Swept volume.

Discussion in 'Piston & Spring' started by Tinks, Oct 24, 2018.

  1. hmangphilly

    hmangphilly Can't Re Member

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    Is any one using superdomes in target shooting ?
    or are they just about making power in short stroke springers ?
     
  2. Tinks

    Tinks Another tricky target at Lincs.....!!

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    Me........! and JSB 8.4 when I can afford to buy enough of a decent batch. Have used domes for the last 4-5 years and achieved a couple of fairly respectable PB's with them (50 HW97 springer in a Daystate round and 55 BSA scorpion in the Lincs intercounty), they are very consistent weight wise, well made and very clean, work best from a thoroughly clean barrel and then only clean when groups open up at 40+ yards. Downside is the relatively low BC which means they do move more in a slight breeze compared to JSB's.
    Shot this yesterday outdoors at whatever range it says on the plate, with Superdomes in my working 97 lol (it's short stroked to 77mm and does 765fps with domes and 770 fps with JSB 8.4's).

    IMAG0612.jpg
     
  3. Tinks

    Tinks Another tricky target at Lincs.....!!

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    Hello Tony, thanks for joining the discussion, I have just about come to the conclusion that the shorter stroke produces a too narrow pressure pulse, along with extreme pressure (100+ bar) which is difficult to seal with an o ring in a dynamic application. I was fortunate enough to shoot the gun Mick is using at Tall Trees a few weeks ago and immediately noticed the long stroke of the cocking lever, that combined with the longer 385mm 77K barrel must have made things a little easier. I would be VERY interested in one of your 23mm seals when you have them (I don't do facebook so can't contact you through your lost volume group). Would it be ok to pm you through here about your seals, heard very good comments on your LGV 25mm this week?
    As I mentioned in an earlier post in here, majority of reduced bore tunes seem to be TX ot LGU variants which have potentially loads more stroke avaialable than the 77/97, currently I can get 88mm from my 97 before the cocking mechanism binds in the action.
    Working on it again tomorrow, so may have some more news then (good news is I got a new TX200 mk3 spring here ready for an LGV soft tune and a Titan#10).
     
  4. bigtoe

    bigtoe Member

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    No they dont use them for anything but setting the spring and making sure its not to long....i call it the superdome effect, if they produce more power than JSB's there is to much spring, a leak, or both in the gun...so they are a fault finding pellet, and for that use they are utterly superb. For target shooting, utter shyte.
     
    Last edited: Dec 18, 2018
  5. bigtoe

    bigtoe Member

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    TX spring in an LGV, TX spring fatigues to much and quite quickly...MUCH better spring is the genuine 16J 97 spring around the same length as the LGV spring and some fit right on the guide just perfect. Also the seals on the LGU and LGV are designed to limit power,. To keep costs reasonable on my seals I had to lose the machining on the face, this was actually a blessing as I was able to test my -5mm stroke seal in the TX at 25mm both .177 and .22, flat face vs recess over the port, vs cupped, vs cupped with recess like the LGU. Soon as you add that recess over the port you lose power, sometimes well over 1fpe so 30 to 40fps on a 177, thats a lot to lose. The flat faced seals can lead to easier detonation so careful with the lube BUT produce peak efficiency and with a little undercut on the lip minimal slide resistance when sized right down. I run them dual seal by default now backed by an O ring which again showed performance gains over the stock seal and my seals on their own.

    Also, not what you want to hear BUT i used to think O rings were more efficient than a poly seal, they are not, even dual O rings will not out perform a poly seal, especially one running at near bore size backed by an o ring also. What O rings are though is more consistent, the shot to shot consistency will tighten.

    https://photos.app.goo.gl/aME8hLFFNvjrRKei8

    [​IMG]
     
  6. Tinks

    Tinks Another tricky target at Lincs.....!!

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    That was one of my concerns when I looked at the numbers regarding this spring in an LGV at 89mm stroke, as the safety factor came out around 1.3 (anything below 1.5 is being overstressed and can wear out quickly or start to take set).
    I have a personal favourite spring which is fitted to both my short stroked guns (HW97 at 77mm and 21mm preload and LGV at 78mm and 15mm preload) BSA Airsporter, seems to suit these with a high enough spring rate over 7N/mm and plenty of coils so as not to feel harsh or snappy, tried the LGV springs in these and for me preferred the firing cycle from the BSA spring. I know it's very subjective and personal preferrance, but I like it and both my guns have a piston landing speed of below 2m/s and less than 0.6J landing energy.
     
  7. Tinks

    Tinks Another tricky target at Lincs.....!!

