I installed a weaver railed bushmaster gas block on my Colt 6700.  It has three set screws on the bottom that lock the block to the barrel as opposed to the tapered pins on the old sight/block.  My first time to the range with the new setup I was suprised to find that my point of impact was off by about 3 inches at 100yrds.  I found that my groups were not as tight either after getting it back on zero.  Any of you have any info. on this phenomenon?  Is it possible that the new gas block could negatively effect the accuracy of the gun as compared to the original setup?  I put it on to allow the unobstructed use of a scope.  

ps. I should also mention that I had the Bushmaster detachable front sight installed while I noticed the point of impact change.  Could the different weight of the new block/sight cause the issue?        

anything you do to the barrel can move the point of impact.  That's why free-float handguards are so popular.

As to why the groups might have opened up, there are many potential reasons.  For starters, make sure that the new port is properly aligned and that the set screws are tight enough to prevent even the smallest amount of movement.

Thanks for the response.  I'll take a look.