In the field, when a pipe breaks at the briar side, the stem is then tapered/whittled to fit/jam into the pipe to continue smoking capability (pre-online shopping).
Then, in order to best protect the briar point of entry, a brass shell casing was used (by tapping out the center/primer). The right size shell fixed the split wood and made a firm opening for the whittled stem to be inserted.
Many army mount pipes since then pay tribute to this original idea with a metal cap over the briar. Sometimes it's just a flair in the plastic, as a tribute shaping. The stem is still/always tapered like it was whittled. That defines the army mount pipe style.
The Peterson Spigot army mount add tapered metal to the stem... but this wouldn't have been done/needed in the field.
Here's a link from a previous discussion of the army mount with photos.
http://pipesmagazine.com/forums/topic/army-mountslikeloveem