i am wary about new calibers until they are proven. call me old fashion, but the thought of spending that much money on a rifle then having to fire form the brass just to shoot it sucks.
Every 5.45x39 rifle is completely dependant upon cheap import ammo to go BANG; there is no indigenous supply nor are reloading components available. The bullet is a non-standard diameter that has no corollary in any US chambering.
6.8SPC brass, on the other hand, is available from at least three suppliers; Remington, Hornady, and Silver State. Bullets are readily available in component form from at least a dozen manufacturers.
Between the two, I believe that 6.8SPC is far more likely to be readily available in the USA, say 25 years from now.
the 6.8 will penetrate too much in your neighborhood, your walls, the next door's , etc., plus ammo cost is a ton right now.
An expanding .277 bullet will penetrate no more than an expanding bullet in any other intermediate chambering, and IMO will provide more reliable (consistent) performance than 5.45x39 milsurp projectiles. I can also buy 6.8SPC self-defense loads for $18/box of 20, which is about the same cost as any other non-surplus chambering.
I have a 5.45x39 rifle and enjoy it. But I am under no illusions as to how much it depends upon cheap imported (corrosive) milsurp to remain viable. I am also not willing to bet my family's well-being on the premise that the terminal performance of a yawing 5.45mm 53gr FMJ projectile is as reliable as a controlled-expansion .277 bullet. I have looked at the gel tests (and sponsored one Brassfetcher test of 5.45 60gr Wolf) and do not see the reliable performance from the round that I want in a self-defense load.
You also need to bear in mind that the cheap milsurp is only cheap because some of the former ComBloc countries have been selling their stockpiles as they converted to 5.56NATO. Once the last of their stocks are sold off, there will be no more cheap milsurp. That will leave you with Wolf as the sole US source of supply. And when the political winds dry up the flow of imported 5.45 ammo (either in the guise of the continuing UN assault upon arms trafficking, or in the guise of a US Gubb'mint ban on 'non-sporting' import ammo), those who banked heavily on the chambering will be left with nothing but the stockpiles their hoarded.
Don't believe it? Just ask the MBR aficionados who believed that surplus Port and SA would always be around for pennies per round. Last I checked, a SA battle pack of 140 rounds was selling for $75/each. At least the MBR folk can look to the .308 reloading community for supplies; something that the 5.45 milsurp aficionados cannot do.