We have a 3 rail gas block with detachable front flip up sight or a gas block with integral flip up sight available that can be installed.
The three rail gas block can only be installed on barrels that do not have a permanently attached flash hider or muzzle brake. To install the gas block the front sight base must be removed first. Remove the upper receiver from the lower. Remove the bolt carrier assembly, charging handle and handguards. Free float handguards are left in place.
Remove the gas tube by drifting out the roll pin in the front sight base with a 1/16" drift punch. Pull the gas tube back into the receiver until it clears the front sight base. Then remove it by pulling it forward by the front sight base and out of the receiver.
Install the gas tube into the three rail gas block with the gas hole in the end down towards where the gas port will be in the barrel. To install the pin it is best to make a slave pin holder. Drill a hole in the end of a piece of round steel stock with a number 48 (.076") drill bit just deep enough to hold the roll pin. Use the slave punch to start the pin and seat it with a 1/6" drift punch. It is easier if the gas block is supported on a piece of wood so that it is stable.
Place the front sight base on a piece of wood to support it with the ejection side down to remove taper pins which come out left to right. Start the pins with a 1/4" drift punch to break the pins free then drift them out with a 1/8" drift punch. Tap the sight base off the barrel with a plastic mallet.
Install the three rail gas block with gas tube onto the barrel. With receiver upside-down place a level on the bottom of the receiver and the bottom of the gas block. With the parts leveled tighten the set screws for the gas block with a 3/32" Allen wrench.
Install the flip up front sight onto the gas block with a 7/64" Allen wrench. Reassemble the rifle. Sight in the rifle with the rear windage set in the middle. Adjust windage if necessary by loosening the set screws and moving the front sight. After the rifle is sighted in remove one set screw at a time and reinstall them with a drop of # 242 Loctite Thread Locker.