cocking force with the three arrangements I have used on this gun….
Conventional Valve…. 237 gr. hammer with 2.0″ stroke, cocking force over 28 lbs…. ie almost impossible to cock….
SS Valve with 3/8″ front chamber…. 140 gr. hammer with 1.4″ stroke, cocking force under 19 lbs…. quite comfortable and linear….
SS Valve with 5/16″ front chamber…. 107 gr. hammer with 0.7″ stroke, cocking force about 9 lbs…. but nearly impossible to control the hammer bounce….If anyone has any doubts about what the SS Valve can do to reduce the cocking force…. you can put those doubts to rest…. :
Bob
Now that I have my backstop rebuilt, I decided to finish up the tuning on the .457 Hayabusa with the new SS Valve…. I connected it up to my SCBA tank, with a digital gauge in between, so that I could monitor the fill pressure and pressure drop per shot accurately…. I then tried various SSG Gap settings with both the 336 gr. Lee FN and the 216 gr. Lee RN, plus a couple of shots with a their 143 gr. Roundball…. Here are the results….
I then tried a few shot strings at various settings, stopping the string when it fell 4% below the peak velocity….
The velocity with the roundball was 1092 fps (379 FPE), at 2 turns of gap on the SSG…. With the same adjustment I got 956 fps (438 FPE) with the 216 gr. at 0.96 FPE/CI…. The 336 gr. hit 819 fps (501 FPE) at 1.06 FPE/CI…. Tuned that way, I got a 4-shot string with the first 3 shots within a 1% ES (8 fps)…. When I turned down the power the efficiency hit levels I have never seen before in a Big Bore…. At 8 turns of gap with the 216 gr. I got a 4-shot string averaging 919 fps (405 FPE) at 1.19 FPE/CI…. At 6 turns of gap with the 336 gr. I got 6 shots averaging 770 fps (443 FPE) at 1.27 FPE/CI, and the first 5 shots were within a 7 fps (1%) ES….
I am extremely pleased with the way this gun turned out…. I lost a few fps at the top end, but I never tune my guns that way because they are super loud and complete air hogs when you do…. I have mounted a Fitco 2-7 x 32 Scope on it, and I plan to sight it in at 50 yards…. Now if my son gets drawn for a Limited Entry Doe Tag, we’ll be all set…. We see plenty of does close up when Grouse hunting, but rarely see a buck and when we do they are never in airgun range…. I hope we’ll find out what she will do this fall, using the 336 gr. at 500 FPE….
Here is the completed .457 rifle, along with the .410 shotgun upper which fits the same lower assembly….
The rifle weighs 10.5 lbs. including the scope…. Tank is a 500 cc, 250 bar tank from England, total reservoir capacity is 640 cc (39 CI)….
Bob
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