[Hectic]

The silence would also be dependant on how heavy of a spring you run.
for me i run ~500fps a 455x6.01 inner a sobo pad no air break and the stock filled in with some veils (easy spot to keep em for different cammo in the field and helps to muffle the echo within the stock. Its not silent but outside of 50feet youd be hard pressed to hear it or at least pin point the direction.
Other way around. A bigger bore=more volume, so more volume wasted, so lighter ammo.
why are you wanting to go for a stock type bore? Most of the best barrels (read consistant, smooth bore) are 6.03 6.02 and 6.01.
Tho i will also piint out that 6.01x455 has a volume of 1307.82697mm3 and 6.05x455 is1325.29304mm3 so a wider bore wont make as much of an impact on volume matching as length, short barrel bigger cyl=more over/wasted air volume, so youll use a heavier ammo. But base velocity and ability to apply hop also have an effect. For example. No matter how short the barrel and big the cyl. If your gun shoots an m90 spring on .20's (~300fps) you wont be able to effectively shoot ammo much heavier than a .25 as the bb will just fly too slowly. its been prety well shown that in and around 300fps is the optimal velocity so if you have a 400 fps gun a .30 will give you ~330fps so thats close to optimal, .32's work well and for some (say rhoped or some other good hop mod) may be okay with .36's tho time to target will be a little longer and any gain in effective range or accuracy will likely be negated by folks bein able to matrix your shots lol