Z-Michigan
Member
For subsonic always it makes sense but if you like the 5.56 baseline the WT provides a little edge...
http://www.wilsoncombat.com/762x40WT.htm
If subsonic is not in the menu another one that you might want to consider is the 6x45 that it is super easy and ballisticaly very accurate and versatile.
The 7.62x40mm offers very limited gain over the 7.62x35 (300 AAC Blackout) and requires custom modified mags to do so. Last I checked only Wilson was loading factory ammo for it. It's not a bad idea, but is less versatile and far less widely available, for very limited gain.
BY FAR the most exciting AR-15 option I've read about is the 6.5 MPC, which is a .223 case necked up to 6.5mm. Ballistics with light to medium weight bullets are nipping at the heels of 6.5 Grendel, without requiring special mags or bolts (and the weaker bolt-head dimensions that brings). But it's another one where factory offerings are all but nonexistent. I don't mind reloading and mail ordering some parts (like the barrel), but I do mind relying on either custom gunsmithing or only a single manufacturer for anything related to a cartridge.
http://sskindustries.com/6-5-mpc
The 300BLK is not the end-all be-all, but it offers a useful and versatile option, and in a somewhat circular fashion its increasing availability makes it more attractive, and vice versa.