If you're building an AR-15 for anything beyond standard 5.56 NATO duty, you've probably run into this three-way debate. The 6mm ARC, 6.5 Grendel, and 6.8 SPC all promise more knockdown power and better long-range performance than 5.56, but they go about it in different ways. Choosing between them comes down to what you actually plan to shoot — paper, steel, or game — and how far you plan to shoot it. This guide breaks down the history, ballistics, recoil, and practical trade-offs of …