A shooting range or private/single player server where you can get a feel for the different guns and different classes. Could be set up like a shooting range and would allow you to enable commands like Bullets path/trajectory. Targets could be set at various ranges and either be classic targets or stationary bots this would allow people to fully get used to the rifle, musket and flintlock.
I feel there's no real way to practice with the weapons without being in an actual server, even when experimenting by shooting at trees it's hard to understand the trajectory and the RNG based bullet deviation. This practice mode could be released with the Crosshair/reticle update all in one. I think that most people who get this game new struggle with adjusting to the aiming system and get frustrated because of the random bullet paths whereas people who have had the 1000 hour experience tend to stomp all over the new players. The RNG of the musket can be really frustrating at a time, wondering whether the bullet is going to go left, right or travel in straight line, I think adjustments to the aiming needs to be a thing, like increased stability whilst crouching, or standing stationary. But whilst moving aim becomes incredibly inaccurate.