Just to set things straight, there are many variants of Assault Rifles, and the UN uses them all depending on the availability of resupply at their location, and the compatibility with the TSFs.
Also, while the Japanese Type-87 did have a variant for precision shooting, by removing the 120mm cannon and including a longer barrel and scope, it is not a true sniper cannon. There have been
no details released about other countries having a precision or marksman variant of their assault cannon, including the US AMWS-21. They have sufficient accuracy and range for the purpose, and can be fired with precision in semi-auto mode as per what Stella did in the show. There is the Squad Support Gun used in the European Union, but its more like an MG or large caliber cannon.
This is an sniper cannon...the 1200mm Over the Horizon Cannon:
http://images1.wikia.nocookie.net/__...1/Tama_pow.png