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This barrel will be for a Remington 700 Chambered for the .308win
i will be using 168gr BTHP Sierra MatchKings exclusivly... So basically Federal Gold Metal Match, i will be Reloading my own ammo to similar velocities as Federal GM.

My question is what would the optimum rate of twist and barrel length be for the most accurate long range shooting. I know some people swear by shorter barrels (sub 20") and some people swear by 24" and up. Why? is there a reason? what do the ballistics show?
 

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A lot will depend on your definition of "long range". A lot of recent testing on the .308 has shown minimal-to-no velocity loss from 24-20 inches. If you don't plan on sending anything heavier than a 168gr bullet past 700 or 800 yards, a 20" barrel would be fine. If you're wanting to load something longer to send out further (155 Scenar, 190 SMK, etc), then you may appreciate a longer (24") barrel.

Regarding twist rate, the standard Remington 1:12 will definitely work with a 168gr bullet. I personally prefer the 1:10 like my Savage uses, as I have the flexibility to shoot anything I want. I know Tac-Ops uses a 1:10 twist to put 168gr FGMM into tiny .1" groups, if that helps. Of course the 700P Tune-up Mel tested shot tiny .25" (or less) groups with 168 FGMM, and had a 1:12 twist.

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For all intents and purposes the last 4 inches of barrel length are about the last 100 yards of accurate distance.

For example a 155 Scenar in a 24" barrel at 2950 goes Mach at 1300 yards, from a 20" barrel with the same load recipie goes Mach at 1200 yards.

So if you are not desirous of the last 100 yards of reach, the last 4" of barrel are not necessary.
 

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I'm glad you brought that up, Mitch. I was just going to ask what people's thoughts and experiences were on shooting the longer 155gr Scenar/Palma in a shorter barrel, say 18" to 20". What kind of velocities are typical in that barrel length with the Palma type pills, and what would be a good load to start with?

If this ends up straying too far from the OP's question, I'd be happy to take this to the Ammo section.
 

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Addressing the twist. I prefer a 1:10 for 30 caliber for the reasons stated above. As a general rule you can not over stabilize a bullet. There are some exceptions, but they do not apply to the 30 caliber range of rifles.
 

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There seems to be a myth about shooting with very fast twist barrels.

This myth implies that accuracy is lost, energy is lost, or velocity is lots because of the higher twist rate.

At the very farthest scales (farther than 800 yards) shooting a mandated bullet, there may be some very tiny accuracy loss. And if you are not shooting in competition, PALMA, F-class, or at these distances there is no "real" loss in the kind of accuracy we strive for in tactical shooting. That is, a faster than needed twist barrel might loose that 0.05 MoA that determines who takes the trophy home.

For parctically evey other situation (excepting one) a faster twist barrel brings more advantages than it bring detriments. It can shoot more bullets, more load recipies, heavier bullets, longer bullets, green bullets, salbot, and others; that the just-barely-enough twist barrels cannot.

The one situation where that fast is too fast a twist is when shooting fragmenting varmit bullets with thin jackets. These can actually fly apart from too much spin.
 

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What’s the muzzle speed of the 168 smk in .308 18” barrel? 1:12?
There are way too many other variables to accurately answer that question. Even different barrels from the same manufacturer can be different speeds, not to mention different loads and even the same round from same manufacturer in a different lot number. Best way to know your velocity is to run your loads over a chronograph.
 

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What’s the muzzle speed of the 168 smk in .308 18” barrel? 1:12?
If you want a ballpark number, like with Federal's Premium Gold Metal Match 168 SMK's, it might be something like 2,550 fps, BUT . . . it's a mighty big ballpark for as ddd-007 says, there are a LOT of variables that can result in MV's significantly faster or slower. The best thing you can do is fire a couple rounds of them from your gun through a shooting budy's chronograph, then you should have a much better idea of what the MV is like with that barrel.
 
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