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PostPosted: Wed Jan 14, 2015 3:58 pm 
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The transfer port is a factor, but how depends on the gun. For some earlier and/or cheap guns like a B3 it's merely a port to pass air and it needs to be big, like big as the bore. Most guns use it differently, it's more like a storage area. Picture the piston compressing all the air into the port, then that air launches the pellet. That would be ideal but reality is the pellet moves before the air is fully compressed and the piston doesn't stick around to maintain that pressure, but it does happen for a brief moment and it works well. So just like the piston weight, the port has an ideal size too. You can't change the length, at least most of us can't, but we can change the diameter. If the port is too small then pressure gets too high too fast which contributes to piston rebound, and since the shot happens so fast you won't get all the air possible to the pellet. If the port is too big the piston will slam into the breech harder yet never build peak pressure. So again there is an ideal port size, but there is a window that works well, which is good because most ports are very rough which impedes flow so you really need to drill it out and sand it to get peak power. Just drill/sand the minimum amount needed and you should be fine. For example a typical generic Crosman port is .125",so I'll drill to .1285 first, and if not enough I'll go to .136. If still not enough I'll go to .140, but that's the outer limit and often the gun loses power there. Going from 125 to 140 is actually quite a big jump and I'm surprised it still works, but I'm guessing that's your window with that gun. Bigger guns need bigger ports, so a magnum will have .150 +- .010 or so.
The barrel length question is thanks to that old Cardew book. The problem is those tests were done some 40 years ago when a 5ftlb springer was probably a magnum, so it doesn't apply to todays guns. If you have a generic 18 or more ftlb gun then only cut the barrel if you have muzzle damage that needs to be removed, otherwise you lose power. Cutting 5" off is a complete waste of valuable fps and greatly increases cocking effort. If I had to guess I'd say an inch of barrel per ftlb in .177, but that's an estimate. In an 18ftlb gun with an 18" barrel you will lose a little by cutting off one inch, and it makes me wonder if 177 magnums could use a longer 22" barrel?
That QB57 can be quieted by making a sheath for the spring, or should I say lining the piston, and/or using spring tar. Plus buttons will help some, as will using heavier pellets.


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PostPosted: Wed Jan 14, 2015 5:53 pm 
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Location: Vancouver
Thanks for the further elaboration Chevota. After reading some of your writings in other forums last night, as well as some of the Cardew stuff and a few other forum threads (especially a couple on airgunbbs) I decided that I'd chop off my QB57 barrel and see what happened. It's not exactly a huge risk with such a cheap airgun, and worse comes to worse I could order a replacement barrel for a bargain price. I didn't want to spend a lot more time on this one, so after frowning at it for a while, hacksaw in hand, I chopped it down from 16" to 10". Re-crowned it on the lathe. Mounted it and did some testing.

Right off I saw a drop of almost exactly 30fps with JSB Heavy 15.89gr. And it felt a bit harsher, and sounded slightly louder under my ear. So I ran some other pellets through it, and quickly found that it seemed to quiet down and feel smoother when shooting 11.9gr RWS Hobby pellets. Which is weird, because it was definitely louder and rougher with those previously. And they ran slower with the full length barrel. Which, when I thought about it, made sense. About a 25% lighter pellet than the JSB Heavy is going to suffer more velocity loss due to friction in an excessively long barrel, when the piston has stopped doing its work and the pellet is just coasting. The Hobby actually went up by more than 50fps! The only 14gr pellets I have around are the Industry things, and those didn't change velocity at all, and shot harshly. Accuracy is unchanged with the JSB, and while my group jumped up and to the left a full inch with the Hobby, the group with those was identical in diameter to the JSB. So it looks like I've found a way to cheapen the eating habits of my newest airgun, with the Hobby pellets costing me half what the JSB Heavy go for! That's a pleasant outcome, even if not what I'd expected. And after re-building the shroud for the shorter barrel I like the handling better. Shorter feels more natural with this bullpup.

I doubt I'll be playing with the transfer port. It's very shallow, a little less than 3mm thick there. Diameter is a hair under 5mm, or about 0.190". It's slightly rough, but not really a concern with such a wide diameter as air is probably blasting through there with minimal resistance. If anything it might be interesting to try a stepped tubular shim, turned out of steel with maybe a 0.5mm lip to keep it in place from the inside. But frankly it's more effort than I'm willing to put into this thing without knowing with reasonable surety that there would be a benefit. I'll leave well enough alone and shoot Hobby pellets and be happy.


