I have been flip flopping for a couple weeks now. Here are the options and my pros/cons.

300BLK
I like that I don't need to get any more mags and can swap my BCG and charging handle. I do reload but don't have any rifle brass sizing equipment or 300BlK dies, brass or projectiles. Ammo is not readily available. Also I dont plan on running a suppressor.

6.8 SPC
I have heard that if you want 7.62x39 performance and are not going to use 300BLK suppressed then 6.8SPC is the way to go. I don't have any reloading supplies or ammo for 6.8.

6.5 Grendel
I like that this is more of a long range round. Same as 6.8 I have nothing for it.

7.62x39
I have thousands of rounds of 7.62x39. It is on every shelf and is about as cheap a round as you can get. I have heard the 7.62x39 has some performance issues but that may be in the past or with specific equipment.

I guess if I had a top two it would be 7.62x39 and 300BLK but I could be swayed between all 4.

It will be a 100 yard range toy, maybe some hog hunting, and possibly as a home defense with a red dot on it.

If you already have the ammo 7.62x39.  Buy new mags, it sounds like they've got the bugs worked out.  More case capacity than .300", and can then convert to 6.5mm Grendel w/ just a bbl swap.

So 7.62x39 and 6.5 Grendel use the same mags and bolt? I like that!

The same bolt.  Dunno if 6.5 will feed to full capacity out of 7.62x39 mags.

No... 6.5 Grendel variants (6.5 Sporter, 6.5 SAOD, .264 LBC, 6.5 CSS, etc.) and 7.62x39 MIGHT use the same bolt (depending on who chambered the barrel), but they DO NOT use the same magazines. REAL 6.5 Grendels use a Grendel specific bolt (deeper bolt face than a 7.62x39 bolt), and you'll have much better luck using 6.8 mags if you just have to use something other than an actual 6.5 Grendel mag.



If you want 7.62x39 in an AR, I'd say get the 300 BLK. You get basically the same ballistics without the potential reliability issues and proprietary parts.

So let's say I was going to go with 7.62/6.5 direction. Would it be better to get a Grendel bolt or a 7.62 bolt?

Is there a big advantage to having a different BCG for each caliber? Or would itake that big a difference to have one carrier and swap it whenever swapping uppers?
Eventually I would just get a dedicated carrier for each caliber but trying to keep up front costs down as much as possible. That said I don't want to be penny wise and pound foolish.

You don't need a new BCG - just a new bolt.