smg762 said:

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

Guilty as charged, your honor.  

Oy vey, it's not a 7.62 nato mp5 the 51 are just very cut down g3's a guy called bill Fleming came up with way back in the dinosaur days where you had to buy shotgun news at Safeway and send check or money order to buy gun parts.

ETA: there's no such thing as "extra BC". You are extremely confused about how firearms and ammunition work, you should work to rectify this.

We have tried explaining these things to you multiple times, I personally am done doing that. You're just plain wrong and muzzle energy doesn't mean what you think it means nor does it work the way you think it does.

You never once explained why the blackout would perform better with the same energy as the FK. If you could give a reason then brilliant..

Yes the pointy bullets will hold BC and trajectory for a bit longer...other than that any differences are trivial.

Within PDW distances, 900ft lbs is 900 ft lbs...its as lethal as anything else

Like i said it would have some merit as a 556 PDW for sharing ammo 'in the field'. 

As for the MP5 ive always known it was a HK51, there were youtube vids of that gun for ages.

The main plus of the .300BLK might be that long bullets offer space for an efficient penetrator to be used in comparison to short stubby pistol-type bullets. 

Said "finer projectiles" not only tend to fly better, shed velocity slower, buck wind better, penetrate more deeply and reliably, and a few other nice fun things.

Or put another way you can get an mp5 and an aksu with similar length barrels and with the mp5 even very good shooters will struggle past 100-150 meters to put shots on target, and those targets are far more likely not to go down even if you manage to connect with a solid hit at longer ranges.

Take an aksu with the same length barrel chambered in 5.45 and, especially with an optic, a trained user isn't going to have much trouble reaching out to 300 meters reliably consistently and with the ability to place good hits that will drop people between one step and the next consistently!

What's even more interesting though, is that steel case 5.45 7n6 round is shockingly close to the same weight if not a little lower weight round for round as your average brass case 9mm subgun food.

Now I'm definitely not saying that you'd want to use 5.45 in this for a variety of reasons due to case geometry not being super compatible with this particular system but the aksu makes a good comparison piece because it illustrates how you can get quite a bit more out of a system that may actually be lighter than mp5 itself and the ammo weights are also disturbingly close between 5.45 and 9x19 while the difference in reach and lethality at distances beyond 100 meters is night and day!

This is why guns like the mk18 sig LVAW and etc exist and get used while subguns are incredibly more niche.

This potentially gets you to something mp7 / p90 like without the dodgy I just may have to magdump to get this guy down terminal ballistics.

If only u could solve the flash problem of 556 in 6 inch barrels.

Could one heavily port the barrel with holes...and have some kind of long flash hider running around the holes

Pretty simple it's already been designed look barrel on the aksu at the end 

Like Wilson said, the aksu muzzle device is one possible route.

Going to an 8 inch "barrel" with a pepperpot reflex suppressor is another.

Or you could just roll yourself a round for this specific implementation that gets you most of what you want without having to go to quite the extremes this design represents.

All 3 of these approaches are viable.

A Fourth approach would be to use something akin to a cbj style sabot for a 5.56 projectile in a blackout case that'd work pretty decent.

I'm not able to do the math (especially without knowing what a cbj style sabot would weigh etc) but I think this would be one of those cases where you could come out substantially ahead even after losing the sabot's parasitic mass because of the swept volume increase so long as you could keep the cartridges effective RC high enough.

Oh man ... I realized that we already have almost the perfect loading for this already, we just need to dust it off and tweak it some.

5.56X38 FABRL !

Iirc it was getting absolutely hilarious velocity at only 38k psi or something insanely low in it's original instantiation. If we push 50k or 55k psi which should be doable with modern powders and even lightweight cases you could sling absolute monsters of a pdw round out of this thing relatively easily.

6x35 TSWG