A sniper rifle by definition generally does not allow for automatic fire (bolt-actions for the great majority of sniper filfe / semi-auto can qualify as a sniper rifle in some cases - Dragunov arguably) since the main role of the sniper is to ensure an accurate and precise placement of bullets (BB in this case) at longer ranges. Thus, Full-Auto is incompatible with this role since it is not precise enough.
Basically any AEG with a long inner barrel can be made into a DMR platform with the right upgrades (optics are optional but recommended
Hence you are looking for a DMR rifle. Arguably, the best DMR aeg would be the Real Sword SVD but again it is semi-auto only. M14, G3, M16, SR-25 might better suit your needs