The P90 according to my research has magazine issues. That being aside the 6.8x43mm (6.8 SPC) is a rifle cartage, the 5.7x28mm is a pistol/carbine cartage 6.8x43mm OAL = 2.315 in (58.80 mm) 5.7x28mm OAL = 1.594 in (40.50 mm) That is a difference of 145% when scaling the 5.7x28mm to 6.8x43mm it can be done but issues will show up very fast. I have looked and I have only seen the horizontal to vertical conversion in pistol cartrages
There is an IFFY concept. Barrel harmonics and trigger pull affect accuracy. In fact bullpup weapons real strength is their size and balance. I have read that all ammunition is more accurate with a specific barrel length, for instance you can achieve the maximum accuracy with a 16" barrel while a 20" barrel has reduced accuracy with ammunition. Barrel harmonics do not affect accuracy dramatically but you can notice on the high end a 1moa increase due to barrel harmonics, if memory serves me. A longer barrel is intended to be used for increased range and higher kinetic energy. Bullpup trigger pull is notorious for being 'mushy' and therefore affect accuracy, Very few bullpup rifles have good trigger systems. As a result for maximum compact ability and balance you should go to a bullpup. For maximum accuracy (negating human inconsistency) conventional rifles shine.