As for game data (with the normal disclaimer that i might have got it wrong somewhere
)
Bullpup full-auto-mode does deal more damage per bullet (30% higher), have a higher rate of fire (10% higher) with a lower recoilrate (30% lower) per bullet than the semi-auto-mode. However, full-auto-mode does have an aimerror that semi-auto-mode does not have at all (semi-auto-mode also have a higher bullet momentum).
All ranged weapons, except flamethrower, also recive an additional aimerror and spread if you move while you shoot (higher the faster you move. The extra penalty is reduced by 40% if you crouch. There is no extra penalty if you do not move).
Both fire-modes also have the same basic bullet spray. But you can reduce the spread by crouching (15%), using the reflex scope (50%!) and using semi-fire-mode (15%). You will also get a decreased accuracy for each bullet during rapid fire (goes for both fire-modes but only up to a max of six bullets at which point it does not decrease anymore). This will last until you hold your fire for atleast 0.5 seconds (at which point the bullpup will be stable again).
If you want to maximize damage per second for close up body shots with the bullpup it is better to use full auto-mode compared to fire semi-auto-mode as fast as the weapon allow.
If you want to minimize bullet spread for really long range one-shot precision decapitations (maybe in a friendly fire setting?) you would want to crouch, zoom in, if possible don't move, use semi-auto-mode and only fire one (or a few) bullet(s) and then hold fire for 0.5 seconds.
When taking out a siren at medium range (just before she get into scream range) it could be a good idea to crouch, zoom in, if possible don't move and use full auto in full-auto-mode until she dies.