Out of curiosity where did you see that the original intent was for long range shooting?
That was listed as being part of the reasoning behind using large caliber, heavy for caliber bullets for sub-sonic sniper rifle rounds. They would have a rainbow trajectory of course, but they would also reach out farther and still retain plenty of killing power.
The advantage in a sub-sonic sniper round should be obvious to just about anybody.
The "Long Range" being discussed is a relative long range for a subsonic rifle projectile, as compared to the short-range performance one traditionally sees in sub-sonic pistols. The advantage in longer range for any sniping application should be obvious to anybody.
One would think so, at least - but expecting everyone to apprehend the obvious is a sure road to disappointment.
The history of sub-sonic rifle cartridge development for sniping applications is well-documented. - And it doesn't take a nuclear physicist to work out why silence, the longest ranges possible and the good terminal performance would be advantageous for snipers.
When a firearm type becomes very popular, as the AR15 has, it should come as no surprise that hokey marketing devices will tend to surface, as entrepreneurs strive to get their share of profits from the feeding frenzy. Thus we hear a lot of misinformation about the 300 Blackout shooting varmint bullets being "just as good" as a 30-30 shooting much heavier game bullets.
BS is BS, and I call it when I see it as a public service. The 300 Blackout in an AR15 makes a nice short-range target round, but is not really optimal for anything else. It is too large in caliber to succeed as a varmint or small game round, and doesn't have enough case capacity to launch heavier projectiles at reliably effective velocities for general game hunting. It can do a fair job on pigs, small deer and such at short range with 30 caliber varmint bullets- but anybody with a lever-action rifle in 30-30 will be much better equipped.
That's just how it is, when we set marketing hype aside and give the 300 blackout a BS-free, objective appraisal.
If somebody gets all butt-hurt over objective reality then that is their problem, not mine.