good on you for being on the right track with those bullets.
i had inquired with hornady some time back about using them in the bushmaster, and they stated that the 250XTP (#45200) was designed for usage in the Colt and as such isnt engineered to withstand the kind of pressures and velocity that the bushmaster operates at with standard loads.
the bullet is engineered for a terminal velocity between 800 and 1600 fps so if you can get them running well at around 1200-1400 fps (hot colt loads - think ruger #1 or TC type usage - velocity's out of a carbine) they should perform great (Terminal performance if you were to hunt with them) and be an fantastic mild-moderate load to shoot out of that MGM setup you're working with.
one place to search - i would hit the cast bullet forums and see what they're doing for low velocity loads for the bushmaster. i bet someone there has some useful data you can dig up - and hopefully share back with us once you find it