Hey Harve, as Im shure you know, Marcelo just got back from "across the pond" on buisness. Im shure he has alot of "catching up" to do. This thread may fall down,,give him a chance to catch up w/ his personal obligations. He is a guy w/a true love of our sport & will shurley respond to someone of your high standing.Tim.
But I have been debating this in my head for sometime.
There are two ways to detune a gun, one of them would be terrible, the other would be just dandy, but it is expensive and/or longer to do.
The easy way, but, as is often the case with easy solutions, not the best, would be to insert a smaller/weaker spring.
Having done that some time ago, I can tell you you pay heck for that, gun becomes sensitive to the extreme, and when you think of it it is logical, pellet is slower and gun has time to recoil, vibrate and move much more in the longer time it takes the pellet to exit the barrel.
What I think would be the "proper" way, but I cannot test this for now, is to short-stroke it. Just taking a piston and putting in a longer rod. As long as the rod needs to be to achieve only the swept volume needed to achieve the level of power desired. This means that the trigger to muzzle exit time gets shortened, the recoil and the vibration gets also shortened and everything should prove to be smoother and better. The trick is, I insist, to make a piston with a longer rod.
Ideally, you should be able to get a 12 ft-lbs D54 in 0.177". That would be an ideal platform for a spring-FT rig, or a Brit-legal hunting gun. In a world of similars, it would be like a turbocharged FWB300.
I am sure Marcelo will post his opinion on this soon.
I know your level of accruacy surpasses mine but you're causing me to ask some questions here lol.
Are the ten meter purpose built springers all short strokers? I know that some have short barrels in sleeves but not all. I can't compare my Diana 75 as its a dual opposed piston design, but my HW55 essentially became, more or less, what the R7s are today. (I keep going inverse of what I'm trying to say argh)
While the operational length of time from the trigger release to the pellet exiting the barrel in a springer is longer per the stroke, the time between a longer stroker and a short stroker has got to be in milliseconds. I would guess too short a time for the shooter to react to one versus the other? That's my feeling anyway. It would then be imperative to follow through completely on the shot. I mean, I have a hard time envisioning there being enough time between the two designs (other than on paper) that a shooter would find himself less perfect with the one than the other with regularity.
I'm a believer that nothing matters nearly as much as follow through. Maybe that's where I'm stumbling here?
Try the email on this post. I just got a new pc so things are still being learned. Vistabulous NOT
As usual, you do not ask easy questions, but let me do my best to address the issues:
The greatest 10 meter gun ever built was the FWB300, no question about that. It was reasonably inexpensive, it was pervasive and prevalent and it was in production for a long, long, long time.
Having said that, you would need to have seen the internals of one of those things. Three things called my attention when I opened one of them:
a) The length of the stroke is barely 4", the piston diameter is about 1 1/8", if memory serves me right; and the piston "seal" is split into two parts that do two very different functions: On one hand is the sealing function performed by a metal ring (like in a car or motor-bike), on the other is the "last thousandth compression/shock absorber" function that is performed by a bumper in the end of the piston.
b) The spring is wound in two sections, one right handed, the other left handed. Supposedly this reduces torque upon firing as the two sections would have opposing torques that would cancel each other.
c) The "detentes" that hold the action in place just befor firing are very very soft. In line with the reduced power (6 ft-lbs), but also thinking that he gun will not stop recoiling before the pellet leaves the barrel.
If you take all this into consideration, you arrive to the conclusion that those three things are what's need to be accomplished. As Tom so aptly points out in his post: reducing spring power may cause more problems than solutions.
All these things can be accomplished easily with the D-54 and I have read dozens of shooters that would die for a 12 ft-lbs. FWB300; so what is the problem? Cost. Diana tells me that for even dozens of shooters a short stroke piston would be prohibitively expensive if done at the plant, and that they are very happily selling the 12 ft-lbs. weak-springed version in the UK.
I suspect that deep down what appeals to the UK shooters is that they can swap springs and get full powered versions easily and that would not be possible with a short-stroked version. You can always say that you got a gun with the wrong spring, but it would be hard to get gun with the wrong piston.
