Jungle Carbine vs Tromboncino... a difficult choice!
The Tromboncino is my most used weapon, even competing with tallied SMG-time. It's just so much fun, difficult to use, and rewarding. The added functionality of the grenade is good for variation, though I have a handful of cases where it's been very useful (blowing up a tank, finishing off an enemy behind cover etc). My specializations are RRRL, the machined bolt is mandatory and the extra movement speed is useful in close quarters. Using the 2X, which IMO is ideal, the benefit of quick aim is negligible. You should always be engaging within the one hit headshot range.
Enter the Jungle Carbine... quick aim again is pretty much irrelevant, which opens up for high velocity bullets (500 -> 570m/s, compared to Tromboncino's 500). Both could in theory net you a kill but we are talking a few frames difference between scope time and bullet travel time, still, you'll be shooting more often than scoping (since it doesn't unscope between shots), so the benefit of velocity bullets is there more often.
Then the Jungle Carbine gets tricky. As with the Tromboncino, faster movement when aiming is a big deal. However, machined bolt will aid you much, much more than the extra movement. I've had situations where there are five heads present and I've knocked all of them, by that point the faster bolt adds up to a lot of time gained. Having 10 shots is insane, that's 5-10 kills from one reload, compared to the Tromboncino's 3-6 (or the Commando with 4-8/5-11). Difference in RoF between the two though is too small to matter at 95 vs 100 (Tromboncino), netting the Jungle Carbine a much higher lethality potential. Thus, I forgo the movement speed in favor of machined bolt.
Since I go with machined bolt I also get slings and swivels, not a big deal, but nice I suppose, especially with my next point.
This is the kicker... Quick reload vs. hip-fire. Quick reload is nice when going from 0 -> 10 bullets, the Tromboncino has a big advantage otherwise that could bring up its lethality potential compared to the Jungle Carbine. However the Jungle uses two stripper clips. This means if you abort the reload after loading one clip (switch weapons), you get back very quickly with 5 bullets, essentially giving you a fast tactical reload (slings and swivels plays a part here). Even with 5 bullets the Jungle is very close to the Tromboncino in kill potential. It is quick enough to make the specialization of quicker reloads less valuable, when needed just do a tactical 5 bullet reload, and try to reload whenever possible to keep it topped off (single bullets above five, stripper clip at five).
Once you get a grip of the reload, you can enhance your grip further and instead go for hipfire! This is a lot of fun to have. When standing the hipfire bloom is a bit too big to be reliable for headshots. However, when you crouch it is comparable to a standing revolver in hipfire accuracy. That is good enough to actually headshot someone in close quarters as we know from using the revolver (though at half the RoF). It's great to have for finishing people off in close quarters, keep moving, crouch and fire, and the kill is yours. It's a very fun tool to have, especially since we have machined bolt and 10 bullets already. Being able to use hipfire in very close quarters compensates further for skipping quick scope and faster ADS movement.
Hip-fire with enchanced grip is identical between all the carbines, though the Commando gets stuck with 8 bullets (with a higher RoF, 109), and the Trombone misses out on faster movement in ADS. You don't want hipfire to be your primary aim method for any weapon (apart from shotguns), so it's situational and should be at a minimum cost. The Jungle Carbine is arguably most viable for hipfire since both other weapons lose something that's always valuable (more bullets and movement speed) compared to quicker reloads as explained above.
That leaves one of the biggest benefits of the Jungle Carbine is a whopping 30 meter extra one hit headshot range, at 100m. Obviously this can make all the difference in your engagements. Up to this point the Jungle Carbine has all the effectiveness of the Tromboncino and then some, pretty much as viable if not more in close combat with the addition of an extended range. The commando is also inferior in most ways from 50 meters and out, which is obviously quite short in comparison and not worth the minor RoF advantage in close IMO (as you'll be losing a lot of those anyway).
So with that I think I've convinced myself that the Jungle Carbine is just straight up better and potentially more fun than my beloved Trombone, and I will be speccing LLLR. I will miss the grenade though. It is nice as a medic to have something to blow up fortifications blocking your path, and I do feel that loss when using the Jungle. But the rule of frequency of use edges the grenade out considerably when taking into account the primary fire performance of the Jungle.
The king is dead, long live the king!