You will need a riser mount for the Trijicon ACOG 4x32mm Rifle Scopes w/TA51 Mount to clear am FSB. The mount at the link below should work.
https://www.opticsplanet.com/american-defense-manufacturing-ad-b3-c-cantilever-acog-base.html
Yes, you would be able to purchase and install the Trijicon RMR Mount for ACOG Scopes with bosses (linked below) to this Trijicon ACOG TA31, and it would allow you to mount an optic that uses the Trijicon RMR optic footprint.
https://www.opticsplanet.com/trijicon-rmr-mount-for-acog-scopes-with-bosses.html
The Trijicon ACOG 4x32mm Rifle Scope will work for your application.
Trijicon does not state why the weights vary between models of the Trijicon ACOG TA31 4x32mm Rifle Scope. They are made from the same 7075-T6 Aluminum. I recommend contacting Trijicon for more information regarding the weight differences.
The Trijicon ACOG TA31 4x32mm Rifle Scopes will be shipped in the same packaging as buying from Trijicon.
The Trijicon ACOG 4x32 Red Dot Scope w/ Center Amber Crosshair, .308 Ballistic Reticle model can be found at the link below.
https://www.opticsplanet.com/trijicon-acog-4x32-red-dot-scope-w-center-amber-crosshair-308-bal.html
The Trijicon ACOG TA31 4x32mm Rifle Scope reticle will work for either 55 grain M193 ammo or 62 grain M855. Trijicon states the following regarding the reticle and different ammo. "The real issue is not the bullet weight or barrel length, but muzzle velocity, ballistic coefficient, and the distance from height above bore. The difference in trajectory from bullet to bullet or barrel to barrel is usually less than the MOA thickness of the stadia lines."