My Hayabusa PCP - The Experiment Continues

[rsterne]

I got my .257 cal barrel about 10 days ago, did the basic machining on it, and then waited for my bullet molds to get here so that I could cast some bullets, measure them, make a chambering reamer, and finish the barrel off.... That all happened this week, and I got to test the gun for the first time today.... My SCUBA tank was at about 2800 psi, but I did manage to run a few short strings through just to make sure I had enough hammer preload adjustment, which I'm pleased to say I do.... I may not have much extra available once I try a 3000 psi fill, but it looks like it will be enough.... Here are the initial results, using the 73.4 gr. solids I cast from my new Lyman #257420 (65 gr.) mold....



With the preload at my maximum reference point (still shy of coil bind), the maximum velocity I got at 2800 psi was 947 fps (146 FPE), easily over my goal of 130 FPE.... I also tried a few of the 87.6 gr. solids from my RCBS 85 gr. "Cowboy" mold, and they shot 878 fps (150 FPE) at just under 2800 psi.... Even higher peak FPE should be available at 3000 psi, hopefully I'll find out tomorrow after my ShoeBox compressor tops up my SCUBA tank.... The curve at 2 turns out on the hammer spring preload is decent, 7 shots starting from ~2600 psi within 4% ES averaging 890 fps (129 FPE) at an efficiency of over 1.0 FPE/CI.... I shot a few of the 66 gr. hollowpoints made from the Lyman mold, and at less than 2700 psi they hit 965 fps (137 FPE).... I expect I'll need to set the preload somewhere around 1 turn out to handle the 3000 psi fill....

So far, I'm delighted with the results.... This gun should be a long range hammer once tuned up....

Bob

[remtom1200]

What are you using for a backstop these days?

[rsterne]

A piece of 3/16" steel plate, two layers of OSB in front, and then about a inch of magazines.... It's due for cleanout tomorrow so I'll be inspecting the steel plate.... Oh, there's two more pieces of 1/2" plywood behind the steel plate as well....

Bob

[remtom1200]

can you take a picture of it after you get this rig tuned? I bet it will be swelled.

[rsterne]

Now that my SCUBA tank was topped up, I could complete the testing on the .257 cal version of my Hayabusa.... The first step was to tether the gun to the tank and fill it to 3000 psi.... Using each bullet, and starting with the hammer spring at just shy of coil bind, I shot two shots (returning the gun to 3000 psi for each bullet), and in each case, the second shot was slower.... I reduced the preload in one turn increments, always starting at 3000 psi, until the second shot was faster, and then shot a string until the velocity had fallen 4% below the peak.... That occurred 3 turns out from coil bind, indicating that was the maximum usuable preload with a 3000 psi fill.... I also shot a full string with each bullet at 4 turns out, recording every shot that was within 4% of the peak, and then graphed the results.... Here is how the power settings affect the velocity....



Also included on the above graph is an average of the efficiency values at each hammer spring setting.... As is typical, adding preload past a given point (2 turns out) at a set pressure (in this case 3000 psi) only wastes air without any significant increase in velocity.... At 3 turns out, the efficiency averaged just under 1.00 FPE/CI, and at 4 turns out, the average was just over 1.20 FPE/CI, which considering the gun is still putting out about 130 FPE is pretty good, IMO.... The maximum power worked out to just over 160 FPE, with the 87.6 gr. bullets at 908 fps.... I'm extremely pleased with that result.... Here is a graph of the shot strings at 3 and 4 turns out....



With the lighter bullets, at 3 turns out, the first shot was the fastest, indicating too much preload, however with the 87.6 gr. bullet, a bell-curve was starting to appear.... At that power setting, the gun was averaging about 140-145 FPE, and the efficiency was running just over 1.00 FPE/CI.... With the hammer spring preload backed out one more turn, all the bullets produced a bell-curve, but the lighter ones were still missing the first few shots, producing 11-12 shots between 3000 psi and about 2300, with the peak occurring at about 2600 psi.... With the heaviest bullet, the 4% ES string started at about 2800 psi, and produced 14 shots ending at 2000 psi.... All the bullets averaged about 130 FPE, with the 66.0 gr. bullet having an efficiency of 1.08 FPE/CI, the 73.4 gr. working out to 1.12 FPE/CI, and the 87.6 gr. producing 1.24 FPE/CI....

If I wanted to have the string include all shots within a 4% ES, I would have to reduce the hammer spring preload slightly more with the lighter bullets, and that would lower the velocity a few fps and extend the string to about 13 shots.... Alternately, I can leave the gun the way it is, and have 10 shots within 20 fps.... which considering the long range use the gun is likely to see, is probably the better choice.... Those 10 shots average 907 fps with the 73.4 gr. solid (134 FPE) and 940 fps with the 66.0 gr. hollowpoint (129 FPE), and I would stop shooting at 2400 psi, achieving an efficiency of 1.16 and 1.12 FPE/CI repectively.... I still have to test the 77 gr. hollowpoint from the RCBS mold.... and all bullets have to be tested for accuracy at longer ranges before a decision can be made about final tuning.... Here is a photo of the completed .257 Hayabusa, with the barrel and main tube still "in the white".... I won't be doing any bluing until the .308 and .357 (9mm) versions are assembled and tested....



