Hmmm... running low on ammo in that rifle I see. Running a tad low in that clip, MG almost dry? Well at some point you will have to reload. My background comes from CS like games, where reloading is the difference between life and death, the slower reloads in this game amplify that.
The German rifle has an interesting feature due to the stripper clip, that is if you expend your entire five round clip you reload all five in the same time that it takes to reload ~1.5 bullets from a partially spent clip. What this means is that it would seem better to shoot off your entire clip before reloading, however this is not always the case. If you have enough ammo you can also decide to intentionally waste a shot on the groud or air if seems neccessary.
In the case of the Berhier rifle and carbine you can only reload when it is empty, so it is sometimes smart to shoot of 1 or 2 shots rather than waiting for enemies.
My suggestion on when to reload the rifle is simple: When you start to charge, have a full clip. When you rush their trench have a full clip.
The reason why you want a full clip when you first enter noman's land is the often times you will have the opportunity to fire across noman's land at counter sniping enemies. You will also potentially need bullets in order snipe MG nests.
You also really want all rounds when you are in their trenches. Due to the long reload times no bullets = death, with the possible exception of using the bayonet. You can interrupt the reloading by sprinting or meleeing.
MG's have a pretty fast reload (3-4 seconds) so if your allies are not rushing and you don't see any imminent threats I would suggest reloading if you have less than 2/3rds of your rounds. If you have above that there really isn't much of a point and it is too risky to reload. You should never Reload when your team is rushing, you should be holding down that LMB instead. I would also suggest reloading before you move up after your guys have taken the first trench line.
The pistol's reload is the fastest in the game by far. Keep in mid that the pistol takes up to 3 bullets on target to kill. Factor in that the pistol is also pretty inaccurate and you will probably miss at least one shot. I recommend reloading the pistol whenever you can. If you shoot once and are safe, reload.
When to never reload ever
1) when you are under fire. Sprint away first.
2) When you have a enemy in your sights. Even if you have a pistol with 1 bullet take the pot shot, you might just assist that rifleman.
3) When you see an enemy charging. Line up and shoot him. If you don't have bullets run back or duck and reload.
Most of the time this happens when you rush the enemy's trenches and your buddy got shot in the charge. Here you are with your pistol or rifle up against another dude in a straight up 1v1. How do you win?
Well there is no sure fire way to win a 1v1, especially against a rifleman or grenadier because of the one shot kill potential. If your opponent is a machine with perfect reaction time and accuracy you will lose every time, but there are things you can do to make his job harder.
Try to strafe left to right, then right to left. Jump up and down, break your bolt throws with a sprint to throw him off. Being a jiggly legs NCO is your goal. They will have a hard time shooting you if you are all over their screen. Try to get behind them and shoot them in the back. I have found very few players who can overcome someone who has jumped behind them.
Also, don't bother with iron sights, just don't. Iron sights are great fro those long ranges shots, but in a trench, it just makes it hard to track your enemy.