If you take your D75, a great gun, but one that did not impact the market greatly because it was not truly mass-produced nor long lived, and measure the length of the stroke, you will surely find a stroke of similar length to the FWB300.
For comparison, the stroke in our D-48/52/54'S runs at almost twice that.
One of the things that pushed me to conclude that the short-stroking was the "proper" way to go was precisely thinking about Harmonics: In a short stroke, almost all harmonics will be high frequency, whereas in a weak-spring-long-stroke, harmonics will be low frequency.
High frequency harmonics are easily cancelled by changes of materials and interfaces between stock-receiver-barrel, low frequency harmonics are not easily cancelled, as they travel well through almost all materials.
Maybe the amount of energy in the harmonics would be the same, after all the energy in the pellet is the same either way so, if Newton is right, the energy in the recoil should be the same. The question here is if the frequency of the harmonics will be easier or more difficult to deal with.
You are perfectly correct in that in airgun shooting follow-through is almost everything; as a matter of fact, this is the main reason why the D-54 is so successful in turning mediocre shots (like myself) into better shots: Follow through is done by the gun. Not by the shooter.
But my feeling is that resonant frequencies in the lower region will be more problematic than high frequencies.
As much as I hate to admit it, this will all be theoretical for the time being, as I have no way to test it. But I'll get around to that one of these days.
Tom @ Buzzard Bluff (Login TheOldBuzzard) 208.54.200.85
Case in point
March 5 2008, 1:40 PM
To prove your assertions:
I recently bought one of the Turkish Daisy 1000s when Wal-mart was blowing them out just to see if they were as bad as I'd read on the net. To keep the story as short as feasible let me say that it was the harshest cocking and firing cycles I've ever experienced with the dirtiest barrel I've ever seen. But as also indicated by my reading it did evidence a desire to shoot within a single zipcode.
Last week it finally got to the top of my list. I tore it down to find work equally as crude as ome of the early Chinese efforts. Even the rear plastic plug that serves as the rear spring block was cast with one side @ 1/8" lower than the other producing the horrible cant in the spring I'd been able to see thru the cocking slot. The ends of the spring were't even ground flat and I dug enough swarf out of the piston seal to start a scrap iron savings account. The breech seal was toast from being installed haphazardly. I called Daisy and made arrangements to return the Chinese scope that wouldn't focus at any range and get replacement breeh and piston seals. Breech seals they have but the lady I spoke with indicated that she'd see if the repair department might be able to find a main seal for me since they didn't support internal parts. I called again this past Monday to order other parts and enquire about the previous items. They had sent it out----to the wrong address.
I spent almost a full half day on the internals of the gun trying to get all of the rough edges addressed. Since I had lost confidence in the viability of the piston seal, the breech seal and the transfer port appeared oversized to my eye I decided to reassemble it using a shorter and weaker spring from the ubiquitous Chinese underlever family to try to reduce the horrible piston slam at the end of the stroke to see if I could learn anything while awaiting parts. I also addressed the trigger with satisfactory results. And I did something I've never done before----I lapped that filthy barrel with JB bore paste until it didn't feel quite like dragging a screwdriver tip across a file when I pushed a pellet thru it with a piece of brass rod. Witness marks on the pellets' skirt indicated a much smoother bore with good sharp edges remaining on the lands. I also touched up the crown with some fine polishing paste until it evidenced better uniformity thru a good magnifying glass in conjunction with an eye loupe. I reassembled it and shot some groups. It still slams the piston into the end of the compression chamber indicating that my assumption regarding the integrity of the piston seal and possibly transfer port size was justified since I had made a suitable temporary breech seal that betrayed no losses. Other than that the cocking and firing cycles were much more civilized. But it wouldn't hit the side of the barn from inside the barn! The accuracy had fled! I've been at this game long enough to be pretty sure when I've screwed up so I'm 99.44% sure that I only helped the barrel. So what does that leave as an explanation for the departure of accuracy? AFAIK the greatest change was to the frequency of the firing cycle. But I'm open to suggestions! Could the pellet still be in the barrel now when the piston slams into the breech end? Had it left prior to switching springs? Here's what I intend to try if the *&^%$# weather will allow. I still have a Vortek muzzle brake that allows tuning the vibration nodes of the barrel. I want to do some tests using it to see if I can get it shooting straight again before I put new parts in it. IF it works it will serve to indicate that there is a work-around in the case of reducing velocity and harshness by only altering the spring in a gun. That would be a LOT easier than fabricating a new piston rod! And it would be SO easy to modify a standard muzzle brake to work the same----even without access to a machine shop.
