I'm brand new to long range shooting and have started a 308 build on a Remington 700VTR, what is the best way to break in the barel to get the most accuracy out of it, and where do I start to do a load development to find out what is the best performing combination of powder, primer, bullet wieght/style, and brass. and finally when should I stop trying to reduce my group sizes, and be happy with what I have.
Your questions cover a lot of ground, and there is some debate amongst experts on the answers. I'll give you a brief version of my answers, which should get you on the right track.
Premium/custom match barrels should require no "break-in." They come from the barrel maker lapped and ready to go. Assuming they were chambered and mounted properly by the gunsmith, they should settle in to very small groups and consistent velocity within the first 10 or so rounds - sometimes less, sometimes more. With these barrels, frequent cleaning of the bore will not be required to maintain accuracy- they should be able to shoot hundreds of rounds, maybe thousands, before a cleaning would be beneficial to accuracy. You can clean more often in the first 10, 20, or 30, 50, etc, rounds if you want to, but it's probably superfluous.
If you ever get the barrel wet or are in a moist environment, always ensure the bore is dry to prevent rust.
The theory for the above is that a premium match barrel with a honed surface will not progressively collect copper or foul. It will foul some, but it will get to a "steady state" condition, and then the bore will be essentially the same round after round, producing great consistency and accuracy. If you clean one of these barrels "down to bare metal" (see cleaning method note below), it is not uncommon for it to take 5-20 rounds for it to "settle" back in to the old accuracy and consistency.
Never use any abrasive cleaning products, including JB bore paste and similar products, on a premium/custom match barrel. You will only harm the bore. Use powder and copper solvent only. I recommend using the VFG compressed felt cleaning pellet system (with adapter for Dewey rod) available from Brownells and the appropriate K-G cleaning products. I use KG-1, 3, and 12 for various bore cleaning tasks. Always use a bore guide and be careful to not damage the muzzle crown.
For barrels other than the type discussed above, such as a factory barrel, you may have to do some "work" to get it to shoot accurately. Bore surface defects may catch and collect copper from the bullet jackets and degrade accuracy. These can be burnished out naturally by shooting if you clean every several rounds. For one of these barrels, start shooting as you normally would. If accuracy is not where you want, clean using the same cleaning methods outlined in the previous paragraph to get the barrel down to bare metal (or as close as you can get), and continue shooting. In severe cases, something like JB bore paste might have to be used, but I would still counsel to avoid abrasives unless you have no other avenue.
I recommend starting with a recipe that is known for accuracy and performance in the cartridge. You can spend a lot of time, money, and effort trying to find the perfect load for your rifle but chances are good one of the top 3 loads that everyone else uses will be best in your rifle as well. You increase your chances of success by using high quality components (e.g. Lapua) and precise and careful reloading techniques. In .308, I recommend the following two loads for accurate long-range shooting:
Choice 1: Lapua brass, 210M primer, Lapua 155 gr Scenar, Varget powder. You should be able to load this up to over 2850 fps (2925 fps is common) with very good accuracy. 44.0gr is a safe starting load.
Choice 2: Same but 175gr Sierra MatchKing, start at 42.0gr Varget. Most people get 2700 fps from a 24 or 26" barrel.
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