I've never seen a suppressed .50 caliber rifle in OFP, and to have someone make on would be great. They do exist in real life, but of course the suppressor's purpose is mainly to hide the muzzle flash. For instance, the suppressor on a UAA Model 5100 takes the sound level from about 175 decibels to about 140. Still quite deafening, but the lack of a muzzle flash permits more sniping.
As a sidenote on decibels
: 140 decibels is about as loud as a shotgun blast or jet taking off 100 feet away from you. Since decibel levels are logorithmic, that means that for every single-decibel increase, the noise level is multiplied by 10. If I'm not screwing up on my math, that means that a unsuppressed .50 is about 100,000,000,000,000,000,000,000,000,000,000,000 times louder than a shotgun. Yes, that's 35 zeroes ;D!