As often occurs, the answer to one question just leads to more questions ;)

The answer of this question:
http://answers.google.com/answers/threadview?id=445464

mentioned "The launch velocity of a conventional powder gun is limited
to 3 - 4 km/s." (quoted from this reference:
http://deed.megan.ryerson.ca/~fil/I/Papers/iaf2004.pdf ).

So, my question is:
What limits the obtainable velocity of a projectile from an explosive
cannon/gun launch? 
(please be specific enough that I could calculate a decent estimate
for the velocity limit for given barrel, explosive, and projectile
specifications)