The logic is that you're more likely to use the gun during a crime - regardless of the crime, whether or not it's victimless, or even if the law is unjust.

Yes, a drug purchase is a voluntary transaction, and yes, I believe drugs should be decriminalized. However it's undeniable that black markets are violent and that innocent bystanders get caught in the crossfire. The sentencing guidelines are there to punish and/or deter that.