I looked for any recent threads on this discussion and didn't find any, so bear with me if this is a repeat of another thread. I've got .223/5.56 covered and I'm thinking about what I want to do next. I've listed some options in the poll above, but that is certainly not an exhaustive list, just the ones that seem to be more common among the articles I've read and people I've talked to. The question is based on using a standard AR15 multi-cal lower. One thing I know is that the 7.62x39 may not be a good option in a standard AR lower, but that's all I have on hand at the moment, so I'm leaving it on the list. I'm leaning toward the 6.5 Grendel based on what I've read about it's performance out to long ranges over some of the other choices. I don't have any long range setups at the moment and it sounds like a good one to use for long-range fun, especially if you do your own hand loads. I'm not a hunter, so this would be strictly for learning more about long range shooting. I understand these cartridges can keep going well out to 1,000+ yards, which seems pretty impressive to me. I also understand it can be difficult to find ammo. My second choice is a 9mm, just for cheap fun. Third, I would love to do a 7.62x39, but from my research, it looks like it's best if you get a lower that will specifically work with standard AK mags, and it looks like those are limited choices at best at the moment. Maybe 5.45x39 would be a better choice. .300 BLK sounds interesting, but I understand it's ideal for suppressed applications, and at this time, I'm not planning on running a suppressed setup. So I'm curious, if you were looking at your next AR15 based build, what would you choose and why?