Is there a mod to change all weapons to machine gun?

@phoenixxxxxx : You can use CPMA with a custom <map>.ent file to do that. The .ent files are text files that can dynamical replace items in a map without the need to recompile the BSP.

Also with CPMA you can use a series of /callvote and /recon options to make similar changes mid game (if you really wanted to).