I think the Semi-Automatic would be best for being a Squad Designated Marksman or the Spotter. We all know that a BAR-10 or VSR-10 is probably going to be more accurate than an ECHO 1 M16, plus, they're cheaper!
A highly upgraded JG BAR will preform on the same level as a highly upgraded TM VSR because you replace all the same parts for either brand. The JG is just $100 cheaper.
ANYWAY... back on topic, my answer for a true sniper, not a DM or spotter, the best route is definitely bolt action wether gas or spring you will still have better performance than an AEG with the same budget in it. Bolt action is also MUCH quieter. But all that changes for a DM, an AEG would be better for the ROF. As far as spotters I would prefer a full auto AEG because in airsoft the spotter job is to protect the sniper and take shots that are inside the sniper's MED. In my case though i usually don't take a true spotter I take another sniper, we are set up about 20-30 ft. apart and comunicate with hand signals and by radio.
do you think that a Real Sword Dragunov would be able to perform in the rolls of both a DM and Sniper, because it seems to be sort of a crossover between the two.