Quoted: 18" the shorter the stiffer. Also get that sucker fluted! A fluted barrel might get hot faster but it cools down faster once it gets hot. And if you're feeding it 75gr bullets then go 1:8 twist not 1:7 because 1:8 will reach out farther more accurately than a 1:7. Example: the military SPR barrel is a 1:8 twist as opposed to the M4 which is a designated 1:7. This is because at a distance the 1:7 twist begins to keyhole and throw bullets in somewhat random directions. I've seen it happen and its ugly, very ugly.
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I read bullshit on many levels in that post, starting with fluting and then to bullet/barrel twist recommendations.
Fluting doesn't really do anything for a barrel other than add eye candy (I'll be the first one to admit that I've done it before). It doesn't make it more rigid, instead What it does is make it more rigid than a similar unfluted barrel of equal weight. The weight savings are minimal, a 6 fluted barrel used by a match shooter I know gets a weight savings of nearly a 1/2lb on a 30" barrel. Fluting after lapping will actually change the bores internal dimensions under the flutes, which is why custom makers like Broughton and Mike Rock actually flute thier barrels before lapping. Cooling is not a reason to flute a barrel; it still takes a long time to cool and actually increase your mirage when looking through a scope.
The military SPR barrels are 1:7 twist and will stabilize bullets up to 80grns just fine. Some, such as the Krieger my on my SAM/R stabilize the 90's just fine and I have used it in 1000yd comps. A 1"8 twist barrel generally performs well up to 69grns. I say generally because I've seen others shoot 77's and 80's just fine. I've seen Broughton 5C 1:8 twist barrels shoot 90grn bullets with no problems.
As for a SAM/R'ish clone. It's easy to do, here's a brief list.
-Yankee Hill rifle length FF tube-$128 from brightflashlights.com on the EE
-Rock River NM barrel, 1:8 twist-$225 from Adco
-regular A2 flash suppressor-$5 from just about anywhere
-Yankee Hill flip up front sight/gas block-$75 from Brightflashlights on the EE
-flat top upper receiver complete- $115 from Adco, but can be found cheaper
-gas tube and pin, $8 from anywhere
-bolt and carrier-$110 from Adco.
That will get you something similar and the RRA NM barrels are known to be good shooters. By substituting a few things you can easily make a Mk12 Mod1 SPR clone by going with the White Oak SPR barrel from Adco ($255) and a PRI gas block (Adco $55), Ops Inc type brake and collar (from Adco) and a Yankee Hill Rail Mounted flip sight ($64 from Brightflashlights).
I have a correct Mk12 Mod0 SPR from MSTN that I regularly shoot out to 700yds with 69grn and 77grn loads-I have 0 problems getting there. I also have a correct SAM/R that I use to 1000yds with 90grn loads (69's and 77's for anything closer)-I have 0 problems getting their either.