I shoot lots of Wolf WPA 5.56 ammo, for training, it's good stuff in my book. ~$220 delivered/1k
5.56 vs the BLK in the SD role, I don't have the factory 110 TTSX LV in factory ammo yet, the Blackout is NOT in the SD or PDW role yet.
The 110 TTSX Blackout round should be a great SD round, Barrier blind, expansion, and penetration. I havn't shopped for SD ammo in the x39 round, but I'd guess a Barnes TSX or TTSX would rock too.
The Blackout, it's a different weapon system than the 5.56, training will show sub-standard performance, from reliability(of many componets), to sub-par ammo. With the cheap pulls, it is cheaper to reload and shoot than the 6.8. Recoil impluse is different, site recovery is different. I also have lots of 5.56 brass, and I've converted many to 300 BLK cases for my use. I bet I am really close in cost reloading for the 300 BLK vs buying x39 ammo, I do loose some time, but 1050's are pretty quick.
I use 6.8's in the home defense role now, and 5.56 as backup/bug out systems, 85TSX is stuffed in lots of mags.
Shooting indoors, big difference in blast with the 6.8 vs the 300 BLK, and the BLK offers a nice compromise between the 5.56 and 6.8, and my 8 inch Blackouts fit perfect in my X3 Lo-drag and GunslingerII Back packs.
We are an AR15 home(and we have an extended AR15 family too), lots of 5.56s and no AK's, the Blackout makes more sense here due to Magazines, and lots of AR parts, and Brass. With 125 Ballistic tips, it's sub MOA, and the Blackout is reliable in my systems. More AR's(in any caliber), is always better in my book!!! Commonality is a huge plus for us. My daughters are not really the type that buy guns and ammo, but they know AR's, and knowing one rifle platform Well has it's pluses.
I still shoot at 3gun match's, currently I'm still using a 5.56, but since I can still use cheap brass, and (within 1 penny) shoot cheap 30 cal bullets, it loads almost as cheap as 55fmj 223 ammo. I have haven't prepped, and shot enough to see if shooting major will benefit me over shooting the 5.56 with a comp(my rifle has no muzzle climb), I've made major with a 175gr bullet, but for me I think I'd need a 147 min, then more gas for a comp. I loose most of the Brass, loosing 200 cases of SSA brass isn't very fun either. You don't see many AK's at our match's, it's all AR15's. Currently I'm holding my own with the 5.56 at matches, it's hard to give up whats working.
The Blackout is not a 6.8, but it's no slouch either, I've made a place for it, and it's probably here for the long haul like the 6.8. I have a G barrel coming from H, if I can ring steel 300M past the 6.8, maybe it will be a keeper, time will tell, keep the good stuff, sell off the posers. Guys with AK's, they have it going on too. Nothing wrong with that system, I would feel well armed with one, but I'm pretty smooth with the AR family(I'd be rough on an AK for a few minutes).
What is 7.62x39 selling in your neck of the woods, it's over $200/1200 rounds here, but I don't check prices often, but I cry when I see the 5.45x45(I think I got that right)ammo, $130 for 1200 cases, yes, that is cheap, and I've thought about an upper, but I'm good with 5.56 and 6.8 mags, and not cleaning up after corrosive ammo is fine in my book.
Blackout or 7.62x39, pick you passion, I went AR15(5.56, 6x45, 6.5PCC, 6.8II, 300 BLK), I've got the bases covered, and home plate too.