Bob

[rsterne]

I topped up my SCUBA tank and tested the 77.2 gr. Hollowpoints cast from the 85 gr. RCBS 'Cowboy" mold today and added the data to the graph.... Here are the shot strings starting at 3000 psi for all four bullets.... The hammer spring preload was 4 turns out from coil bind for all of these strings....



You can see what is typical of what happens as you increase the bullet weight.... the peak of the curve moves to a lower pressure.... I included all shots above 96% of the peak velocity, which resulted in 14 shots for the heaviest two bullets, 12 for the 73 gr. and only 11 for the 66 gr.... but you can see that the reason for the shorter strings is partly because of the 3000 psi fill pressure cutting off the top of the curves.... That may not matter, as I will likely reduce the ES range to around 2% which will drop the strings to about 10 shots anyway....

All these bullets produce an average of about 130 FPE over the shot string, peaking at about 135 FPE.... which is a ton of energy for a quarterbore.... Really all that remains is to do some accuracy testing to determine which is the most accurate at long range.... I plan to strip the gun and reassemble it with the larger breech, valve, and hammer in preparation for the initial testing of the .308 and .357 cal barrels shortly....

Bob

[rsterne]

I've been working on assembling the .308 version of the Hayabusa over the past few days, and I ran into a problem.... I can't get enough hammer strike to make the velocity peak at over about 2300 psi.... This is despite having a heavier hammer with more travel, and a heavier spring than the .257 version.... The new valve has a larger throat, at 0.312" instead of 0.281", and I used a Disco poppet instead of the MRod poppet on the smaller valve.... After fitting an even heavier hammer spring, again with no success (and the gun was so hard to cock it literally took two hands).... I decided I better pull the valve to have a look, and here is what I found....



The sealing face of the poppet started out dead flat.... Note that I was using the latest Disco poppet, with a hole in the top....



The face of the poppet was extruded into the throat about 0.040".... The stem had slid back, and I thought that might have been partially due to the hole in the top allowing air pressure to act on the end of the stem.... The face of the poppet had completely taken on the shape of the raised seat, and was sealing on a diameter larger than the outside of that seat, which was 0.375".... The head of the poppet was actually belled out to a diameter of about 0.44", and was sealing on nearly the outer edge.... No wonder I couldn't drive it off the seat.... Under load, it was probalby wedging into the throat as well.... I talked to SeanP on the phone, and he informed me that the molding force required for Delrin was 270 lbs, and my calculated seat force was nearly 300 lbs. at 3000 psi, so that is what we think is happening.... the force is causing the Delrin to flow....

I had one of the older Disco poppets, without the hole, and wanted to try that, so Sean suggested that I try fitting a metal ring around it to keep it from expanding, which I did.... I also chamfered the edge below the ring slightly, hoping to prevent the seal diameter from getting bigger than the OD of the seat on the valve....



I put the gun back together, and there was no difference.... Upon disassmbly, I found that the face of the second poppet looked basically just like the first, it had a big groove pounded into it, and was extruding into the throat of the valve.... Interesting, the groove on both poppets is deeper and narrower than the shape of the slightly raised seat on the face of the valve.... There is some serious flowing of the Delrin going on.... I will be pulling the gun apart and reinstalling the valve from the .257 cal version.... The throat is a bit smaller, but at least the hammer will open it....

Bob

[rsterne]

Well, installing the .257 valve at least got the gun working in .308 cal.... At coil bind I got over 200 FPE on the first shot, which makes this rifle more powerful than my DAQ .308 Exile with the same 119 gr. Lee RN bullets.... My SCUBA tank wasn't quite at 3000 psi, and dropped with each fill, so all I could do was some preliminary testing.... but it was quite satisfying....



The gun is quite sensitive to hammer spring preload after 2 turns out from coil bound as that is where you start to get "play" in the spring.... so there was a big drop from 2 turns out to 3, and that happens to be right where I need to be.... I have a 131 gr. hammer with 1.45" of travel driven by a 3.0" long spring made of 0.051" wire.... so the gun is quite hard to cock.... I'll do a bit of playing around tomorrow and see what I can get for a shot string....

I know there is more lurking in this gun once I figure out how to get the bigger valve working.... However, I already accomplished my goal of breaking 200 FPE at 3000 psi....

Bob

[SeanMP]

Excellent Now that you have a working valve your back to not needing as much spring.

photo (2).jpg

I still think there is merit in this design

[rsterne]

That is not unlike a Hatsan valve.... I was wondering if you captured the Delrin completely like this....

Valve.jpg

The blue circles are an O-ring, likely a quad seal as it would get squashed flat by the pressure.... Would that work on a 5/16" throat at 3000 psi, or would be still need to go to PEEK?....

Bob