I was talking to one of my friends recently about pistol caliber uppers and he told me that when he went to a Colt Armorers course the instructor said that they beat the lower to death because it needs a heavy buffer assembly and heavy bolt group.  Is this true?  

The reason I ask is I have a RRA M4gery and would like to purchase a pistol caliber upper so I can shoot it at an indoor pistol range near my house.  However, I don't want it to beat my lower to death though.  Thanks.

The heavy bolt and buffer have virtually no effect on lower reciever life.  What does is the fact that some of the bolts are not ramped and will mess up the hammer pin holes and break hammer pins.

If you get a ramped bolt or have your bolt ramped you will be fine and won't incurr any negative effects.  

The ramp is like that of an Ar-15 .223 carrier (the sloping surface on the bottom of the carrier that re-cocks the hammer.)