Game's science system ensures that you can "permanently" (until first team wipe) improve your firepower. However, how effective it actually is?

Base damage of various ammo levels is known from update notes, and is integer. And a beast was confirmed to have 15 hp. As such, you can count amount of shots it takes to kill a single beast.
- Yellow (light) ammo deals 2 to 3 damage, which kills a beast in 5 to 8 shots, usually 6
- Red (medium) ammo deals 2 to 4 damage, killing in 4 to 8 shots, on average in 5-6 (high values spread)
- Blue (heavy) ammo deals 3 to 5 damage, killing in 3 to 5 shots, usually in 4
- Green (ultra) ammo deals 4 to 7 damage, killing in 3 to 4 shots, usually in 3

All weapons only differ in amount of shots they fire at once as well as reload time/firerate - damage of a single bullet is the same. Exception is razor which deals reduced damage (more on that later). And any bullet regardless of its energy will only damage you once.

Additionally, bullets are able to ricochet from surfaces. This is not directly based on their damage (they must hit at certain angle, plus win a coin toss). However, each ricochet reduces remaining bullet damage by one. This means that stronger ammo types may results in more max ricochets (however unlikely it is) so you must pay attention to not hit allies - however, bullets that do ricochet hit beasts for more damage, and a powerful enough bullet can ricochet after overpenetration (see below). Meanwhile Razor ricochets are less likely at yellow/green ammo, as 1-damage bullet never ricochets.

Overpenetration is also a thing - especially notable when you hit something behind a killed astro-creep (which has only 1 hp). Any bullet that kills a target does not instantly disappear, instead merely losing as much damage as target has. So if beast had 2 hp remaining and your bullet had 5 energy, bullet will continue flying forward albert with just 3 energy remaining. Beasts lined up in a tight corridor are susceptible to this.

As a drawback, all weapons except shotgun lose firerate on higher end ammo, the higher bullet damage is, the slower you will be firing them. Lucklily fire intervals are also known: 1x/1.2x/1.4x/1.6x for yellow/red/blue/green ammo.
This means that your DPS stays about the same when upgrading to red ammo, but you can conserve ammo better and be more free to applying more of it. And going up to blue and up to green both have the same effect and actually increase DPS (to 1.143x and 1.375x conversely). Great DPS as well as ability to spray more bullets towards aliens mean you will be much better at killing. Unless astro-creeps are enabled, of course, since they always take at least one shot to hit (and due to them being in the air, overpenetrating bullet likely won't not hit anything behind them) - and even then effect against beasts does not diminish.

The bread and butter of a marine, rifle is your default weapon, a weapon that was in the game from the start and that all other weapons were based upon. It is what the game was made in mind with, and as such shows decent performance in all situations without showing unique traits or excelling in some situation. Jack of all stats, they call it. This weapon was, in fact, the reason you are now able to equip two weapons - back when you could only carry one, 98% of the time Rifle was chosen above anything else.

This is a weapon that has no weaknesses, and picking is a decent choice at any ammo level, game settings etc. Picking any other weapons over it is a matter of whether you want to have some specific advantage. If you are new, feel free to pick rifle+anything and experiment.

Stat wise, declared fire interval for a rifle should be 0.075f. (13.33 shots per second). However, rifle was depleting 500 (yellow) ammo in ~41 second (plus-minus 1 second) during a test. So it's real reload is about 0.08s, or 12.5 shots per second. This is listed here for comparison.

This is a secondary weapon you have selected by default, and it differs the most from the rest. It's usage depends a lot on your ammo power.

Shotgun fires random spread of 8 standard bullets at once, using 8 ammo and going on reload that is, on yellow ammo, lasts 8 times as long as one of Rifle. What's unique is that on stronger ammo levels, shotgun reload time is not increased (as it is long enough already). As such, shotgun scales much better with your ammo power than other weapons. (DPS wise only, obviously - you will still be using 8 ammo to kill a single beast, unless you happen to hit two with one shotgun blast or if you fire into a large horde where almost all bullets hit).

At yellow/red ammo, shotgun is strong enough to guarantee beast kill if all pellets hit, and often kills with just 6-7 (better to not hope for that and aim to hit all pellets anyway - you don't want to require a second shot). This makes it decent at close range and not really useful otherwise. Still works as a reaction weapon (when you have one chance to get rid of enemy), but using rifle/razor might save you better.

