Universal Ammunition essentially replaces all forms of weapon-specific ammunition that was present in previous installations of Dead Space. This was done to accommodate the introduction of the Total Construction Bench, since the large number of possible weapon permutations makes weapon-specific ammunition impractical to keep in the game. Dead Space 2 Multiplayer also uses Universal Ammo.
It is unknown how Universal Ammunition works in-universe, or whether it even exists in-universe. It is entirely possible that it is merely a concession to gameplay, and that canonically, different devices continue to use different ammunition.
While it may seem efficient due to the fact that there is no longer a need to hunt for specific types of ammunition, it comes with the cost of making your ammo shared throughout all guns. Therefore, running out of ammo for one weapon means running out of ammo for all of your weapons. To counter this, ammo is easier to find than in previous games.
Universal Ammo is stored in your inventory in stacks of 20 units (or clips) each.
Universal Ammunition works on a magazine size basis. In other words, the main reason why more powerful weapons tend to use up more ammo is because their magazine size is much smaller. Because of this, increasing the magazine size of any weapon essentially increases the number of rounds/blasts/swings that the weapon can fire/blast/swing before running out of ammunition, assuming you reload only when your magazine is empty.
How many units of Universal Ammo each reload consumes is dependent on how much of your magazine is spent. It seems that the game splits your magazine into 4 sections of an equal 25%, and every section you use up to fire your weaponry equals one unit of Universal Ammo. The upper and lower weapon on a weapon platform consume ammo separately (Although they reload at the same time) . Since they can both use up to a maximum of 4 units of ammo, if all of the ammo in both the upper and lower weapon are used, reloading will consume 8 units of ammo, if the ammo in both is half used it will consume 4 units of ammo etc. This is disputed, however, and the exact ammo consumption mechanism is unknown.
Because of the way Universal Ammo works, reloading will sometimes consume more ammo than you may think it does. Shooting one round from a military engine with a standard tip and then reloading will consume one unit of Universal Ammo, regardless of how large your magazine size is. It is better to rely on the number displayed above and below your weapon when switching weapons to determine how many rounds you actually have left.