The Force of Your Magazine Spring
Your magazine spring's rate is a very critical piece of information on your design because most magazine springs must be compressed down to solid height. This means that if you aren't able to match the loads applied on the spring with the amount of travel for it to reach solid height, either it will be too strong for the load to push it down to the required loaded height or it will be too weak which means your load can exceed the max safe load and therefore, exceed the max safe travel. If you exceed the maximum safe load of your spring, it will be over stressed and probably take a set, which in effect, will cause your spring's free length to be reduced, therefore, not being able to push back up to it's original length so the last bullets or rounds won't be able to be placed into the chamber.
Just like on a regular compression spring design, a magazine spring's force and rate may be significantly affected if the number of coils, wire diameter, length or outer diameters are altered. Please see the force chart below for reference.
If your magazine spring design is too weak, you would have to eiter reduce the outer diameter, reduce the amount of coils, thicken the wire size, or make your spring longer without adjusting the amount of coils it already has. Reducing the outer diameter will also reduce your spring's index therefore, giving you more force. The same happens with increasing the wire diameter. As to decreasing the amount of coils, take a slinky and a key ring for example. A slinky has numerous coils and therefore, it is very weak, but a key ring, which has less than two coils, has a very strong force. Increasing your spring's length will get you more force because the rate remains the same but you increased the free length, thus giving you more travel . Therefore, since rate works by travel and not loaded height, it'll take your spring more force to reach the loaded height thus giving you more force.
In case that your magazine spring design is too strong, you must either increase the outer diameter, increase the amount of coils, make the wire thinner, or reduce your magazine spring's length. Increasing your spring's outer diameter will increase your spring's index as well, therefore decreasing your spring's force and rate. The same will happen when making the wire diameter thinner. Increasing the total amount of coils of your spring will weaken the strength of your spring as stated above in the slinky and key ring example. Reducing your spring's length will give you less force because the rate of your spring doesn't change. Therefore, it will take your spring less load in order to reach your desired loaded height.