Or that's the current plan anyway----subject to change on arising to a new day.
Abrazos, Tom @ Buzzard Bluff
As you can see from the answer to Harvey, I do thinkk you are right, harmonics get horrible when you reduce spring power.
Curing an illness, is not, IMHO, as good as preventing it. I do think you can cure the harmonics, but I feel very strongly that it would be better if we could avoid creating them altogether.
Rick and I have been talking about this very subject only we've been attacking it from a different angle.
When all is said and done, what changes the pitch of an instrument or something in which it could be said, has a voice?
In order to produce a proper staccato you need two things; the "gate" must be small and the pressure must be abrupt. You need an ultra quick attack through just the right size... transfer *port*.
Whoever coined the phrase "tuning" air rifles was so far advanced that individual must laugh if they're still alive to read what some of us say on these forums.
I think what we're supposed to be doing is tuning for the right pitch of sound of a given air rifle. That means that once everything else can be attended to, the "voice" of the rifle still is heard through the transfer port. (actually being out of tune has something to do with that dissonant vibrato but that's another argueable debate lol)
Hector, as you said, you raise the pitch to eliminate long wave tranfers through the instrument. ( I can imagine a group of Monty Python extras charging King Richard's coconut horses yelling Sack the Hum!!)
In order to reduce piston slam, like your rifle has, Tom, could you find a way to adjust the transfer port size?
I don't know if there's such a formula, but if there was, wouldn't it be some equation with variables for fpe energy per linear inch compression of the spring, piston weight and diameter versus port diameter?
I suppose spring diameter and and coil count would have to part of it somehow but my head is starting to hurt.
Hector, Tom, my deepest thanks for this discussion. Rick, naturally to you too as you got this mind ball juggernaut rolling lol. I have more questions but they can only follow if the answers are going in the direction(s) I suspect they might.
Funny you should ask that. I was handling a friend's 52 yesterday (which is the same gun, fancier stock). I liked it quite a bit and is a great hunter, I'm sure. For me though, that thing de-tuned to 700fps would be the cats arse.
I agree with Hector that the true way to de-tune is to short stroke it but it's very time consuming and requires the use of a lathe which I don't own any longer.
I detuned a Diana 45 just on Sunday and got it down to 625fps and the thing is sweet. Cocks like an R7 but in a fuller figured size for those so inclined.
After checking JM's site, I notice he still has the GRT kits. I would take one of those springs/guide and cut off a few coils and see how she does.
Yeah, Hector, you're way ahead of anything I've even considered. MZ, a detuned 48 series could really be something.
What would you think of a detuned 48 (done right for the shorter cycle time) with a barrel cut to carbine length and choked? Top it off with a barrel sleeve (not a shroud) glued on to add stiffness to the barrel?
My guess is the forward weight would be nominally different than stock, depending on the cut down length and the thickness of the sleeve. Weight wise, a possible wash compared to factory barrel weight.
Maybe a carbon fiber sleeve would not only be nice, but be stiff as it could be. If glued with some elastomeric glue it could also function as a vibration dampening thingy; in my beloved D52, the sleeve was glued with hot melt glue by heating both pieces and then sliding them (Muzzle plugged) till the whole thing set. The result is a single piece of hardware that does not vibrate even when producing 18 ft-lbs in 0.177"
For certain swept volumes and pellet weights, 12" of barrel is more than enough. But I owuld not reduce it much more than that.
