Place this line into favoriteAnims.xml in your menyooStuff folder.

	<Anim dict="anim@shooter_stance" name="stance_clip" />


Install using Jennie's Custom Anims mod. Get it here:
https://www.gta5-mods.com/misc/custom-animations-add-on-customanims

Make sure you follow their guidelines on how to install that mod properly and how to install animations using it.

Disclaimer: Will take a little finess to get the rifle fitted appropriately, the hands are tightly fit to AK platforms.
I don't know if ARs will work as nicely, but you can try it out.
It should be alright.