The nice assortment of weapons in Crysis gives you the appropriate options to deal with specific situations. Matching your weapon choice to the situation will not only help you to survive but will also save ammunition.
Weapons are divided into three distinct categories: short range weapons, assault weapons and explosives. Only two of each type can be carried although this only comes to bear when considering which assault weapons to equip as this is the only weapon category that offers more than two types.
Short Range Weapons
These will stay with you throughout the game (obviously) and are the alternate weapon to the pistol (press 1 on your keyboard to toggle between pistols and fists). Even without Strength mode engaged, fists can be used to bludgeon enemies to death although it will take several hits to do so. With Strength mode engaged, KPA soldiers sent to the sandman with a single devastating punch.
Alternatively, you can sneak up close to enemies using Cloak mode, then use your fists to grab them. Then carry them off to a secure place. Then switch to Strength mode and punch them out with one hit.
These will also stay with you throughout the game as there are only two short range weapon types so you won't be swapping out your pistol for something else. What you will be doing is eventually picking up another pistol so that you can wield one in each hand effectively doubling your firepower.
A good pistol strategy is to use them in close combat when facing only one or two enemies at a time to conserve machine gun ammo.
This is your default assault weapon. More powerful than the Korean FY-71, the SCAR can sport a wide range of attachments and has a larger magazine capacity (40 rounds). However finding enough ammunition to make the SCAR worthwhile using up an assault weapon slot for is problematic. A good SCAR strategy is to simply use up all its ammunition in the first half of the Contact level of the game and then switch to the FY-71 (which you will acquire very early in the level as your second assault weapon). Then once you come upon the command post on the eastern peninsula (one of your secondary objectives in the Contact level), you'll encounter KPA soldiers carrying shotguns. Kill one of these soldiers and then swap out your SCAR for the shotgun.
This Korean machine gun is very useful because lots of ammunition for it can be found throughout the game and hence it will probably become your assault weapon of choice. Make sure you search around all your kills as FY-71s will be one of the most commonly dropped weapons that can be picked up to acquire rounds (or actually acquire the weapon itself if you don't have it yet).
The FY-71 is not as powerful as the SCAR and it also has a smaller magazine capacity (30 rounds) so once you equip it, you're going to have to be a better shot and use more controlled bursts to avoid being forced to reload your weapon at a crucial point in a firefight. If you're out in the bush and you can't find the weapon dropped by a felled KPA soldier, equip your binoculars and use that to search the ground. The weapon drop will be highlighted in blue through the binoculars. (This feature is not available in Hard and Delta difficulty modes.)
The shotgun will be your close quarters weapon of choice since it can usually deliver an instant one-shot kill at close range. With this baby equipped, you'll have no need to try for head shots anymore. Just aim in the general direction of bad guy, pull the trigger and he fall down and go boom.
A good strategy for shotgun usage is to designate it as your second assault weapon of choice whenever you're headed into a lot of close range combat, for example, inside an enemy base. Then swap out the shotgun for the precision rifle once you head back out into the field or the bush. Shotguns can be found in use by KPA soldiers on the Contact level in the command post on the eastern peninsula (one of your secondary objectives).
Another in the assault weapon category, the submachine gun is found around the third or fourth level of the game. It has a larger magazine capacity (50 rounds) and a higher rate of fire than the FY-71 although the latter of those two features will also have you eating up ammo faster. Another feature of the submachine gun is an infrared laser point attachment which makes for better aiming. However be advised that equipping the laser point attachment might give away your position to enemy forces (much the same way equipping the flashlight will).
The submachine gun can be used simply as an alternate machine gun when FY-71s weapon drops and ammo caches are scarce. If you notice the KPA soldiers are dropping a lot of submachine guns instead of FY-71s, it might be a good idea to temporarily swap out your FY-71 for the submachine gun.
The precision rifle is usually found in guard towers and is extremely useful because its sniper scope is more powerful that the simple assault scope. With this equipped and Cloak mode engaged, you can wreak havoc on enemy bases by hiding out in the perimeter and picking off KPA soldiers one-by-one. Since ammo is scarce for the Precision rifle, the best strategy is to use it to take down the enemy's heaviest firepower, e.g., guard tower sentries, mounted gun positions, etc.
This high-powered electromagnetic rifle is slow loading but worth the wait as it can usually deliver a one-shot kill to a KPA soldier even when shooting at the body. However, ammo is scarce and the round carrying capacity for the Gauss rifle is only 20. What you'll want to do is save your Gauss rifle for the aliens as a well aimed shot can usually bring down aliens with one shot as well. This weapon becomes especially lethal when you equip it with an assault or sniper scope. Absolutely a must-have for the end level of the game.
A good Gauss rifle strategy is to acquire it and then save ammunition for special circumstances. Then if you start running into more Gauss ammo than you can carry, use some of it up on 'regular' enemy kills.
First acquired on the Awakening level, this mow-em-down Gatling gun takes a while to warm up but after it does, essentially anything in front of you is hamburger. Use it when you find it but definitely do not consider carrying one around permanently as ammo is scarce. Also very useful against vehicles and is a good alternative to use against enemy helicopters if you don't have a missile launcher handy.