Air Gun Home :: View topic - New Old Project - Hayabusa PCP

rsterne · Moderator

I wasn't very happy with the hammer spring I was using, it had no preload and must have required 40 lbs. to cock it, so I searched for a longer spring of smaller wire.... I couldn't find anything ready to use, but I did manage to find a 10" long spring made of 0.062" wire, and when I finally got it in the mail, the first thing I did was "set" it to collapse the coils so that in future during use it wouldn't get even shorter.... It's a good thing I did, because it lost 2" in length, ending up at only 8".... I cut 2" off and tried it, but the preload was too much, I could hardly install it, and it turned out that I didn't need that much, so I cut it down to 5.5" and that worked out great.... I can still max out the gun with the 336 gr. bullet at 855 fps, with a bit of preload left, and even at that setting the gun was much easier to cock.... I tried it with the 216 gr. bullet and it did 979 fps (460 FPE), and I had to back the preload down 6 turns to get a proper string, and at that setting it was MUCH nice to cock.... I shot three strings at different preloads, and the results are below.... The caption on the pressure line is wrong, that was at -8 turns....

At 6 turns out I get three shots within 10 fps and two more before the velocity has dropped by 4%.... The first 3 shots average 965 fps (447 FPE) and the 5 shot string works out to 0.86 FPE/CI, not too bad for that much power.... At 7 turns out I got 6 shots averaging 934 fps (419 FPE) at 0.90 FPE/CI, and at 8 turns out I got 7 shots averaging 922 fps (408 FPE) at an efficiency of 0.96 FPE/CI.... Any of the three strings would be fine, but the first one, with 3 nearly equal shots at almost 450 FPE with two more within 4% looks very tempting....This pretty much completes the development of the .457 version, when I get a chance I will test the .410 shotgun with the new spring....

Bob

Bob La Londe · Veteran Member

I've been doing some studying on your air shotgun project. Well parallel thinking. I think you were the one who said you need roughly 1cc per FPE of air plenum. Well 1/2 oz of shot at 1100 FPS is 588 FPE according to Chairgun. I consider personally consider 1100 FPS to be the low end of velocity for effective shotgun hunting. Basically what you get out of a promotional field load.

I've also done some study on shot load efficiency and see where larger shot is more effective at range than smaller shot. I think we all know that, but it bears pointing out.

900 FPS will obviously drop a deer at shotgun ranges with a big enough pellet, but at some point we need circle back around to why people want a shotgun. For me its to put a cloud of shot into the air in a uniformly distributed pattern to give me a larger kill zone for wing shooting. If we fire 3 40/41 caliber lead balls we could certainly drop a goose if we could hit it. I've met people who could wing shoot a bird with a rifle, but there aren't many of them. Never mind that lead is not legal in the USA for Ducks, Geese, and a few other species.

I think the key is to work backwards. Pick the load that is the minimum to do the trick and go from there. Steel is lighter than lead. Go for the same volume (not weight) as lead in a size adequate to drop the target species at short range. By dropping the weight slightly you get more velocity which helps with shot placement, but it also reduces energy. Bump the shot size slighty and the energy is increased per piece of shot but not per load. The pattern density or size is not as good.

Then I think the shooter needs to compromise with their shot distance. A few years ago I hunted (powder shotgun) with a couple other guys every day of dove season. My shot to kill ratio was abysmal maybe 40%. The next year I went solo or with my son and my shot to kill ratio was 80-90% over the whole season. I had a limit on closing day with only one bird un-recovered. The purpose of writing this is to point out that a hunter who takes his time, picks his shots, and isn't rushed by other people trying anxiously to get the shot first can get a lot of close range shots.

I think that a smaller pattern and more practice will be in order for any reasonable air-shotgun hunting. Also, picking shots that are easier and closer and not wasting shots on longer range targets will be a must.

Before I go any further in my thoughts... 1/2 ounce is 218 grains. I found that a Sam Yang 500CC gun can get about 4 shots (maybe 5) with a 210 grain bullet in the 679-681 range. Your comments in previous posts indicates that a 36" barrel could in theory add 22% more velocity, but would also use more air. That brings us to 828fps in theory. 1-2 shots?

Now about selecting a load. I figure (just guessing) that 900 FPS is a minimum. Steel is roughly 70.4% lighter than lead. Your 218 grain load drops to 153 grains (apx). If you can accelerate that to 900 FPS that's 275ft lbs of energy. This seems much more doable than 588 FPE. We have to bear in mold that less energy is less energy. I am thinking larger shot (bb maybe) at full choke. This means close accurate shooting, but possible. Maybe 4-6 shots with 500cc of air at 3000 PSI.

The closest shot example I could find on-line was an Evanix Tactical shooting 143 grain (our target weight is 153) pellets at 720 FPS. That review said 12 "usable" shots, but did not say how many full power shots. 22% increase in speed for a longer barrel gives us 878 fps. That's getting close to my self imposed minimum.

