A hammerless firearm is a firearm that lacks an exposed hammer or hammer spur.
In rifles, using a firing-pin rather than a hammer is a substantial improvement because the time from trigger pull to firing can be less. This makes the rifle more accurate, because the rifleman's muscular tremors have less time to move the rifle off-aim.
One of the disadvantages of an exposed hammer spur is the tendency to get caught on items such as clothing; covering or bobbing the hammer by removing the spur reduces this tendency.
Read more about Hammerless: Early Hammerless Designs, Pump Shotguns, Handguns