My Colt AR-15 manual describes using the bolt catch in loading - Why?

I can't really imagine what the lengthy procedure is that involves holding the bolt catch down. Latching the bolt open before installing the magazine will make it take less force to install the magazine because the rounds in the magazine will not be contacting the bolt carrier. The AR-15s are not double action since pulling the trigger will not c0ck the hammer. The hammer is only c0cked by the bolt carrier moving towards the rear of the rifle.

Inserting a magazine with the bolt locked to the rear was the way I was taught in the military in the late 80's. It serves two purposes. First, by locking the bolt to the rear, the operator can visually inspect the chamber to ensure that it is clear prior to loading. Second, it is surprisingly easy to insert a full 30 round magazine and not get it locked in with the magazine catch. This is because of the pressure of the rounds in the magazine being in contact with the bolt carrier. The magazine may stay in the mag well momentarily but it will drop out with the slightest jarring or movement of the weapon. I have seen it happen in combat, most embarrassing.

If you are just shooting for fun at the range and you don't bother to load the magazine to full capacity then you will probably never notice this.

There is another side to the story however. It has been noted that vigorously inserting a full magazine on an open bolt can cause the top round to dislodge from the magazine and remain loose in the chamber area or upper receiver. If this occurs and the operator does not note the condition, a double feed malfunction will occur when the operator releases the bolt to chamber a round. More than embarrassing in combat, such a malfunction can cost the operator his life.

Since it was pounded into my soul by the military, I am destined to forever using the open bolt loading procedure, good or bad.