At blue ammo picture changes, as hitting just 5 pellets guarantees a kill, which makes it a bit easier at close range, slightly increases this "close" range - and at this point DPS scaling raises shotgun well enough to be quite strong at horde clearing in close-medium to medium range as well. Especially in tight corridors where a bullet that hits a wall will likely ricochet further. This use is further improved by green ammo, where 4 pellets is enough to kill a beast and overpenetration happen frequently.
As such, at strong ammo shotgun can be picked to clear a corridor of beasts, or even a small room full or lurkers. At close range, one-shotting the beast will get rid of it's danger in a quickest way possible. In beast-only environment (without creeps) shotgun is perhaps the best clearing weapon at green ammo.

The weakness of a shotgun is astro-creeps. Those critters would take 8 ammo to shoot conventionally, and while you can just ignore it and time your shot so it removes a creep as well as sends 7 more bullets towards horde, it can be unreliable, especially with ping issues. It's recommended to switch to faster firing weapon against singular creeps/beasts, and equip shotgun when all them are coming from about same direction, letting you shoot creeps down without stopping clearing.
Additionally, capacity for ammo conservation with shotgun is quite limited - which means you may want to pay attention to your remaining ammo more than with other weapons.

Hammer 4x, or h4x for short, is not actually a hax. It is a specialized "sniper" weapon that is strong at picking beasts from afar, but can be lackluster at closer combat fight.

Hammer 4x sends 4 bullets at once with next to no spread, before going to reload for 4 times the rifle reload time. Weapons have recoil after each shot, and so Hammer's shooting pattern ensures it fires four bullets onto your target before recoil kicks in. If you follow pattern "shoot - compensate for recoil - repeat", h4x is potent at destroying enemies at long range.

Drawback is that at close range, you have exactly same DPS as a rifle, but will have to spend 4 or 8 bullets on any target (and reload for as long, too) before being able to fire at another one. As such, when multiple beasts come close and personal, you want to switch your weapon immediately. If there is just one beast or if they are in right corridor where overpenetration can easily happen, you still can keep firing, but at any rate you want close-range weapon to pair with h4x.

Ammo level is important to take note of. At yellow ammo it will take exactly 2 full shots to kill a beast. At red, sometimes it may kill in 1 but don't count on it. At blue, it is often in 2 but sometimes takes 1. Finally, at green ammo you will always one-shot a beast. It's really neat to see a lurker in your infra vision and remove it without it waking up! (that being said, if there is a second lurker behind it, 1 bullet likely will overpenetrate and wake that one up, but at long range you can kill it easily too).

A middle grounds between Rifle and Hammer 4x, Hammer 2x (or h2x) is decent at long range fights while not sucking completely at close range, it it still surpassed by h4x at the former and by rifle at the latter, however.

Hammer 2x works exactly same as h4x except it only sends 2 bullets and reloads for 2x rifle reload time. This reload time is, at least for me, not enough to readjust after every shot without losing much of firerate. This means that raw DPS at long range will be less than that of h4x. However, in many cases you will be able to actually spend less ammo killing a single beast, which somewhat evens this out against small number of targets (where overpenetration is not much of a thing)

At close range, h2x is... usable. You can generally reaim from first beast killed to another before it reaches you; astro-creeps also can be snap fired and killed relatively easily. However, it still is less usable than a rifle/razor. At least if you don't expect too many enemies, you are not forced to "switch weapon or die".

Shots wise, at yellow ammo beast will die from 3-4 shots, at red 2-4 (usually 3), at blue 2-3, at green 2 shots. So below green level, sniping with h2x can be more ammo-efficient than with h4x. At green, there is no difference, but the ability to also shoot down random astro-creeps may come in handy.

The survival tool, this weapon comes with substantial close-range usability - at the cost of your ammo supply and actual damage dealing capability.

Generally speaking, Razor works just like a Rifle, with two exceptions - it fires less damaging shots (no more than 3/4 of normal damage, but normal knockback on a beast hit), and it has reduced reload time (also 3/4 of normal reload time). This has several consequences.

