POTD: The AR-300 Blackout

@Christopher B I’ve always thought the 6x45 (5.56 necked up to 6mm) was and is superior to the 5.56 for knocking things down.
Agree with your opinion that hybrid cartridges about to revolutionize the industry.
I’m thinking of a 6x45 or a 6.5x46 operating at 80k psi running out of a slightly modified AR platform to allow for longer heavier bullets (Revolution DI) giving Creedmoore or 260 ballistics in your after mentioned small package. Subs can be added just as easily.
The .277 Fury is a long list of questionable compromises designed to fix a problem that probably doesn’t exist, at least from what we’ve seen in the Ukrain.
Watched the Garand Thumb video where Kevin Owen’s was talking about clearing out a building with a 11” AR and they had SR25’s in a pack on their backs for when they got to the roof and engaged the enemy at longer distances. Imagine doing the same thing with a rifle as compact as the small AR but better than the SR25 at any range.

@KsKLR Blackout is a mature cartridge that's fully developed and flushed out. With it and options like 6mm ARC on the market there's no need to design yet another new cartridge. The machinery and tooling to make 5.56 NATO is prolific and can be easily converted to produce Blackout. Sig's new spear and high pressure cartridge will fail because of logistical momentum and the combination of too much novel technology, while Blackout, Grendel, or 6mm ARC are small steps that repurposed existing products and processes. Regular troops and security personnel don't engage targets at long range, the preoccupation with shooting targets 600,700,800 meters or more is totally misguided and puts every armed person in a DMR role when that makes zero sense. Real world combat takes place at 100-300 yards so delivering more energy on target at those ranges without changing what works about your primary weapons platform makes the most sense. Creating more internal stress that shortens a rifle's service life and causes faster throat erosion and more recoil is a bad idea. A simple powder swap in a hybrid case can push standard off the shelf 300 Blackout to 2,350-2,400 fps range, dramatically improving barrier penetration and terminal performance with a simple barrel and ammo package rather than a full rearming.

@Christopher B Now go to YouTube and watch videos of M855A1 being shot at gel and armor from 10" guns at range. And it's not the only round capable of that kind of thing, by far. So long as you design ammo to fragment reliably in mind with anticipated muzzle velocities (and don't use bullet construction developed for 20" barrel velocities in 10" guns), it is very devastating.

And if 5mm low-impulse cartridges "became obsolete" in real combat, don't you think at least one of the major military players would have switched to something else? Instead we've seen Soviets switching *to* 5.45 based on what they've seen in Vietnam, followed shortly by Chinese switching to 5.8.

The other things you're completely ignoring is weight of ammo and recoil. Quite bluntly, you can carry a lot more ammo for the same weight in 5.56.