Guns are great! Space guns are even more awesome, right? So why does everyone seem to get so upset when you get some shiny new toys & go try them out.. & why are you so bad at using them?
For the first part of this text I'll explain the details of how to use guns while sitting in the pilot's chair (& other weps as appropriate), then I'll get onto differences & optimisation. For ship tactics (eg rolling while attacking a harder target, or fleet combat) you'll have to look at another page: this page is purely for personal combat weapon use
Part 1: aiming the weapon. If you can't aim the wep effectively, whether vs a lockbox's stationary lock or a live target, you won't get a hit & therefore the dmg required, & dmg is almost always the key to winning encounters. So first take a look at your HUD insert pic here
In the centre of your HUD there is your 'boresight', which never moves & is aligned with the longitudal (main thrust) axis of your ship, no matter which ship you're piloting
If you target an object you'll see a target lead cursor (a big fattened O) appear on your screen: this is what you have to fire at (/not/ the target itself!) insert pic here if you want to guarantee hits. It takes into account your relative speed & maneuvers vs the target's, so a powered flying target will be quite a bit more difficult to hit at range than an unpowered static rock, esp if you're flying around using fancy dogfight maneuvers too
The third thing you should pay attention to is those tiny white/blue dots that appear to move around at random insert pic here. These dots indicate where your main guns are currently pointing, so if they're outside that lead target reticule they're unlikely to hit. The reason they're moving around like a drunkard's eyeballs is that they indicate your (fixed mount, AKA non-turreted) primary weapon system's gimbals: all weps now come with a gimbal system that allows for a v limited (IIRC a 15deg cone) amount of movement for a certain amount of target tracking; this can be both a blessing & a curse (I'll come back to that in a bit). While the gimbals allow for a certain amount of shots to land on target, even if imprecisely aimed, they're very crude & can have their own drawbacks.. plus they take time to align depending upon the wep's 'heaviness rating': see below
Part 2: Firing the weapon. Obvious, right? You just click & hold until the target is destroyed. Umm.. not quite 
There are 2-3 parts related to firing a wep efficiently to destroy the target in as quick a time as possible (there are other details too, but I'l get to them)
First you need to hit the target (obviously!); Second you need to damage the target enough that it's no longer immediately a threat (it may temporarily run away), quickly enough that any potential backup it may call for isn't immediately a problem, & Third you need to destroy/disable the target completely (or at least damage it enough that mission objectives are met, if relevant). While these may sound like simple steps, their suitable application is crucial in successful combat (you want to come out the winner, right?)
OK, so you fit your ship with the heaviest most dmg-dealing guns you can. Always a winning tactic, right? Err.. no.
First: Most weps in X games have 'bullet (I prefer to call it 'shot') speed' inversely proportional to how powerful they are (the wep's 'heaviness rating'), so the more powerful (ie dmg dealt on hit) they are, the slower their shot speed & alignment time. Therefore plasma cannons all around are highly unlikely to get more than a single hit on a fast fighter or similar before the gnat pummels you with its faster-firing weps, if it survives the initial blast. Usually resulting in 'game over'. That being said, stock beam weps are effectively insta-hit (given the targeting mechanics detailed above), but are so low-powered that their dmg is often next to useless as they'll take forever to disable even a single scout before its shields recharge & it comes back again
Second: heavier weps require more time to recharge their internal batteries/dissiapate heat than lighter ones, so they'll be /much, much/ slower to fire more than one charge at a time. Again, vs speedy opponents, this is the worst choice for you
Second-and-a-half: turrets (if you have any) do really really badly if you fit them with heavy weps unless you're engaging capships/stations from range. Don't /ever/ rely on such turrets to deal with anything more maneuverable, even a heavily-laden freighter: if you only have 90% misses with such turrets I'll be impressed! Use turrets for anti-missile & anti-fighter/drone ship defence primarily, so equip them with the longest-range fastest-firing weps you have access to. While flak turrets have an AoE effect, their RoF is poor & their range more or less equals other weps, so I've ignored them for now; perhaps someone else has made better use of them?