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PostPosted: Wed Jan 14, 2015 7:32 pm 
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Interesting result Gerard! I'd just never put much thought towards springers before, and this thread has really opened my eyes. I should get those books and do some reading.

Jim


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PostPosted: Thu Jan 15, 2015 6:50 am 
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Gerard: Lower powered guns don't need as much barrel, and .22 needs less than .177, so between the two a 10" just might be ideal. I would not make the port smaller since it's like the B3 I mentioned which needs a bigger port. I would consider opening it up to 5mm, or maybe 5.25, but I would certainly radius the inside edge and smooth it with fine sandpaper. I would also fix any seal damage it may have, and reduce dead space at the seal/piston face. Cutting off some barrel reduced the guns weight so it will kick more, be louder, and probably more hold sensitive. You could add some weight somewhere if you wanted. I suppose filling that hollow forend with silicone would both add a bit of weight and help dampen the sound and feel. You could also drill a hole in the stock under the pad and insert a steel rod. Just be sure you secure it well so it can't budge, like a couple big course screws threaded in at an angle behind it. Or maybe use a big lag bolt as your weight, as in cut the head off and make a slot to screw it in. I would also place the weight up as high as possible to keep the CG of the gun as close to the center line of the piston as you can. For example putting weight in the pistol grip would be the opposite of ideal. A steel muzzle brake would probably be the best place to add weight, and it should balance it out better for you, but weight at both ends is an even better option.


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PostPosted: Thu Jan 15, 2015 12:10 pm 
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Gee, sounds like your preference is for a heavy airgun. I was actually saying that I LIKE the slightly reduced weight, with the QB57 feeling more balanced now that the barrel and shroud are shorter. At almost 6 pounds it's by far the heaviest airgun I have, so I certainly won't be adding any hardware to weight any part of it. I will be putting some measures into place to damp the spring noise. Probably making an Ertalyte spring guide today as this material (a dense technical plastic with great stiffness and strength and very low friction, more slippery than Delrin but not quite so slick as mushy old Teflon) seems well suited for this purpose. While it's open I'll consider what I have around the shop which might do for a sleeve around the spring.

After that I can add a bit of sound-absorbing material (light weight, not sure what yet) to the voids in the stock while leaving enough room for the trigger mechanism to move freely, and I'll be building up some sort of laminated acoustically absorbent structure on the exposed part of the cylinder in the area of the spring and piston, as well as wrapping the cocking lever such that it seals against this covering when locked closed. The opening clear through to the spring under that lever is the single biggest source of noise from this airgun.


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PostPosted: Thu Jan 15, 2015 4:22 pm 
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I prefer lighter too, I was just giving some thoughts on noise reduction and reducing the kick. Lots of people like extra weight for those reasons and add a pound or so to an already 7lb gun. 6lbs is very light imo, my lightest (not counting my Daisy Red Ryder) is ~6.5 w/o the scope which is a Benj Titan/Trail mix. My heaviest is ~8.5 w/o scope, and ~9.6 with.
Are you going to make a spring guide or sheath? I make sheaths, but really it's a piston liner. It keeps that end of the spring from vibrating, then the oem guide handles the other end.


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PostPosted: Thu Jan 15, 2015 4:48 pm 
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Chevota wrote:
I prefer lighter too, I was just giving some thoughts on noise reduction and reducing the kick. Lots of people like extra weight for those reasons and add a pound or so to an already 7lb gun. 6lbs is very light imo, my lightest (not counting my Daisy Red Ryder) is ~6.5 w/o the scope which is a Benj Titan/Trail mix. My heaviest is ~8.5 w/o scope, and ~9.6 with.
Are you going to make a spring guide or sheath? I make sheaths, but really it's a piston liner. It keeps that end of the spring from vibrating, then the oem guide handles the other end.


The kick doesn't really bother me. Sure it's there, but not serious. Noise reduction is definitely of interest as this is the only airgun I own whose power source is so noisy, while also residing right next to my ear and pressing against the bottom of my cheekbone. So yes, I've sheathed the spring today. Found some good quality plastic (someone gave us a snowman from a very fancy chocolate shop and the box grabbed my interest right away for the nice plastic) and wrapped it about 1.25 turns with the stuff, just shy of the full inside length of the piston's minor inside radius. It's a sloppy fit for the spring, but mainsprings expand slightly on compression so it seemed good to leave it like that. I also put about 30 grams of lead/tin solder (left over from a weight I was testing in trying to isolate a buzz in an electric upright doublebass headstock) in the front of the piston to replace the hardware I'd cobbled together previously. Shorter and a better fit, turned down to where I had to press it into place lightly with a dowel. And following your recommendation I bored out and polished the transfer port, but only to 4.8mm Easy to go bigger should I want to, difficult to go smaller later.

