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).

With OSP you can do them individually. I think the command is like "replace_bfg #" where the # would be the value for the machine gun. It's in the OSP docs. Obviously you do it on the server then map_restart.
Edited 19 seconds after the original posting.

Alternatively you can also disable the weapons in Q3 itself by calling a lot of these prior to game start:

set disable_weapon_shotgun 1
set disable_weapon_rocketlauncher 1
set disable_weapon_plasmagun 1
set disable_weapon_railgun 1

etc. It won't make more machinegun ammo no, but it's something built-in