Third: the killing blow.. or is it? S/M class ships have a chance to bail depending upon how outfought they've been (along with a lot of other factors); L/XL ships will never bail: they must be boarded to take control. If you do a lot of dmg with high-power weps quickly enough, or plink away at their hull for long enough, you may decrease a S/M ship's bail threshold enough that the pilot jumps out & leaves you with an empty, partly damaged ship that you may wish to pick up later. If this happens, don't worry about others grabbing your prize first: it'll remain in place forever, ignored by other hostiles unless/until some random missile left over from a local firefight happens to destroy it (or was already targeting that ship when the pilot bailed)
Once the vicinity is clear of any local hostiles you can EVA over to that ship & claim it for yourself quite simply (covered elsewhere insert link here), then fly it back to any friendly station as temp storage until you decide what to do with it.. or you can designate a marine to do the same, but automatically (& likely causing more damage in the process). Don't forget to check its local consumables storage to see if there's anything you may wish to transfer to your main ship before forgetting about it or selling it off tho.. & remember that an intact hull is worth a lot more if selling for scrap than a damaged one, so get your EVA repair laser out for use!
Part 3:Hitting the target (prescision shooting).This is not as obvious as it may first appear, even (perhaps even especially) with a static target such as a lockbox's lock
Remember what I said about main wep gimbals above? This is where they can become a curse! If you're too close to a targeted object the gimbals (& therefore your main weps' aiming points) are likely to go haywire, esp if those weps are mounted far apart on your ship's hull/outriggers, due to them not being able to converge on the target's Centre-of-Mass (CoM) within their firing arc. You won't be able to get them to settle down for long enough to get that accurate shot you wanted, esp in a M-class ship. Solution: back away a little until they steady themselves (if in an M-class this may result in you being too far away to see the locks clearly, in which case call in a suitable S-class ship or simply use the EVA suit's laser)
When attempting to open a lockbox using ship weps (it's perfectly possible with an EVA suit's hand laser, altho more time consuming & potentially dangerous), select a single one of the lightest wep you carry & use middle mouse button to fire at where your cursor's indicating (practice by shooting a single shot into empty space, which also ensures the gimbal is unlocked from the boresight). Mining lasers are good for this, but almost anything that does as little dmg as possible is suitable. Once again, back away until the gimbal steadies, then shoot at the lock
Chances are you hit the box rather than the lock itself on this try. There are 2 parts to this: firstly if you're desperate for the loot that box may contain (eg mission cargo), you can EVA & use the suit's repair laser to fix the box's hull again; & secondly you probably had the box itself targeted (you can't target individual locks), which automatically directs any ship gun firing (not the suit's, fortunately) to focus on the target's CoM. You don't want this: you only want the lock(s) destroyed, so ensure you have the box unselected as a target before you try precise shooting again. It may take a little practice & some retries, but lockboxes' contents can be quite lucrative early-midgame (altho often contain expensive illegal items), so worthwhile practicing with.. also, see below
If you're in a situation where you find yourself taking on a capship in your fighter (it can happen!), it's not always an impossible dilemma. While you're highly unlikely to achieve a successful boarding op, disablement/destruction is usually on the cards. As always, find a blind spot, take out engines ASAP to deny it maneuverability, then start paying attn to its turrets. The latter is the second reason why those auto-correcting gimbals can be a curse: as soon as you select a turret as a target they're likely to go haywire again (as their target's CoM is too close, & you can't back off), so you'll have to be a bit more clever. Deselect the turret so your gimbals steady down (while remembering where it is ofc), then maneuver yourself so you're still within its blind spot but at least some of your guns can bear. Most of your shots are likely to hit the main shield/hull, but enough should get thru to damage/destroy that turret: watch for the bright white outline that indictates subsystem hits (remember you only need to take it down to <25% of its HP for it to be disabled enough that it won't shoot), giving you another spot to work forward to to gain an eyeball on the next turret emplacement. Continue slowly like this until all fangs are pulled, then you can concentrate on ship shields/hull or simply GTFO as you wish. Don't be surprised if this type of battle takes you 1/2hr or longer realtime (excluding reloads if you play that way): it's a slow manual process & to be expected while working lonesome!
As always, this is a WIP & all helpful comments are welcomed! - Dom