First off I'm new to the whole AR thing. I built my first .223/.556 last year and now I'm looking to build my favorite bolt gun a Weatherby .257 in an AR platform. I use this caliber on deer and I shoot 115 BST.
What are my options? I'm open to any and all suggestions. Thanks in advanced gun)
You have exactly one option, it's a wild cat 25 SAUM in the large frame AR10 or DPMS equivalent version...I'll get to that in a second.
You're closest bet in the AR15 platform is a 25WSSM and that's not even going to be close to what you really want in ballistics...but nothing will.
With the 25 WSSM you can potentially push the 100gr bullets at 3250 +/- depending on barrel and powder choice. 115s at 3000, depending on seating depth, barrel length and powder choice.
Neither of which are close to your 257 Weatherby Mag pushing 115s at 3400+
Now, if you want to have a wild cat upper built for the AR10 platform, (which means you're gonna need another bran new rifle) you can try and have someone like Carl at Accuracy Systems build you a 25 SAUM by necking down a 7mm SAUM. You'd probably be able to push 115gr bullets close to that, but the hassle of converting brass that much to run through a semi - auto is not really worth it in my book.
Best advice, give up on the idea of those kind of numbers in an AR until someone like NEMO decides to release components to build wild cats on their "OMEN" platform which is a 300 WinMag AR. That would be something you could chamber a new barrel for, and run any of the magnums you want...but it's going to cost you...A LOT.