First, reduced damage will result in a different number of shots to kill a beast. The way damage system works, first a bullet damage is selected (a random integer value in a range - this part is the same as with other weapons), then it is multiplied by Razor's multiplier (0.75x) and then rounded down. As such Razor loses out much of damage, each of it's bullets having 50-75% of a normal bullet's energy (average damage is reduced by 40%/33.3%/33.3%/31.9% for four ammo grades).
- Yellow ammo will deal 1 to 2 damage, which results in 8 to 15(!) shots to kill a beast depending on your luck - obviously, rolling 1 for 14 shots (so it takes 15th to kill) is, while technically possible, highly improbable. Usually it takes around 10-11 hits (values spread is high).
- Red: 1 to 3 damage, killing in 5 to 15 shots, on average in 8 (very high values spread).
- Blue: 2 to 3 damage (rolls: 2, 3, 3), killing in 5 to 8 shots, usually in 6. Slightly better than a normal yellow bullet.
- Green: 3 to 5 damage (rolls: 3, 3, 4, 5), killing in 3 to 5 shots, usually in 4-5. Slightly worse than a normal blue bullet.
As you can see, damage of a single bullet is weak, especially at weaker ammo. And even with increased firerate, DPS is lower than that of a rifle (80%/88.9%/88.9%/90.9%).

Second consequence is increased knockback. Bullet hit is still a bullet hit, and a beast affected will be thrown away from your fragile body. In a close quarters, razor is the weapon to go if you aim not on killing those beasts, but on keeping them at bay. This will not work if there is too many of them (there will gather too many to knock back all of them and/or you will run out of ammo), but as a mean of escaping a tight situation this can do fine. Additionally, more bullets fired mean you will hit more of them which helps when it's difficult to aim. This also helps at hitting those elusive astro-creeps.

Notably, at higher ammo grade knockback effect is reduced substantially (at blue it already fires slower than yellow rifle). But at least there you will be killing beasts instead of merely pushing them away, and you will likely have more max ammo reserves too, so it's still usable as a survival tool.

Now that all weapons have been discussed, time to select weapon pair to go into the vessel with.

Weapon combination selected by default. Shotgun will seldom be used at weaker ammo, leaving you with mainly the rifle - not the best what you can have.
Once you upgrade your ammo to blue, shotgun will be your main tool of destruction, and rifle will be used to pick off astro-creeps or snipe to afar, which is generally decent.

Decent weapon combination early on, use rifle in general and switch to razor when in danger. When you have upgraded, it kinda loses some of utility but is still usable.

Doesn't have much variety between weapons, as h2x can do mostly ok at close range and rifle can do almost as well at sniping (also, you likely won't be doing much sniping anyway at lower difficulty). Selecting this basically means you are ok at using either weapon, and only switching when it's convenient/you have time to. Comfy, but not much use for, compared to the next combination.

A decent combination, rifle for use everywhere except at long ranges. Works generally fine in most situations, not much to add.

With weak ammo, this has little use - both weapons are meant to be used at close range (shotgun because it can't oneshot otherwise, razor because it's meant to get you away from tight spot).
At blue ammo onward, this does get practicality: shotgun will be able to kill at better range and will be your main damage tool, while razor will kill astro creeps and can occasionally save you from being flanked (despite great DPS, shotgun still has big reload time so two beasts coming from opposite directions will be a problem).

Actually decent combination even at low ammo - shotgun can be your "reaction weapon" (instakill a beast you weren't expecting, as long as you don't miss) and h2x will be used to kill everything else. At high ammo, h2x will become your sniping and anti-creep weapon.

Shotgun kills from close range, hammer from long. Generally seems obvious, and excels over previous combination... as long as there are no astro-creeps. If there are creeps, this loadout suddenly loses many points of appeal, tho of course is still usable (even without some teammate picks creeps off you can just accept to spend 4 ammo on each or 8 on a several).

Close-range survival tool to escape tight spots, medium-long range weapon for killing. Decent combination early on, and works with stronger ammo too. Both weapons are usable against creeps, tho razor is obviously better.

Seems about same as before, but your long range weapon is weaker as short-medium range killer. Still can work, especially at green ammo where a single h4x shot always kills a beast. Two weapons cover each other's weaknesses, resulting in decent combination.

The only combination I would never recommend using. Both weapons fill the same niche of long range coverage. If you want to have long-range coverage, pick either of them plus any other weapon - this way you will have something to switch to for close combat.

That's it. Hopefully this guide will help you understand weapons in the game and select the loadout (or two, for different ammo grades) you like.