No I am 100% certain you could build a more efficient gun. Lets say 10% increase in efficiency. That gives us just over 900fps, but for only 3 or 4 shots.

Basically you wind up with a 25 yard gun (maybe) with a small pattern. It would also be either unwieldy with an extra chunck of barrel sticking out the front or a more complex build if you went with a semi bull pup style action placing the action next to your face when shooting.

I think its possible, but the shot count makes it unpopular. Even on duck hunts I'll take more than 3 or 4 shots. Lets get back to plenum size. If your cc per FPE is dead on we are probably looking at 1 or 2 shots with a 500CC air reservoir at 3000 PSI. One thing I think might make it better. A high pressure regulated bottle. It sounds simple at first glance, but lets get back to plenum size. If your cc per FPE is dead on we would need a 250CC plenum. So... with a 2500CC plenum at 3000 psi and a 500CC reservoir at 4500 PSI maybe we could get back up to 4 or 5 shots or so.

Now I know I strung a lot of stuff together to get there and some of it may not apply, but I think its atleast a good rough guess.

I think its possible. I think an air gun builder like yourself could build it. I don't think I could with what I know. I think its shot count would make it a novelty unless you had a large SCBA bottle in the blind with you Maybe tethered with a long high pressure hose.

~~

As to a method of holding the valve open I have an idea. A barrel port with a tube to a detent that hold the valve open. When barrel pressure drops (load leaves barrel) the detent spring pushes the detent back and the valve closes. I think that would be pretty wasteful of air, but it could give you the hang time you need to get the velocity.

rsterne · Moderator

I agree with much of what you say, but there are a couple of basics about PCPs that you are missing.... There are practical limitations on the power you can generate with any PCP, based on pressure, barrel length, and caliber.... 588 FPE in .410 bore is going to need either a lot of pressure, barrel length, or both.... I will certainly NEVER see it from my Hayabusa.... Holding the valve open past the point where the projectile has passed the 50% mark in the barrel, right up to where it exits, will only result in an increase of about 3% in velocity, while using twice the amount of air.... Almost all the extra air is simply wasted.... Obviously, leaving the valve open after the bullet has left wastes even more, for NO benefit except more noise.... Dump valves are a complete no-go, IMO....

Using steel shot to increase the velocity MAY actually be of some use at the limited ranges I am contemplating.... This gun only has one use for me, and that is Grouse, mostly Ruffies, and only at close range.... I'm not sure which would be better, using less #7 1/2 lead shot (say 3/8 oz.) to increase the velocity, or going to #6 steel shot and staying with the full (volume) load, at that same ~ 3/8 oz.... It would appear that the number of shot per load (~135) would be the same either way, at the same velocity.... The question then would become which would have the greater retained downrange velocity, energy, and penetration....

Bob

Bob La Londe · Veteran Member

Bob La Londe · Veteran Member

One more thought. At these speeds I am sure muzzle erosion will be a factor with steel shot. From stuff I have read, gunsmiths I have spoken with and friends who hunt steel more than I do. A straight barrel with an extended screw on choke tube should take the brunt of that and is replaceable.

While a regular barrel might have some damage at the force cone, I do not see you even using a force cone in a barrel for an air-shotgun if your wad is sized right. I would very much recommend using a plastic cup wad that is a good fit to the barrel.

Bob La Londe · Veteran Member

I re-read this thread regularly because there is so much information in it to absorb. I thought about my original idea for valve timing, and its still just about that simple, except it would need two barrel ports. When it passes the second port pressure equalizes and a very light spring could return the detent to the free position.

My big concern is that pressure right in front of the load will be building so exact tuning of the mechanism would take some experimentation, so that the timing is just right.

Also, instead of holding the valve you might consider holding the hammer. It would have the benefit of guaranteeing no hammer bounce, but would need to be much more robust due to the mass and energy of the hammer. When the detent releases the valve spring gently pushes the hammer back.

I know this makes for an overly complex gun, but wouldn't it be awesome if successful. Hint! Hint! LOL.

rsterne · Moderator

Dump valves are very inefficient, and will only work at constant pressure without allowing the velocity to drop with pressure.... Timed valves (eg. cam or electronically operated) can be more efficient, but will have the same problem, the velocity will drop with pressure.... Both types, therefore, are only usable, IMO, in regulated PCPs.... If I understand you correctly, you are thinking of a second barrel port, part way along the barrel, to trigger the closing of the valve.... Since you want the valve to close with the pellet about 1/3 of the way down the barrel for good efficiency (1/4 to 1/2 is the normal range), and there will be a time lag, the port will have to be closer to the breech than where you want the valve to close.... It only takes the pellet about 1 millisecond to travel from the mid-point of the barrel to the muzzle.... If you can figure out a way to get the valve to close in a fraction of that time, then maybe the idea has some merit in a regulated PCP.... In an unreglated PCP, it would still have the declining velocity curve found in a timed or dump valve....

Bob