One of the defining musical elements of drum corps is the exclusive use of bell-front brass instrumentation. Throughout the years, the horns used in drum corps have evolved from true, valveless bugles to modern multi-valved brass instruments. These changes have effectively eliminated bugles from the activity, since the current three-valve approved instruments are more akin to band instruments than true bugles. Competitive drum and bugle corps have not used true bugles for several decades. Traditionally, corps use three-valve vertical-piston horns in the key of G, but horns in other keys are also allowed. From highest pitched to lowest they are: sopranos (trumpets), mellophones, baritones and euphoniums, and contra-basses.

All these instruments can appear in any key, with G, Bb, and F being by far the most common; the name of the activity is not dependent on the key of the instruments. Sopranos are essentially trumpets, but tend to have a narrower bell flare and larger bore than the trumpets used in other venues, a characteristic found in most of these horns but most obviously in the soprano. Although many corps have recently started using Bb trumpets instead of sopranos, most purchase trumpets that are specifically designed for outdoor use and resemble sopranos in shape and sound. Mellophones are only one of many midrange or alto-voice horns that have been experimented with, but they have become the most widely used because of their consistent playability and tone quality compared to the alternatives, which include marching French horns, alto horns, and flugelhorns. A contra-bass is essentially a tuba configured so that it can be carried over the shoulder with the bell facing forward.

Until 1999, American drum and bugle corps hornlines were required to be pitched in the key of G. That year, the DCI rules congress passed a proposal to allow any key of bell-front brass instruments on the field (no Trombones-no valves, no French Horns-not bell front, and no Sousaphones-not really a bell-front instrument)(Open Class opted for a two-year moratorium on the proposal. Corps could first use instruments in other keys in 2000). DCA followed suit in 2004. This allows music to be arranged truer to its original form and gives corps access to more affordable and higher-quality horns, along with a much wider resale market for used instruments. Hornlines now are most commonly pitched in Bb, with mellophones usually pitched in F.