the left side of this photo shows a wrench design I saw auctioned on eBay... while functional, I think it needs improvement to prevent slippage during the installation of barrels... remember that these barrels in most cases have been in place for 60 to 100 years and will require considerable torque to remove... we don't want to mar the receiver now do we?
The design on the right would be my modification to allow tighter capture of the receiver before using the wrench. I would cut the wrenchhead where the red line indicates and then drill two holes for use with socket head screws where the yellow images are shown... this would allow a maximum grip on the receiver before the handle is moved. NEVER use anything in between the 'forks' of the receiver... because if you do you will likely ruin the trueness of the breechblock races...
make sure the inside surfaces of the wrench are polished smooth.
The wrench handle should be at least 16 inches long... break initial torque with a 2 lb mallet on the handle until you see movement at the draw lines or witness mark...
To see other wrench designs that I have collected, use this URL to download a 350K zip file containing those graphics...
http://www.lugerforum.com/wrenches.zip
make your barrel vise blocks out of acraglass on a real barrel that has been generously lubricated with release agent... use powdeered resin in the barrel blocks after cleaning out the lube and clamp it HARD.
this should remove your barrel without marring it or the receiver if you are careful...
Let me repeat my original caveat... Changing Luger barrels is not a job for the amateur... I recommend that a pro do it... but if you have the equipment and can afford to live with results... then good luck to you.