Also made a new spring guide from Ertalyte.
Image

A snug fit, but the stuff is so slippery it pushed through easily. Put things together... and can't fire it. Not with the trigger anyway. I'm puzzled, as I copied the spring guide length and rear section length exactly from the factory steel guide, so there's no change there except taking up the slack on the ID of the spring. Gun cocks just fine. So I tried adjusting the little bolt at the back of the sear block which is the only trigger adjustment on this gun. After a few turns in, bang, there went the pellet. Into my putty trap as I don't like shooting out walls and windows. Turned it back about half a turn, loaded and tried again. Still wouldn't fire. I'm staring at it a lot, searching for solutions online, but so far nothing. So I'll pull it apart again and try to figure out what the heck I did wrong.

For what it's worth, the pellets are going further into the duct seal than previously and the noise is dramatically reduced thanks to the wrap and the plastic guide. A massive reduction. Haven't tried firing next to my face yet as that'd be awkward using a screwdriver to do it... Seems the sear just isn't being pushed by the rest of the mechanism for some reason. No clicking, no resistance, the trigger just goes all the way back and nothing happens. I didn't change anything about the trigger group so a bit of a mystery. Anyway, once sorted out I'll run a Chrony string and see what's happened to power with the slight increase in piston mass, 0.8mm port enlargement, and very slippery spring guide all contributing.


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PostPosted: Thu Jan 15, 2015 5:08 pm 
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That Ertalyte sounds like interesting stuff to have in the shop - where do you get it from?

Well done on having the gun lined up with the trap while you were working on the trigger. A springer that won't fire (or might fire when you least expect it) is something of a personal nightmare, especially as half of them can't be decocked these days.


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PostPosted: Thu Jan 15, 2015 6:00 pm 
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Exactly. In my searching for QB57 trigger/sear problems I came across one guy who had his discharge into the ceiling on his first day trying the thing. He was very grateful the pellet didn't hit something more important.

I got a length of Ertalyte in 1" diameter a couple of years ago from Sabic Polymer Shapes in Coquitlam:
http://www.sabicpolymershapes.com/polys ... /home.html
You can't just go there, have to call ahead and order something on a credit card then arrange to visit or have it shipped. I think they have branches elsewhere too. Loads of types of plastic. The Ertalyte wasn't quite what I wanted for a low-friction, high-pressure static application - a doublebass with an adjustable neck joint for easy string height changes on the fly - but I got it anyway as the closely related plastic wasn't in stock. I'd been trying to get ErtalyteTX. Turned out as suspected, not quite as slippery as I wanted. I've since found a supplier of much lower friction plastics in Germany and will be ordering a foot or so of their stuff once I settle on a dimension. The German stuff is even harder than Ertalyte while being several times slipperier so it's ideal for a manually adjusted mechanism under a few hundred pounds of string tension. I got away with using lignum vitae for my first such mechanism but the degree of difficulty in turning an Allen key with that very hard, very waxy wood as the bearing is just a few pounds more than I'd like. A good friend who runs a machine shop swears by the German stuff, Igus, which costs a fortune but can handle enormous pressures.

For general machineability I'd recommend Delrin. It'll handle a wider range of feed rates. Ertalyte is somewhat closer to aluminum in texture for turning, though obviously a lot easier on the tools, and the ears. I just mean I try to stick to low speeds and moderate depth passes when reducing Ertalyte as compared to Delrin, which can be shaved off with quite impressive ease. Especially for drilling, Ertalyte for some reason tends to heat up and then get mushy and start tearing. Becomes useless if excessively heated as it embrittles when cooled. So sharp tools, slow feed rates, and if it's a big hole you're drilling take it in a few passes letting the drill cool between plunges. I think it's because Ertalyte is an incredibly efficient insulator. Holds heat for a long time. It's actually got a considerably higher melting point than Delrin, just different machining requirements.

So it turns out the trigger problem was just 'one of those things.' I tore it apart and inspected the heck out of it, found nothing wrong, so put it back together and it just worked fine. Go figure!

As for performance changes, not much to say. Next to the ear it's sounding a bit different but still annoyingly loud. My sound pressure level meter is saying there's a 2dB drop on average, and when holding it away from me for Chrony testing (I rest it on my bench so no need to aim really) it does sound quieter. But against my face no real difference except there's a bit more twang and a bit less bang. Guess the spring is more free to move, hence the increase in twang.

