Just looked in the actual Stats of FFG.
Funny, the "Scourge" Bolter has a little less range, in + and Pen does a little less damage, weights 5 kilos less, no full auto, 4 shots less, and is reliable but lacks tearing.
As i stated earlier: ever thought about that in Bolters the Tearing is part of the damage? at least for DW.
A word on munitions: Boltshells have a relative low rate of fire, heavy Bolter with 120 shots/minute the most, typical Astartes bolter 48/minute Bolt pistol 36/minute. and this is considerd: holding down the target with semi/full-autofire. Non-Astartes Weapons even less.
A standard 1960's Gyrojet rifle has an RoF of 60/minute, but the tend not to be this extremly noisy and the Problem with the velocity of under 20m due lack of starting the thrusters. Sabot would also make no sense, Bolters throw out ammo caseings. What works: due lack of Knowledge and to sound cooler! it was named: Micro Rocket. Makes perfect sense, explain most things. But to be honest its just normal HEAT/HEAP rounds, just with a lower rate of fire than even an 50 year old Vulcan-cannon, and about as much or less devastating power.
Point is, trying to work with pure scientific logic doesn't work. Especially in a SienceFantasy scenario, wich deviates from a broad game
Btw: this little DH/RT vs. DW is a little ridiculous. In the End, despite this x2 T/S bonus they are overall worse than a typical Acension DH guy, expect for their gear.
and to be honest: Marines feel like super tough guys and so but.... well even a little genstealer with his 65WS that scores a good hit kills your defender of mankind in one round. Trust me, my players respect them