Most of the time handloading is as simple as matching the powder and powder charge to the caliber and bullet. However, when reloading for a gas-operated, semi-auto rifle, another parameter comes into effect – the gas pressure at the operating port.
Generally, this is not an issue when you are reloading within the normal “factory” range for a semi-auto rifle with a proper burn rate powder. When you start moving away from the original design of the cartridge, such as a faster load or heavier bullets or sub-sonic loads, you probably are changing the pressure at the gas port.
The pressure can go higher or lower at the gas port, depending on the load. Remember, port pressure does not necessarily follow velocity. Too low of a port pressure will cause functioning problems – failure to eject the shell case or strip a new cartridge out of the magazine, feeding failures, etc. Too high of a port pressure can also lead to functioning problems, as well as signs of damage to the head from the extractor / bolt.
The only way to know for sure is to load up a handful of cartridges and the next time you head out to the range check for gun function. With a little tweaking, you probably can tailor the performance of the load you want to your individual firearm.