I once said this:
Suppression should be on a bullet per bullet basis
Its like this:
A bullet hits near a man, the effect of overwhelming the senses with such a near hit is brief but harsh since the mind knows it came close to a deadly object which just passed and struck close - The 'fear' percieved by the mind is based on a sense of the calibre firing i.e. a pistol bullet wont be percieved as deadly as say, an AT rifle round and less than explosive 20mm round etc... Also, an effect of the vibration from the impact of the round
Now,
2 factors ingame -
Suppression amount (SA) - caused by calibre of round or shot
Suppression length (SL) - caused by velocity/ viberatiion of impacted surface, percieved by brain in 'fear'
Every round has a suppression quantity that lasts for a set time - The smaller the round the lower the viberation effect, usually the shorter the supression length, the lower the suppression amount eg. An m1A1 carbine will have equal Suppression amount to a pistol (intesity of suppresive effect), but perhaps more suppression velocity (time suppressed)
A pistol round will be eg. 1x SA and 1/2y SL
A rifle round (Bren, mg, bolt action, semi-auto) will be, say 2x SA and 1y SL
An AT rifle round will be say, 4 or 5x SA and 2y Sl (I'd put this as current amount of suppression for reference)
Now if each shot does this, then the number of shots will also determine how long the suppression - And since suppression 'grows on itself' i.e gets moreintense with further suppression - An mg may have a heck of suppression or just as much as a rifle depending on how many rounds were effective in suppressing the enemy NOT because its an MG in itself - obviously, an MG42 well aimed will have more suppressive effect than an mg34 or .30cal due to alot more bullets hitting close, thus keeping up the suppression effect.
A max suppression amount needs to be set to avoid overwheling the perspective of the player - At max suppression, you are disoriented enough not to want to poke out since you wont know left from right, but the effect itself will not increase with successive near hits, but remain the same or lower based on if shots come close or stop coming close
I wont go into grenades, AT cannon, other explosions or 20mm HE, but they will follow the same principle - This way, a person suppressing can do it with any projectile-firing weapon, and a number of riflemen can suppress a person too, just never with the same firing rate as a well-aimed mg42