I use the Grendel (and scar and FY) in semi-auto fire very frequently. I imagine it kinda causes a lot of WTF moments for other players because they sound significantly different. I think I'm the ONLY person I've ever seen in matchmaking using the semi-auto fire modes. Hell, with the limited player population I might have been the one shooting at you
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The The Grendel and scar have very fast semi-auto rates of fire. Both are only slightly slower than the scar on Full auto. Sometimes I think the FY actually has a faster ROF in single than auto, It's very close.
I have had a lot of success lately, particularly with the Grendel. With the mild semi-auto damage bump, and less damage drop with distance, low recoil and almost no spread... It's a beast.
The scar is exceptionally precise at range in S-A fire, and it feels like it has an almost pistol-like hip-fire as well.
If I recall, the Grendel doesn't explicitly state "semi-auto" in the customization menus or kill-cams like the other weapons do. However if there is nothing listed for the secondary fire-mode the weapon is equipped with semi-auto fire.
*edit
Nothing listed for the 'under barrel' accessory group I mean. Extended magazines eliminate semi-auto. It can be used with barrel or sight accessories.