In the Alpha, suppressors decrease the effective range and damage of projectiles. This does not happen in real life. There are no negative effects on the projectile fired from a weapon mounted with a suppressor. There is usually a very slight (50 fps) increase to velocity and some evidence suggests it makes the projectile slightly more accurate due to stripping excess turbulent gases from the around the projectile as it exits the barrel. Using a suppressor also makes a noticeable reduction to recoil impulse. It is no coincidence suppressors are commonplace in the US military now among units that can afford them, notably amongst the precision and 'sniping' communities, and of course SOF.
The only negative effects to using a suppressor are a) Increased weight, b) Increased fouling, c) Increased cyclic rate due to overpressure*, d) Cost**.
- Not necessarily a negative, depending on gas system tuning and desired effect.
**Answering the question of 'Well then why doesn't everyone use them?'.
A suppressor works by controlling the expansion of gasses that follow the bullet AFTER it has left the barrel (the rapid expansion of which is what causes the 'bang' from firing). The weapon has already completed it's job of propulsion as soon as the bullet exits the pressure chamber created by the locked bolt face, the barrel, and the rear of the bullet; the bullet then flies unimpeded through the hole in the suppressor while the gasses behind it are slowed by the baffles around the bullet channel. I repeat: there is nothing impeding the bullet. It only takes a little bit of logical thinking.
This is considered common knowledge in the shooting community in the United States; it would be a shame to see a game like ARMA follow the misinformation trend started by games like Call of Duty.