When performing dry-fire practice with an AR-15, there are a lot of reasons you might not want the bolt to lock to the rear. Teaching a new shooter how to properly load, unload and charge an AR-15 is one reason. Another example is repeatedly performing immediate-action drills. You can use dummy rounds, snap caps or other safety aids, but there’s another trick used in training circles requiring far less investment.
Take a penny and slide it between the follower and feed lips of a magazine (being careful not to wedge it too tightly in polymer types). The penny holds the follower down enough to not engage the bolt catch. This allows you to dry-fire, charging the rifle after each shot, without having to remove the magazine or press the bolt catch release between shots. To practice open-bolt reloads, stage the rifle with the bolt locked to the rear with an empty non-penny magazine. When ready, perform an emergency reload and use the penny magazine as the “fresh mag.” The bolt release functions as normal and does not give any unexpected resistance.
To practice immediate-action drills, insert the penny-magazine into the rifle and close the bolt. Upon hearing the “click” of dry-fire, perform the drill. The simplest version is to firmly slap upward on the magazine to ensure it’s correctly seated, grasp the charging handle with the support hand, then authoritatively cycle the charging handle all the way to the rear. From there, resume firing.
As always, it’s extremely important to ensure your dry-fire session remains a dry-fire session. Follow all firearm-safety rules and keep live ammunition and loaded magazines away from the practice space. When you’re done, make sure to remove the penny from the magazine so it doesn’t cause you any problems in the future.