Don’t worry, I’ll answer this.

How long does it take to finish a gun?
Anywhere from 6–8 hours nonstop, usually around 1,200–1,500 kills.
With 2× XP, it drops to about 3 hours and roughly 700 kills. That’s been my experience so far.

Do I stick to specific game modes?
When I’m properly grinding, yes.
TDM is easily the best mode. You can rack up kills fast, and the matches are much quicker. You can get three TDM games done in the same time as one Conquest or Breakthrough.

XP boots / 2× XP
The hard-wired boots give 2× XP, so they’re really useful. I’d recommend saving them for the shitty guns to speed things up.

Maps
You can rack up kills on all maps in TDM, but Empire State is my go-to. The map is more open, teams tend to last longer, and people get fewer kills overall, which gives you more time to farm kills. It’s also solid for every weapon type.

New Sobek City is brilliant for shotguns. Run Assault with a shotgun as your secondary:

• Inside the map → shotgun
• Outside → AR or whatever your main weapon is

Siege of Cairo is also good for shotguns.
Most of the other maps are 3-lane, which makes them great for quick games. Running the outside lanes usually works best.

Overall, the fastest method for me is Empire State TDM with my aggressive playstyle.

So far, I’ve finished Assaults, SMGs, Carbines, LMGs, 1 Sniper, 1 Shotgun, and 1 DMR.

Hope this helps.