Are you changing from a two piece handguard to a free-float rail system? What kind of gas block do you have now, and what kind of gas block do you plan on going to? Generally, First the muzzle device is removed. Next the gas block is removed. If the current handguard is a standard two piece system, that can be removed prior to the gas block. I usually change the gas tube the same time as the gas block since it's just a quick $10-$15 part, so no reason to take the old gas tube out of the old gas block. Next the barrel nut must come off, but only if the new handguard or free float tube requires a new barrel nut. If the same nut is going to be used for the next accessory, then start reassembling. You don't need to pull the barrel out of the receiver, and it's easier to work on the parts of the barrel is left in the receiver. Just work backwards from there. Barrel nut goes back on, either the handguard/free-float or the gas block goes on next (depending on what combo you are using). Last reattach the muzzle device. Worst case scenario. I think if I had a standard carbine with a taper pinned A2 front sight gas block and delta ring assembly, and I was upgrading to a low profile gas block with a free float tube... I would have the work done in about two hours, tops. That would give me enough time to finish the job, clean up my work and enjoy my coffee. The one system that would add a considerable amount of time is the Daniel Defense rails. That's only because all those damned allen screws that are required to mount the rail.