CT cartridges are shorter, but also fatter.  This will reduce magazine capacity.

been wondering that - how much liberty is there in the proportions?

Not as much as you'd likely want/need.

There's potential workarounds but they definitely aren't risk free and would require some level of development.

The much better option is something like shellshock nas3 cases which can be ran up to pretty hefty pressures safely and reliably. I think you might get a slight bit of extra case capacity over brass even.

Being able to run 70k psi Though should do you well.

Oh, did I mention that they're also cheaper than brass cases? 

Since it's a pdw you're probably gonna go through a bunch of rounds so that's not a bad thing.

graylion said:

been wondering that - how much liberty is there in the proportions?

Case proportions vary with projectile length and diameter, and propellant charge.

Presumably what you actually mean is, how small can the case diameter be made?

With the Textron design, case diameter is ~0.195" greater than the bullet diameter.

Caliber ---- Bullet --- Case

5.56mm --- 0.224" --- 0.419"

7.62mm --- 0.308" --- 0.504"

that's actually fine. I had reckoned on ~12mm base diametre and for 6.35mm this gives me 11.3mm (.445"). And one can profit from the higher pressures.

Looking that old 6.5 CT cutaway, I had a simple and possibly stupid idea: since bullet tip is totally surrounded by plastic case... why don't make it as pointy as possible, in order to improve slightly aerodinamics and armor penetration?

It's not a stupid idea.  The 6.8mm projectile developed for NGSW is about as pointy as possible.

I meant pointy as a needle, not only a nice BC. Like, reducing impact surface as much as possible, less than 1 mm^2

You mean like an APFSDS? I guess that it has been debated here that such formats aren't really useful below some 25 mm, and APDS not good below 12.7.

For rifles and LMG, the 6.5 pictured above is basically as good as it gets.

No, I mean the same specifications as XM1186 but with a pointier point, so to speak. With the least possible point surface