Each rifle is different and what works in one gun may not in another. Depending on what you are trying to do or (use it for) will generally establish your end combination. Usually when working up a new loads for a gun I have owned for awhile, I know what factory ammo it likes. Matter of fact, I will only reference my factory ammo data at this stage because it estabishes a base line to go by.
The .257 is currently making a comeback and gunmakers like Ruger, Remington and so on are once again chambering their bolt gun lineups with in this outstanding caliber. But to the best of my knowledge factory ammo for them is still lagging behind and your choices in projectile weight will be limited. Remington makes the 117gr Core lokt, Hornady makes the Custom using their 117gr interlock and their light Mag uses the same 117gr interlock. Federal is using a 120gr Nosler partition and Winchester once offered a 100gr silver tip.
My point is, you will want to mimick the best grouping factory round as close as possible with your baseline reload. This means case, primer, powder and projectile. Finding what powder the factory uses could slow you down a little but the info can be found on the net. This also includes OAL and validated for FPS by chronograph. Shoot your reloads and hopefully they shoot as well as the best factory load you mimicked.
If it does, now you can start tuning overall length and powder charges. Before making too much adjustment to the overall length buy a length checker if you don't already have one and a quality chronometer/scale is a must. Adjust powder in .5 grain increments up and down until grouping falls off. The best grouping combination should be then validated by at least 3 groups of 3 rounds on target.
Now with the best grouping combination adjust out in .005 increments until grouping again falls off or you get to .005 off the lands and grooves. Hopefully with each adjustment your groups tighten up, if not mine uses
41.0gr of IMR 4831 with 117gr Hornady Interlocks OAL of 2.775". Good luck
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