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Thread: .25 cal LW Polygonal Barrel and 50 gr. Cast Bullets?

  1. #1
    Moderator rsterne's Avatar
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    .25 cal LW Polygonal Barrel and 50 gr. Cast Bullets?

    Has anybody tried this combination, and if so, how did it work for you?.... What size were the bullets, 0.250", 0.251"?.... What mold?....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

  2. #2
    Senior Member SeanMP's Avatar
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    I got nothin Bob

    The only polygonal barrels I've ever had the pleasure of using were black powder. They worked pretty good. Less caking than traditional rifling
    Sean

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    Moderator rsterne's Avatar
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    I have a .25 cal Poly LW choked blank sitting here.... I'm thinking it might work well with those .25 APC bullets, which will fit in an MRod clip.... Guess I'll have to machine it to fit my Hayabusa and then I can compare the conventionally rifled LW to the polygonal LW, that's why I bought it anyway....

    Any idea why the cast bullets (sized to 0.250") were so down on velocity in my 2560 with the TJs liner running at 1800 psi?.... All of them (Lyman & Mr. Hollowpoint) shot ~ 40 FPE instead of over 50 FPE.... Not enough pressure for the drag in the bore?.... or blowby?.... They shot as expected in my .25 cal Disco (ie more FPE than pellets because they are heavier) which has a conventional LW barrel (choked), even down at 1600 psi (I could still get 73 FPE).... The TJ's has taller lands, it's 0.243" across the lands instead of 0.246" for the LW (0.245" at the choke).... and the 2560 is tuned for only 52 FPE so it has a much shorter dwell....

    EDIT: I just ran the numbers through Lloyd's spreadsheet, and it appears that what happened in the 2560 was almost predictable.... God I love that program !!! .... When I set it up for the way it is tuned with the 25.4 gr. Kings and the changed the weight to 50 gr. it LOST power from 51 FPE down to about 45.... (It turns out that FPE would peak, with that tune, at 34 gr. pellets).... Then I tripled the bullet drag from 2 lbs. breakaway 1 lb. sliding to 6 & 3 (just a guess) and it dropped to about 42-43.... From there down to 40 FPE was only about a 3% loss in efficiency, which is certainly within tolerances.... By comparison, when I set the spreadsheet up for my .25 cal Disco at 1800 psi and 80 FPE, increasing the weight adds FPE and vice versa (as expected).... The short dwell on the 2560 isn't enough to allow more FPE with pellets over 34 gr., the energy then begins to fall....

    Bob
    Last edited by rsterne; Mar 19 2013 at 01:15 PM.
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

  4. #4
    Senior Member SeanMP's Avatar
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    Thats a good spreadsheet!

    I would have guessed it was a combination of drag and blowby. Firstly I would polish the buhjeez out of the tj's. The several I've seen so far are rough enough to benefit greatly from a really really heavy polish.

    Do you have a close up of a push through?

    LW says the accuracy of their Poly barrels is about 90% of conventional but 2x the life expectancy in PB's.....Your basically breaking new ground in an air application
    Sean

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    Moderator rsterne's Avatar
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    The TJ's has received 100 strokes full length, then 3 per inch less, plus about 25 extra in the chamber and leade, so 100 at the muzzle, 200 at the breech, of Bore Paste, and then about half that many of Bore Bright.... It shoots pellets into basically one hole indoors.... The TJs is on the gun at the moment, so can't push straight through, but next time I pull the barrel for a polish I'll do that.... The TJs and LW seem to be about the same groove size, but the TJs has deeper rifling, so that rifling is engraved deeper on the driving bands.... The groove area on the bullet it NOT smeared, so I don't think it's too tight, but that's just from pushing it in from the ends and then pulling it back out.... I did push a bullet through the Polygonal barrel, and the rifling marks are quite shallow....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

  6. #6
    Senior Member SeanMP's Avatar
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    Dang.... that's certainly lots of lapping.

    Ya, push one through next time you have it off for any reason even just to get the info out there of what to look for.

    I'm definitely interested in seeing that Poly perform. There is just so much that should be an improvement with polygon twist. Did the slug you pushed through conform to the poly shape? Or is it just lightly squeezed on the flats?
    Sean

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    Moderator rsterne's Avatar
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    Just lightly squeezed on the flats.... The rifling marks are only about 1/16" wide, maybe less.... even when pushed through the choked portion.... It was much easier to push through than I thought it would be.... Next time I'll take some photos....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

  8. #8
    Moderator rsterne's Avatar
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    Here is a photo of a 25.4 gr. King and a 53 gr. Lyman pushed through an LW Polygonal barrel....



    Top row pushed halfway and then back.... bottom row pushed through including the 0.002" choked area.... Across the flats they measured 0.249" and 0.247" respectively.... I hammered a larger bullet through the choked portion, and it measured 0.253" across the largest points on the polygon.... so even an oversized pellet like an EunJin would barely have the head touching the high points at the choke.... I'm thinking the polygonal barrel would have less bore drag and possibly produce a slightly higher BC than conventional rifling.... but it certainly doesn't engrave the bullet much.... I'm guessing accuracy would depend, as usual, on pellet choice....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

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