What's the effect of a "low mass" (competition only) buffer? And why competition only?

A few answers talk about cycle time, but the AR15 already cycles a few times faster than a human is capable of pulling the trigger. I.E., you can't pull the trigger as fast as an AR can be bump fired or as fast as an M4 can be fired full auto.

A better reason is because speed competition shooting (3 gun, for example), the rifles are often designed to be very light weight and the rounds used are low powered and with down tuned gas blocks. This is to help with target transitions and keeping the AR on target.

However, the AR system needs a buffer weight and spring weight that scales with the power of the round. If the buffer and spring is too light, you end up pulling the brass out while it is still under pressure, causing a host of problems, and if the buffer and spring is too heavy, there isn't enough energy to reliably cycle the rifle.

For reduced loads used in "race guns", a light buffer and spring is needed for the rifle to cycle reliably. A combo too light for full power loads.

What is the effect? The simple answer is, a lower mass buffer will cycle faster. Why? See BBean's answer.

in a competitive arena... if a part fails you DQ or do over. On a defensive or service pistol if that part pukes you die..

That's the competition only part... If seen a few 1911 buffers self destruct. It took tools to get the pistol apart to repair it.... I wont run the risk on a "work gun" cause there is NO advantage in the part to start with outside speed work

Buffers keep the pistol from eating up the internal parts ... great for guns that are shot fast...they also limit muzzle rise... brings the sights back down for quicker rate of fire