Hey guys I haven't found any links to any military sniper field manuals on this forum. Here is a link to FM 23-10, which is the older us army sniper FM. They had a revision when the M110 SASS came into service and they re-wrote the entire manual when they determined that the m2010/M24E would replace the older M24(a1,a2). Despite being a little old a lot of the information is still useful even in airsoft. The most useful chapters are 2, 4, and 5 for airsoft snipers(imo). The rest of the chapters are still a good read though! Enjoy!
That website also has the basic infantry FM. FM 3-21.8 is also a little dated, however most everything except the MOUT operations are still current SOP's. It might also be a good read if you are interested in learning about how a Designated Marksman should operate in a normal infantry squad. Typically the DM would take the place of the Rifleman(RFLM/R) in the typical US Army infantry Fireteam.