John,
The primary problem with bolt breakage is due to all of the weight added to the non-free-floating railed handguards by adding lights, lasers, forward grips, carry handles and other items. This causes the barrel to sag at the muzzle end, changing the angle of the barrel to the bolt slightly, which causes the pressure to be unevenly distributed on the bolt.
Freefloat handguards would solve this problem, and thus they are a requirement of the SOPMOD II package that the top companies are designing and bidding for. The SIR, RAS II, and URX designs were all created in part to address this problem.
-Troy