When I first called Bushmaster, the guy I talked to told me it really should be an inhouse job. I thought what the hell for, it is just a front sight swap. So I ordered it anyway.
Get a set of punches, and drive out the pins in your front sight base. They move from left to right. Simple terms, push them from the side without the ejection port, they come out on the side with the port. The MAS block does not use the pins.
Drive out the roll pin that holds the gas tube to the font sight. Dont lose it, you need it.
That should leave you with the gas tube in your hand. Push it into the MAS block, and drive the roll pin back in. There are special punches for roll pins, that have a dimple that lines up the pin and punch. But, a punch that is larger than the pin will work fine.
Then just slide the MAS block back over the barrel. Line up the tube, push it through the handgaurd ring and into the receiver. Push the MAS block till it is tight against the ring. Then, there are set screws in the bottom of the tighten them till they just touch the barrel.
For making certain that the block is straight, it is nice to have a receiver block and vice, but not neccesary. If you don't have one. just remove the upper, and set it upside down on a flat surface. a small level works best here. Place the level on the bottom of the MAS block, and see that it is level. Make sure your surface is also level. This is easiest with Flat top guns, but will work with an A2 upper also. jsy make sure the gun is level first, then make sure the block is level. If it is, tighten the set screws in the bottom of the block. Use some LOC-Tite also at this point. Blue is good, as it sets very well, and isn't as hard to undo as Red is.
Put the drift pins from your A2 sight back in the old sight, so you dont lose them. Then toss the sight in a parts bin.
Now, let the LOCtite set and go sight it in. Mine was dead on, using the Bushmaster rear flip up sight, with the front flip up. They stay down most of the time, but work well as a back up the the EOTech. Any more questions, or more detail, let me know. -Kirt