Couldn't you make the double tap work for Flatline and 301? Two shots, so same damage, same fire rate as Anvil?
Double tap doesn't really decrease TTK, what it does is increase "burstiness", or how much damage you can do in a short window. This has advantages in situations like peek battles, or other situations where once you start hitting someone they're gonna go into cover. The downside is that the range is a bit reduced, since the recoil makes the second shot go above the first (for most players). For something like the R301, it having 2 bullet burst isn't really that meaningful -- 2 bullets of r301 isn't enough damage for the "bursty" benefit, and the R301 already has the recoil range limitation naturally because it's full auto. The Anvil sort of already gives the R301 the bursty quality. We could do like a combination of anvil + double tap, where you get a double tap of the anvil thing, but it still changes the r301 in basically the same way as the anvil compared to the base r301, so it'd mostly just be flavor (and it'd take audio work and be too similar to double tap imo).