You will need to go here: http://www.saints-n-sinners.com/files/SA2K4_102.zip to download and install the rifle.
Extract the three files to your UT2004 system folder.
Put these lines in your user .ini file:
Set the priority to one number higher than the highest weapon you have in there now, so it should look something like this: Priority=20
Start UT2k4 and go to "Settings" > "Game" and check "Switch Weapons on Pickup" (you need this checked to spawn with the rifle)
You should now spawn with the rifle everytime and when you run over another weapon you will not switch weapons. Its a good idea to have your Translocator as your 2nd highest weapon so you will not switch to another weapon while running around with the trans in your hand.