rubber bullet will never be used for "dopper practice" because it tends to bounce uncontrollably
up to this day TNI uses 5.56mm FMJ bullet fired from SS series/minimi..it poses a danger because the bullet is inclined to fragmenting when it hits surface..In some instances they use 7.62x39 AK-47 but the bullet still not immune to bounce and injuring nearby soldier
the desired features of weapon/cartridge for this kind of activity are: predictable path of bullet..good penetration to mud and dirt..long range is not necessary..projectile doesn't tends to fragmenting or bounce..
this SS version uses 7.62x45mm round commonly used by Polri in their SS-1 sabhara..basically, this round is 7.62 NATO projectile neck down to 5.56 NATO cases..it has heavier projectile and fewer propellant thus lower velocity bullet that will not bounce nor fragmenting when it hits mud..
the barrel is longer and beefier to sustain constant barrage of fire and produce more stabilized path of bullet
but contrary to what the article says, longer barrel and lower velocity bullet will produce quieter sound as the propellant will be completely burned when the projectile leaves the barrel, also the projectile will never go supersonic speed..maybe the weapon is fixed with some kind of muzzle device to increase the sound decibel