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    Apologies for not keeping this running, but due to a back injury last November, things requiring me to stand up or sit still have been difficult to say the least. Anyone who has seen me trying to shoot recently will probably understand.
    However things have not stood completely still, I decided to replace the 97 barrel with a longer one (found a 15mm barrel from a Diana 24) which provided me with approximately 380mm of useable barrel. Next was just to remove the 97 barrel........easier said than done is a mild understatement, anyone who has had a 97 or 77 out of the stock may wonder why there are 2 blobs of weld either side of the main tube. Simple, there is a 6mm splined dowel holding the end plug into the main tube which is then welded in to aid removal. Once the weld was carefully milled off the dowel came out very easily and the end plug was pressed out of the main tube. Next problem was to remove the barrel, again this is dowelled in place and the hole the dowel is pressed into is burred over at the factory, also it is drilled at a slight angle so as to hold the barrel with half the diameter of the dowel. It was driven part way into the main 6mm dowel hole and milled off, this was repeated until all the dowel was out, the barrel was pressed out of the end block with a hydraulic press (absolutely necessary IMO to avoid damage to the end block) courtesy of a neighbour with a garage business. Some lathe work followed to modify the silencer/underlever catch assembly to allow a longer barrel to be fitted and fill the silencer void, barrel was turned down to suit this along the last 65mm or so and a new crown cut, deburred and polished. The barrel was then pressed into the end block to the exact depth required for the underlever to catch correctly which left about 10mm of barrel to remove at the breach end. I have done this today, turned the diameter to 12mm and machined the barrel lead, gun is now partially reassembled and will be welded tomorrow (only the ends of the 6mm cross dowel before you ask :)), then its back to the testing. From the emulation, I hope to be running 84-85mm stroke for 11fpe, this follows what I have already measured with the 300mm barrel and expect to gain 50+ fps by adding 65mm to the length of the barrel. If all goes well, I may be posting again tomorrow with good news......fingers crossed.
     
    davep, Adam and ksts100 like this.
  8. Adam

    Adam Well-Known Member

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    Wow a lot of work there! I hope the Diana barrel was a good 'un! :eek:
     
  9. FPoole

    FPoole Active Member

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    Better check that the bore lines up with the transfer port. I've seen a lot of pictures of Diana bores that were no where near central to the barrel.
     
  10. Tinks

    Tinks Another tricky target at Lincs.....!!

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    Update......having changed the barrel, I fitted a set of standard'ish internals to go and check the barrel out for accuracy. Turns out to be a good barrel, at least as good as the original HW and as accurate as my other 97. Unfortunately the 23mm set up still would not deliver power. Having received some very good bits of info from members on here (massive thanks to Phil, Rob, Nick and Tony) it now works as it should.
    Turns out the compression tube was the culprit, although no leaks could be found in the tube it was the only component of the build I had only one of, so all permutations of my parts gave poor and misleading results. So I made another sleeved down compression tube this week and fitted it this morning with an 87mm stroke piston and a standard spring with 18mm of preload, couldn't believe the chrono when it said 803 fps with JSB exact 8.4 :eek:.....
    Repalced the spring with one of my 'favourites' with 15mm preload and now it shoots JSB exact at 760fps, JSB Express at 780fps, FTT at 750fps, Superdomes at 765fps :D.
    Still a little work to refine the shot cycle as it feels extremely fast......maybe need to play with the stroke again and/or reduce the piston weight down to nearer 140-150g, still, this is for another day as the next job is to take it out and enjoy it some.
    Thanks again for all the help and advise, I have enjoyed doing this despite the hours of greif and headscratching, seems this is normal for springers eh!
     
    nick2 likes this.
  11. Nick G

    Nick G Active Member

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    completely normal ;)
     
    Tinks and Gary Martin like this.
  12. Gary Martin

    Gary Martin sultans of spring

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    wibble
     
  13. Tinks

    Tinks Another tricky target at Lincs.....!!

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    Tried the rifle at the club yesterday, shot approx. 400 pellets through it and went round the hft course twice, thoroughly enjoyed it and after almost 6 hours of shooting no sign of arm ache. It behaved extremely well, only thing I noticed was the supported standers and kneelers required a different hold from before as the POI was too high if the rifle is rested on the back of the hand near the wrist (as in the picture to the left). Not really a problem, just something else to remember...
    Thought I would share the pictures of all the hard work, not much to show for it really, but immensely enjoyable.
    IMAG0626.jpg IMAG0625.jpg IMAG0624.jpg
     
  14. Tinks

    Tinks Another tricky target at Lincs.....!!

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    Having shot the rifle for a month now I was not too keen on the hold sensitivity (particularly noticeable on the standers and kneelers) so decided to experiment.
    Played around with the stroke at different lengths from 87mm down to 80mm and was quite surprised with the poi variations. The initial build left me with a stroke set at 87mm and this is where the gun was zeroed and shot for a while before "hold sensitivity reduction" started with a 2mm reduction in stroke and velocity reset at 750fps, the poi moved 6mm higher at 20 yards with no change to zero? A further 2mm reduction with velocity reset at 750fps and it moved another 4mm up at 20 yards, at this point I wondered where this would end, I didn't have to wait long. The next 2mm reduction and velocity reset saw the poi almost return to the original zero position, this is where I stopped and went shooting. The hold sensitivity was still there for the supported standers and kneelers.......
    Reset the stroke to 83mm for no other reason than that is where the pellet was striking highest and the hold sensitivty is gone!!! I can only assume it has something to do with barrel harmonics and without further testing will probably never find out. I can now rest the rifle as I like on the supported standers and kneelers and be reasonbly confident of the poi (my input taken into consideration) being at the estimated range and not having to allow 3/4 mil dot just because it's a positional. Can start to learn the gun again now and hopefully score a couple more points.
     
    MickyFinn likes this.
  15. Nick G

    Nick G Active Member

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    That's excellent , I like your methodical approach to development . I recently did something similar with the 21mm set up I have been shooting for the last 18 months , I dropped the stroke back in increments , and ended up far shorter than previous incarnations , and an easier to shoot gun.
     
    Tinks likes this.
  16. Tinks

    Tinks Another tricky target at Lincs.....!!

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    Thanks Nick, it was somewhat an accidental discovery as I couldn't be bothered to remove the scope, made the first change and went to see what effect it had on the shot cycle, the rest is listed above. Will give it another good work out on Sunday, fingers crossed it should prove easier to shoot at the elevated ones as well.
     

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