Daugen, two things cause the sounds we hear from the firing of the .22LR cartridge.
First the sound level depends on the length of the barrel of the firearm we fire them in. The shorter the barrel the louder the muzzle blast as the hot gasses do not have a chance to cool in the shorter barrel. Also the longer barrel allows the gasses to expand along its length and cool down some, so the muzzle blast is less "explosive" as the bullet clears the barrel.
Second comes the "sonic crack" as the bullet in the air breaks the sound barrier. This is the exact same phenomenon as an sonic boom from a jet aircraft. While this velocity varies due to temperature and even humidity of the air, it is somewhere around 1200 fps or a little greater. So a bullet travelling less than 1200 fps will likely not produce this 'crack' sound. These bullets are said to be subsonic.
Ironically the identical round may have a crack sound when fired from a rifle but not produce the sonic crack when fired from a pistol, as the pistol may not be able to wring out the needed velocity out of the powder charge. BUT the pistol will always sound much louder due to the increased muzzle blast.
If we put a suppressor on the pistol and give a place for those hot muzzle gasses to expand slowly into its chambers and cool down before being released into the atmosphere this round can sound like a mouse fart. And since the sonic boom velocity is not reached by the bullet fired from the shorter pistol barrel, no 'crack' sound will be heard in the open air.
Also adding an ablative substance in the suppressor can further reduce muzzle blast sounds. These substances can include such things as wire pulling gel, vaseline, or even water. These substances help cool the gasses before they are released. But if the round from the pistol exceeds the sonic boom velocity then no amount of muzzle suppression will keep the sonic crack from occurring in the open air. This is why you see ammo listed as subsonic or standard velocity etc. Adding ablative substances is called shooting a "wet can". Several db's of sound suppression can be achieved by "wet cans"
As for hollow point vs. solid accuracy, that is a very difficult thing to say. The deviations in accuracy of different rounds in different firearms is greater than the deviations between the two types of bullets. There is some thought that some hollow point rounds may not be as concentric as they could be, and therefore might not be quite as accurate as possible.
I hope this little "book" helped.
