I think there MIGHT be a slightly better way to do the follower these days, but yeah a 6" barrel 5.56 or 300 blk would be... Interesting.

Despite this I do think they have something here, it just probably needs it's own cartridge design or a modification of something we already have to really be worthwhile.

The blk has huge energy in short barrels.. 900ft lbs from a 5.5incher

762x39 would have similar energy

556...about 500ft lbs from a 5.5

but the 7.5FK has the same high energy as the Blackout in these barrels....its a far better choice

7.5 fk just isn't actually all that impressive of a round for one.

For two, your entire line of reasoning focuses on terminal performance metric you frankly don't understand.

Not all muzzle energy is created equal, not all projectiles are created equal, and muzzle energy only weakly correlates to real world performance at absolute best. (There's an entire universe of cases where the higher muzzle energy round performs much worse pretty much across the board)

Like I said, not all muzzle energy is created equal. 

The 7.5fk is in an entirely different class than the 300 blk and not in a good way.

My point was that they both have the same high energy in 6inch barrels, and that 7.5 would avoid all the crazy feeding mechanisms required of the blackout.

As the pic shows, the gun has a massively long grip...7.5 would avoid this

And the 7.5 would obviously have twice the power of 4.6 or 5.7

Im aware that the black has more BC and so on.

You're using "power" in a way that doesn't fit with actual effects in reality.

Nothing you said changes this.

Additionally, there's almost certainly at least one better way to handle the follower situation.

Beyond that the only thing I have to say Is you're still entirely missing why 300 blk would be in an entirely different class than 7.5 fk.

It's not that you haven't had the reasons why explained several times either, because you have.

"Just chambering it in 7.5 fk" defeats the purpose they were going for.

You never specified why the BLK bullets are better.

 as a PDW it wont be used beyond 150m so the extra BC of the blackout isnt too valuable. The only other advantage of 300blk is maybe more penetration.

Then again the blackout bullets might be worse because at 140grains plus they will be slow

Like i said the grip is very long...thats the drawback of the design. 

It isn't that BLK bullets are better.  It's that 7.5 FK is a pistol cartridge.

The purpose of the unique magazine design is to enable use of rifle cartridges like .300 BLK, 5.56 NATO, and even 7.62 NATO, in a weapon of MP7 size and configuration.

556 would be pointless given the huge flash and low energy.

Admittedly 7.5fk is a little 'niche' in terms of availability but it would allow a much more holsterable gun...something resembling those 'glocks with stocks'

That's all true.  And it's all quite irrelevant.

Their design purpose is not to use a cartridge that allows a more holsterable gun.

The design purpose is to allow use of rifle cartridges in a holsterable gun.

Speaking of 762 SBRs, ian has a new video on an MP5 in 762nato.

I think as a military PDW (the crye PDW)  its great because of the ammunition commonality. If you could solve the flash problems of 556 you would have a real winner

Still, the ultimate PDW would resemble a glock w/ stock, and with the 300blk the grip length is not ideal for vehicle comfort etc.

Again id love a scratch-built round with huge penetration....basically a CT pistol round in 4.8mm

Oh, were you the guy behind the 'crist PDW' design?