geno, the HBARs use the same USI A2 type front sight tower/bayonet lug assembly as any AR15A2/M16A2 type rifle. Katman is also discussing modifying his pre-ban Colt Sporter HBAR, so there is no need to use one of those USGI A2 lookalike post-ban compensators. In fact, he should already have a USI A2 type flash suppressor on it.
Katman, from past discussions, I have gained that changing front sight towers is a real pain in the ass. It is not just a simple matter of driving out the pins and swapping towers. Welding or otherwise adding material to the base of the sight tower and then machining it into a functional bayonet lug is probably not only financially impractical, but could damage the barrel in the process. My 2 cents, if you truly want to get your Sporter HBAR as artistically and aesthetically close to an M16A2 as possible, you need to lose the heavy barrel, too. Look around for a Colt government profile barrel complete with front sight tower and bayonet lug. They can usually be found for $100-$200. Rebarreling is a simple task to perform yourself or inexpensive to pay someone else to do it. Then you will be as close to an M16A2 as possible.