The gas block is a relatively small component in the AR15 firing system. It is often not considered by many gun owners as a critical part to change out, but there are some very important reasons to consider a new gas block for increased weapon performance.
The AR15 gas block and the entire gas system is a very simple system. The gas produced in the firing of a round leaves the top of the barrel through a small component, the gas block, which may have adjustable ports to allow for more or less gas to vent through the block. The explosion of the gas through the port keeps the gas block clean, so it is rarely even a factor thought of by newer gun owners.
After the gas leaves the gas block, it turns a sharp 90 degrees and moves through the gas tube. From here, the energy of the gas is used to move the top of the bolt carrier backward, which allows the bolt to pull the spent case out of the chamber and eject it from the weapon. Finally, the energy is used to cause the bolt carrier to fully extend and surge forward with enough force to strip the next round from the magazine into the chamber, ready for the next shot.
Issues to Consider
A central reason to change out the AR15 gas block is to increase or decrease the amount of gas moving from the barrel to the gas system. Some gas blocks may not allow for adjustments, while others may have issues with the port that prevent adjustment through damage or defect.
Additionally, with some AR15 optics, the existing gas block may sit too high to correctly mount the optic. When this is an issue, replacing the gas block is a simple solution.