From a post to ALT.BOOKS.DAVID-WEBER dated October 29, 1998:

Capital missiles, multi-stage missiles, and missile pods

    Capital missiles are more dangerous than cruiser/battlecruiser-range missiles because:

(a) In RMN service, they have marginally higher maximum acceleration (95,000 gravs versus 85,000), which equates to a small but significant advantage in maximun attack range.
(b) They use a good bit of their additional size to pack in superior ECM and penetration aids designed to fool defensive fire control and the homers on counter-missiles, which makes them much harder to stop than smaller, less capable missiles are.
(c) Capital missiles carry multiple laser heads, each equipped with its own extremely powerful set of reaction thrusters and independent targeting systems which operate in tandem with one another. In essence, a capital missile launches a small cluster of laser heads which separate themselves physically from one another but combine the input from all of their tracking systems in an effort to obtain the most effective firing positions for each independent warhead.
(d) Capital missiles carry fewer but heavier lasing rods, which means that each of them produces a lower number of x-ray lasers, but that the individual lasers are much more powerful and, given the multiple targeting inputs generated by the multiple laser heads, tend to be more accurate, as well. (It also means that, against a given strength sidewall, a capital ship laser head has a greater stand-off attack range, since the more powerful laser can burn through at longer distances.)

    In many ways, the MSM is simply the capital missile writ large. It delivers the same terminal payload and has very similar ECM and pen aids. In addition, of course, it has additional drive units, which give it either a vastly higher acceleration or a much longer burn time (with some "restart" options standard missiles lack), which makes it much more dangerous at almost any range. It pays for this by being larger than the regular capital missile, which means that the birds will be such space hogs that it is unlikely any ship in the present-day (Echoes of Honor) size range of a BC would ever mount them.

    The original RMN missile pods were designed for capital missiles. The missile pods carried by the Medusa/Harrington class SDPs were designed from the outset to handle the MSM, as were the ships' standard broadside launchers. Note that an MSM launcher can fire a smaller missile, although the smaller launchers cannot fire MSMs.