I use the proto RR with prof 3. It's crap in CQC, to the point that I've gone back to the AR despite the RR dominance at range.

Let's break it down. To start with, the theoretical DPS is virtually identical to the AR, ACR, AScR, and much lower than the ScR and CR. Don't have the numbers to hand but it's easy to work out, just damage*RPM/60 (CR is slightly more complicated due to burst interval having to be factored in).

Then there's the 0.25 second charge up. So in the first second of firing RR is only getting 75% of the (already mediocre) theoretical damage and the weapon is outclassed by everything.

But wait, there's more. You know how, with a normal gun, if you lose tracking for a fraction of a second (e.g. target went round corner or moved past you), you just stop firing and resume when you have tracking back? Well doing that with RR reduces your DPS to comically low levels. For instance, half a second of fire, 0.25 seconds to acquire target, half a second of fire, 0.25 seconds to acquire target, half a second of fire gives you half as much DPS as AR/ACR/AScR with the same fire pattern, as the rail is losing half of its firing time to charge up. Losing tracking, even more than once, is not at all unusual in CQC and puts the RR user at a massive disadvantage.

People say you can negate the charge up by pre-firing. That can help a little, but it's very situational. It's useless if (1) you've not seen the target before you've shot them, or (2) you've seen them but you're not sure that they're coming into your line of fire, or (3) you've seen them but you're running just before you want to shoot them (e.g. to round a corner). To be honest, the main situation in which pre-firing actually works is the situation in which it's not really needed, which is when you're tracking the target before they've even seen you and you're bound to get the kill regardless.

TLDR Even in optimal circumstances, of pre-firing and sustaining full auto fire (no pauses allowed), the RR is still only getting the same DPS as the AR/ACR/AScR and much less than CR and ScR, which don't need to jump through these hoops to achieve their theoretical DPS. In practice a RR rarely achieves its theoretical DPS and is outclassed by AR/ACR/AScR/CR/ScR in CQC.

Everything I said assumed that every shot hit the target. Try reading my post again.

But wait, there's more. You know how, with a normal gun, if you lose tracking for a fraction of a second (e.g. target went round corner or moved past you), you just stop firing and resume when you have tracking back? Well doing that with RR reduces your DPS to comically low levels.