Velocity with RWS Hobby dropped on average about 8fps. Not significant. Maybe it'll wear in and go up again but no worries.


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PostPosted: Thu Jan 15, 2015 6:35 pm 
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Yes, delrin is just lovely to turn - so easy and smooth. I have plenty of delrin (I'm lucky enough to have found a local supplier, so I can just walk in and pick it up - the luxury!), a little bit of ptfe and a tiny amount of peek. Oh, and a small selection of nylon. I'm always on the lookout for new supplies for my materials collection - I love it when I have an idea and all the stuff I need is already to hand. My go to material tends to be aluminium, but I have a selection of 12L14 steel for things that need to be stronger (plus some of the cheap stuff from home depot, but that's a pain on the lathe), and a small amount of brass for when I want to treat myself. I love the brass, sure wish it was cheaper.

Glad to hear the trigger is sorted, although I'd be nervous about something like that which didn't have a clear explanation. But you did say the '57 trigger was a bit on the scary side!

Jim


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PostPosted: Thu Jan 15, 2015 6:49 pm 
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Mostly it has a bad reputation because the little adjusting screw can work loose. Funny though, because it's in making it tighter that it fires more easily. Another one of those unexplainable things? But a lot of folks seem to report that screw causing misfires, so I apply Loc-tite to it any time I want to adjust it. Simple enough solution, stays where I put it just fine. The way it's shooting solidly now, as it did before the one episode of failing to fire at all, I'm fairly confident. But of course I keep my right arm between stock and cocking lever, firmly pressed back, whenever I load a pellet. Don't want to lose a finger or thumb! Here's the Archer Airguns page depicting (in glorious blurry photographs) the stitched together finger a similar side-cocking airgun inflicted upon its careless user:
http://www.archerairguns.info/2011/09/a ... en-to.html
Funny, but as a kids we used break-barrel rifles constantly and no one I knew ever suffered an injury. We knew the barrel would come up fast if the trigger were touched while loading and probably smash the finger or thumb. So we didn't touch the trigger while loading! Guess we were lucky in not having defective sears. There certainly weren't any anti-beartrap devices back then.


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PostPosted: Thu Jan 15, 2015 10:16 pm 
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Industrial Plastics and Paints (just down the road from me) has a good selection of acetal products, too bad I don't have anything to turn it on, I have a buttload of ideas for this stuff.

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The flaw with experience is, it causes us to forget what we were like when we didn't have any.


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PostPosted: Thu Jan 15, 2015 10:52 pm 
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Handyguy wrote:
too bad I don't have anything to turn it on, I have a buttload of ideas for this stuff.

I know the feeling. A lifetime's frustration of not having a lathe finally got too much for me a bit over a year ago and I caved and bought a Taig. Part of me wishes I'd put the effort into finding a used full size lathe, but the reality is that I would have struggled to fit a full-size into my work area, or even get it there in the first place. The Taig is a lovely thing, and eminently manageable, even if I do occasionally bump into its limitations.


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PostPosted: Thu Jan 15, 2015 10:57 pm 
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Been using my TAIG and bumping into its limitations for about 20 years I think. Still never regretted the choice. It gets done all manner of things I just couldn't do, plus a whole bunch of other stuff I couldn't do well without it. Sure I'd like a 2 tonne lathe... but where would it go in my crowded 9' x 10' workshop with all the instruments and tools and musicians visiting? Nope, I'm a tiny hobby lathe guy and I'm okay with that. Just wish I could do single point threading on it...


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PostPosted: Fri Jan 16, 2015 5:11 am 
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I put the plastic sheet inside the piston so it butts up against itself, and I make it a tight fit so it's as if the piston is coated with the plastic. Plus I choose the thickness so it's is a snug fit so the spring cannot twang. I'd think the looser setup you made would help a little, but you really need to be snug to stop that twang. I like coil spring guns, but that twang is a deal breaker for me which is why I make the extra effort. When I'm done people assume it's a nitro, both cocking smoothness and firing, except it cocks easier so they think a nitro that has partially leaked out. The kick doesn't bother me either, but it bothers the pellet and scope, so something to consider. A gun that's balanced and/or heavier overall is not as hold sensitive. You seem like me in that you like to tinker to get the best performance or whatever you're after, so if you're interested I have pix of said spring sheath/piston sleeve, and other mods if you're interested.


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