@parados,
Rate of fire is the frequency at which a specific weapon can fire or launch its projectiles. It is usually measured in rounds per minute (RPM or round/min), or rounds per second (RPS or round/s).
Several different measurements are used. The fastest and most commonly quoted rate is the cyclical rate of fire. However, heat (possibly leading to weapon failure) and exhaustion of ammunition mean most automatic weapons are unlikely to sustain their cyclic rate of fire for a full minute. That is why lower rates apply in practice and it is incorrect to describe RPM as "the number of rounds a weapon can fire in one minute."
https://en.wikipedia.org/wiki/Rate_of_fire
A 30 round magazine would not make a gun illegal, the gun would be legal but the magazine would be illegal. Did you know that they list guns by their rate of fire regardless of the magazine that would be inserted into the gun?
As an example:
Technically, the AR-15 fires 800 rounds a minute (source: Wikipedia), which is 13.3 rounds per second. This is for the full-auto version. We know that fanning your index finger is going to be much slower than that, so saying 180 rounds per minute is actually a very, very reasonable number. Except that you'd never get there, because you can only fire 30 (or 10 in Colorado) rounds from a standard magazine. (Colorado is actually 15 rounds)
Now we know 800 rpm is not really possible because even the military only uses 30 round mags and not a continues feed to the chamber.
M249:
Rate of fire Sustained rate of fire: 100 RPM
Rapid rate of fire: 200 RPM
Cyclic rate of fire: 800 RPM
The same could be said for the M249 since it only uses a can that holds 200 rounds.
Here is some info for you on the different meanings:
There are diverse measurements of rate of fire:
Cyclic rate[edit]
This is the mechanical rate of fire, or how fast the weapon "cycles" (loads, locks, fires, unlocks, ejects). Measurement of the cyclic rate assumes that the weapon is being operated as fast as possible and does not consider operator tasks (magazine changes, aiming, etc.). When the trigger is pulled, the rate at which rounds are fired is the cyclic rate. Typical cyclic rates of fire are 600–900 RPM for assault rifles, 1,000-1,100 RPM in some cases, 900-1,200 RPM for submachine guns and machine pistols, and 600-1,200 RPM for machine guns. M134 Miniguns mounted on attack helicopters and other combat vehicles can achieve rates of fire of over 100 rounds per second (6,000 RPM).
Sustained or Effective rate[edit]
This is the rate at which the weapon could reasonably be fired indefinitely without failing. In contrast to the cyclic rate, the sustained rate is the actual rate at which the weapon would typically be fired in combat. Sustained rate considers several factors, time spent reloading, aiming, changing barrels if necessary, and allowing for some cooling. Knowing the sustained rate of fire is useful to know for logistics and supply purposes. Machine guns are typically fired in short bursts rather than in long continuous streams of fire, although there are times when they must be fired in very long bursts (see rapid rate below). Sustained rate also applies to box magazine fed assault rifles and semi-automatic rifles. In these weapons it refers to the rate at which the typical rifleman can effectively engage targets in a combat situation. The rate is usually 12-15 rpm, except for barrel changes it considers most of the same factors as for the belt fed MGs.
Rapid rate[edit]
Rapid rate is a rate of fire between cyclic and sustained. It is usually much faster, although less accurate, than the sustained rate and is only used in emergency/final defensive line situations. The Rapid rate is not sustainable for long periods because it eats up a great amount of ammunition (more than the troops are likely to carry on a patrol), the heat generated requires barrel change times to be reduced, and with the one spare barrel usually issued, prolonged rapid fire will result in shortened weapon/barrel life.
Semi-Automatic rate[edit]
The semi-automatic rate is the maximum rate that a semi-automatic weapon can fire with any degree of accuracy in semi-auto mode, usually 45-60 rpm. However a semi-automatic firearm can mechanically function at (about) the same rate of fire as its fully automatic (or select-fire) counterpart, this is assuming the trigger could be pulled fast enough to facilitate such a rate of fire. Firing rates approaching that of a full-automatic version of the same gun can be achieved using "bump fire".