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Thread: Effects of a moderator on a bullet

  1. #1

    Effects of a moderator on a bullet

    Hi members

    Has anybody experienced significant bullet yaw when fitting a moderator to there rifle..

    My Sako 85 shot a group which wasnt even in the same postcode last weekend.

    Using factory Sako 220 grn ammo.

    Going to do more testing at the range this weekend.

  2. #2
    Not sure quite what you mean by 'yaw'.

    Do you mean that your rifle shot good groups without a mod, and now you've fitted a mod it shoots good groups in a different place?

    If so, then I can confirm that all my moderated rifles do that - and although the shift is always the same for each rifle mod on-mod off, each rifle is affected differently by its mod. I'm not aware that this is the result of a change in the bullets' yawing behaviour, though.
    Last edited by Dalua; 17-01-2014 at 23:33.

  3. #3
    Yes and the group also opened up significantly.

  4. #4
    Quote Originally Posted by AndyAitchison View Post
    Hi members

    Has anybody experienced significant bullet yaw when fitting a moderator to there rifle..

    My Sako 85 shot a group which wasnt even in the same postcode last weekend.

    Using factory Sako 220 grn ammo.

    Going to do more testing at the range this weekend.
    The weight of the moderator on your barrel changes the harmonics when shot is fired resulting in a different accuracy sweetspot for your bullet/powder combination, however accuracy may not always be affected but again the weight of the mod can also change the bullet impact point.

    Ian.

  5. #5
    Can you expand this?
    Range?
    What rifle, ammunition, mod?
    Who cut the thread? Barrel shortened?
    "There comes in the dead of night a hand of cold steel that plucks the German sentries from their posts"
    WSC 1942

  6. #6

  7. #7
    The harmonics of a rifle barrel are 17,000fps so a wave runs up and down the barrel this also effects the inner bore, not an issue as you current load if it groups well is shooting within the "optimum Barrel time" (OBT) namely its matched its exit to coincide with the "wave" being not near the barrel end.

    Adding a mod has now got the "wave" going to the tip on the mod and back to the bolt face so in effect the "OBT" will change.

    Changing the "wave" and adding a weight to the barrel is the cause and the point of impact change it the result.

    All smoke and mirrors, Google "optimum barrel time" there are some good pages on this black art

    Also my rifles do the same

  8. #8
    Quote Originally Posted by AndyAitchison View Post
    Same postcode??? & Yes and the group also opened up significantly.

    How big was the group before fitting the moderator?

    How big was it with the moderator fitted?

    Did the mean point of impact move?

    What caliber is your Sako?

    Ian

  9. #9
    I'm sure you have already checked but is the bullet maybe clipping a baffle if the group is poor and erratic that may be an explanation

  10. #10
    I use a Sako 85 Finnlite 6.5x55sw with fluted barrel,very light and can carry all day. Bought the rifle second hand and was screw cut for mod,when used with mod (3rd eye tactical),which is an excellent mod and quite light, could not obtain tight groups. Without mod an absolute tack driver with 140grn Nostler partition.....all down to barrel harmonics I think.Dont think this rifle was designed for a mod as Sako dont supply them screwcut. Your thoughts would be welcome.

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