The 6.5 uses a shorter and fatter case than the 5.56

It delivers a heaver and larger bullet than 5.56 at decent velocity

The P90 cannot be chambered in 5.56 because of the P90s restriction on ammo length

Could a new round be designed that used a shorter and fatter case that would fire 55-60 grain .22 caliber rounds at M193 velocities with only a new magazine and rechambering of the P90?

It would have more recoil and more pressure.  It would reduce ammo capacity to about 30-40 rounds.

It would essentially do away with the terminal ineffectiveness of 5.7 at close ranges as bullets could fragment and still reach 12" penetration in tissue and use bullets with a better BC for flatter shooting and enhanced penetration at longer ranges.

Does this idea exceed the capabilites of the P90 due to structural reasons or the recoil system used?

Good idea or not?  Ideas and input appreciated.

I don't know, but I doubt it.  You'd be working with some pretty severe limitations in cartridge size.  Heck, the 6.5 rounds are already working with limitations in order to be based on the M16/AR15 platform.  

I'd rather see a PS90 in 9mm.... so I can use dirt cheap ammo!  

They would have to re-design the whole gun. Keep in mind the 5.7 case is 28mm long, so once you put in a 55 grain or bigger bullet, there isn't much room left over in neither the case or the gun. This would leave only the option of a super wide case with a horrid neckdown which would probably require every piece in the P90 to be re-made.

I'd be willing to bet the round would be even far less effective than the 5.7, especially when fired from the short barreled variants.

10mm P90!

Quoted: 10mm P90!

Fallout reference?