Every cylinder comes with a length range, stick to the lower range
with a 300mm barrel you want a cylinder for 300-380mm
reason being it makes the gun more energy efficient on heavier ammo.
And personally i wouldnt go longer than 14.5" or 363mm. As i said, the longer the barrel is, the faster it will foul. And the 14.5 is going to be just as accurate as a 455 or whatever. best to use a socom m14 with the short barrel.
As far as a short barrel in a long outer, it really shouldnt matter. Systema has always used 14.5" barrels in their 20" M16s.
it only takes about 2ms or less for the bb to get from the end of the barrel to the flash hider, and if you dont have at least a 6mm grouping at 12", you have some serious issues with your gun lol