Just some thoughts on mods (take 'em or leave 'em)
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I have a Roedale Ultralight alloy moderator on my .308 Steyr Scout. I really like the weight as it is hardly noticeable on that lightweight rifle, and maintains the handling characteristics (such as they are). It replaced a T8 which was effective, but noticeably upset the balance .
The T8 is an over-barrel type and the Steyr Scout has a deeply fluted barrel. There were no problems in getting it to fit at all. I think all centrefire over-barrel moderators have sealed expansion chambers over the barrel, so fitting onto the fluting is not a big issue - the rear bushing just fits onto the barrel outside diameter - there is no gas leakage through the fluting.
My experience has been that the smaller alloy moderators do not appear to be so 'quiet' as the big steel ones! (I have just put an A-Tec alloy over-barrel mod on my other .308, and while I have not had a chance to test it on an open area of ground, it seemed quite noisy when fired within a woodland setting while I was re-zeroing). I have fired a shot w/out the moderator at the same location to see if it was making any difference at all, and it most certainly is reducing the noise substantially!!
All in all, I think it depends on whether the intrinsic balance and feel of the gun is important to you - if so, thee small alloy moderators are much better, if you shoot mainly from prone or off a bipod, this may be less of an issue.