For some odd reason you can’t make the Halo 2 BR or the SPNKr Rocket Launcher your starting load out weapons unless you use the 343 made “Snipers and Arena Breakout” game type. These are the only two modes that will allow you to make the Halo 2 BR and SPNKr Rocket launcher your starting weapons.
I’m not sure if this is a glitch or not. But I do find it kinda stupid that 343 didn’t make it available to make both of those guns your starting weapons if you choose.
Halo 2 BR > H5 BR