Peak pressure isn't the real problem in blowbacks; duration is. The combination of slightly higher pressure, significantly heavier bullet, and
much wider pressure spike due to the additional powder mean that the cartridge must be held close to battery for more time. Bolt weight is the ONLY factor when designing blowbacks, and most designs try to keep it as light as necessary in semi-autos so as to speed up cycling, reduce felt recoil (from bolt slam), and increase extraction reliability. Simply boring out a 40 blowback and running 10MM through it is likely to be incredibly dangerous.
Orion's Hammer (an authority on things 'Blowback') lists the recommended bolt weight of a 10MM at 2.8lbs, compared to 2.2lbs for 40 S&W. That's a difference of over 20% in bolt mass. As powder volume goes up, pressure duration will as well, and regardless what the peak is, it's higher than you want vented to atmosphere (i.e. your face
). This rapidly raises bolt weights to ridiculous levels (54.3lbs for 50BMG; I don't want to even know what the Oerlikon bolt weighs) which is why we have locked and delayed breech designs.
An open-bolt design can utilize its forward momentum to "multiply" its effective bolt mass, but since those are off limits to us civvies, we're stuck with simple closed bolt blowback. Since that design is limited to (essentially) a certain threshold of cartridge power if it is to remain "handy," a locked or delayed breech is the way to go when chambering upper-mid pistol cartridges and above. Delayed blowback tends to be hard on brass and has even less operating flexibility (tolerance of different loads) than simple blowback. If the Light Carbine is to be a "jack of all trades" as far as utilizing different loadings and cartridge conversions, it really needs to be recoil or gas operated from a locked breech.
TCB