This thread has bothered me with what seems to be a misunderstanding of the technology.  Here's my understanding, YMMV, etc.  

Tapered-bore weapons were beneficial in an age when only CPR weapons existed, the technology was still in development, and weapon designers were always seeking ways to increase muzzle velocity.  A tapered bore offered a way to build up chamber pressure and increase velocity with the slower-burning powders of the day.  We no longer use tapered bores because our modern propellants burn about as fast as is possible for a chemical explosion, and anything that reduces their speed of expansion, such as muzzle friction, reduces muzzle velocity.  This is why smoothbore cannon are now back in vogue, because they produce higher velocities.

In an age of electro-magnetic propulsion such a rail guns, there is no such thing as chamber pressure and anything that reduces muzzle velocity is a Bad Thing.  Tapered bores are a relic of the past, and have no place in the far future, again IMHO, except on lower-tech worlds.  

Now that I type that, I remember playing a Mercenary scenario where the troops in battle dress went down to a planet that was TL5 or so feeling invincible.  They were surprised to find that the locals had 25mm or so squeeze-bore rifles that could punch through BD if they rolled well enough.