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Thread: Moderator bushing

  1. #1

    Moderator bushing

    Bit of a simple question, but should the bushing at the rear of a reflex style mod touch the barrel ?

  2. #2
    I was told that they should just brush the barrel, the bushing is apparently there to support the back of the mod, thus keeping it aligned and any force being put on the threaded front. Both my T8s sit like that but I am sure someone more qualified than me will clarify.

  3. #3
    Distinguished Member Ronin's Avatar
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    Oct 2014
    Lancs / Cumbria Border
    Should be slight interfernce fit ;

    Barrel OD (for arguments sake) .780" at point bushing interfaces.

    So bushing needs to be bored to .780" for "just touch" fit.

    Thats what I do on mine anyway (it works)

  4. #4
    I have a t8 which id bushed and doesnt touch barrel and i have a nortstar which isnt bushesd and both accurate so doesnt seem to make a difference but i woild imagine that anything that touches a free floating barrel could be a problem but may be wrong

  5. #5
    I think anything that touches the barrel has the potential to make a difference, however if it does so in a consistent manner then there shouldn't be a problem. It's when something doesn't touch then touches, or touches with different pressures you get a problem.

    I had a bushing on a Predator 8 that was a little too big and there was clearance all around it. The groups weren't bad at all but it was always in the back of my mind that the potential was there for the bushing to touch whilst firing, particularly since I was told it should be an interference fit.

    I had a new bush machined to be an interference fit and the groups did tighten up.


  6. #6
    SD Regular vizslamad's Avatar
    Join Date
    Mar 2009
    Shropshire. Land of rain and more rain.
    I use the same mod on my .243 and .270. On the .243 the bush fits close to the barrel and on the .270 it does not. Both shoot perfectly well the difference being slowly but surely the blueing is being worn off the .243.


  7. #7
    The bushes on my two T8s don't touch the barrel at all.
    This is apparently to allow consitent fitting of the mod based on the shoulder of the thread, rather than on a blue-stripping touch-fit of plastic on steel at the rear bush.

    The gap ain't large, so the bush wil still take the strain off the thread if the mod gets a knock: which is, I think, the point of it. The T8 is very long, so a knock near the back gives quite a wrench to the thread - to be avoided especially if they're on 1/2"unf, as mine are.

    The Northstar is bushless, but much shorter. Furthermore, mine is robustly arranged on a M17x1 thread with a 1" spigot.
    Last edited by Dalua; 04-04-2010 at 09:27.

  8. #8
    It should be as Redmist states,......... why have a bush if it is not going to bear on a surface?, interference fit is what is intended by the designers, your overbarrel moderator is at risk of causing changes in wave resonation in the barrel if only fitted to the thread portion of barrel.
    (The Unspeakable In Pursuit Of The Uneatable.) " If I can help, I will help!." Former S.A.C.S. member!

  9. #9
    Distinguished Member Ronin's Avatar
    Join Date
    Oct 2014
    Lancs / Cumbria Border
    Whether the bushing marks the barrel or not is immaterial, dont have an over barrel moderator if you do not want the blueing to wear.

    The overbarrel (reflex style) moderators are designed to have TWO bearing surfaces that suport the alignment of the moderator on the barrel.

    #1 - the thread itself

    #2 - the bushing

    The thread should have been cut absolutely concentric to the barrel bore.

    The thread on the moderator will be concentric and central to the moderator bore.

    The rear bushing is there to provide a bearing surface with slight interferance fit, if it is NOT touching, it isnt doing its job.

    Lets put this another way if the bushing allows the rear of the moderator to move side to side, the front of the moderator is moving a similar amount (think of the threads as a fulcrum (pivot point) so when your bullet enters the moderator at 3000 fps, and the bore of the mod is able to move the exit hole is moving too.

    You run the risk of damage to the mod at the very least with a loose fitting bushing.

  10. #10
    Quote Originally Posted by finnbear270 View Post
    It should be as Redmist states,......... why have a bush if it is not going to bear on a surface?
    I suppose for the reason I described - viz. to prevent excessive movement of the mod from straining at the thread should the mod get walloped.

    The instructions for fitting the T8 Reflex mods suggest that the bushing be machined to the barrel-diameter at the area of fitting + 0.1mm.

    It seems to me that there is a slight tension between the idea that the bush should align the mod with the outside of the barrel and the idea that the thread (and its shoulder) should be cut square to and concentric with the axis of the bore, rather than of the barrel: If the axis of the bore is not the same as the axis of the barrel, it seems to make little sense to cut the thread true to the bore and then line the mod up with the outside of the tube.

    My mods is screwed up against the shoulder of the thread, so it really shouldn't move relative to the muzzle, bushing or no bushing, should it?
    Last edited by Dalua; 04-04-2010 at 17:09.

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