Swept volume.

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

  1. hmangphilly

    hmangphilly Floppy Quick Phil

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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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    Washingborough, 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.....!!

    Joined:
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    Location:
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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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    Plant City, FL USA
    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.
     

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