Not mine, but:
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"The other answers have adequately covered the fact that simply drilling holes will make the weapon’s report louder, but I’m going to add a bit about a more obscure technique that involves porting a barrel to decrease velocity.
A standard suppressor will not have any meaningful effect on the velocity of a bullet, all it does is corral the escaping gasses to reduce the report. If the bullet leaves the muzzle at a supersonic velocity, it will make a sonic ‘crack’ sound as long as it is moving faster than the speed of sound. This sonic crack is usually eliminated by using subsonic ammunition, but a downside of this is that now you have different ammunition types for the same round and therefore conflicting logistical systems.
With the Heckler & Koch MP5SD(SD stands for Schalldämpfer, German for suppressor) , HK wanted a way to make the 9x19mm Parabellum round* subsonic without using specialty ammunition. Their solution was to port the barrel down its entire length so that the gas pressure was reduced during firing. This causes a lower force on the bullet and lower acceleration, leading to a reduced muzzle velocity. The gas vented from these ports is captured in an aluminum sleeve around the barrel and allowed to slowly escape. The forward portion of the barrel has baffles like a traditional suppressor to help slow the expansion of the hot gas.
*Standard 9x19mm loads have velocities around 400 m/s out of the MP5’s 225mm barrel. The speed of sound at sea level is only 343 m/s, meaning that a standard MP5 shooting 9x19mm ball ammo will be significantly supersonic. The MP5SD shows muzzle velocity decreases of 16 to 26% compared to a standard MP5, giving the MP5SD a muzzle velocity between 336 and 296m/s.
Below is an early diagram of the MP5SD barrel, showing the ported section to slow the bullet and the baffled section to slow the escaping gas (the barrel in the diagram is pointing left)."