Jack told me they couldn't silence the rifles because they had no sub-sonic ammo, but the suppressor quieted the sound of the rifle blast so you couldn't tell where it came from. It also totally eliminated muzzle flash so was especially useful at night.
FWIW, QOB acquired the rights to the Ingrham machine pistol and made three versions: the MAC-9 in .380, the MAC-10 in 9mm, and the MAC-11 in .45 They tried to sell them to police departments by going to a local PD and asking for the guy who could almost never qualify... then they put a 50gal drum of water in the field at about 40 yards, handed him a MAC-10 and said, "Shoot the drum." BRRRRRP and the drum was squirting from 5 to 10 holes. Everybody was impressed but they never bought any.