There here have been several generations with changes to the upper and lower. But we can really lump them into 2 different generations. Lets look at what makes them different. The pictures below show the gen 1 upper on the left and the gen 2 upper on the right.

1. The gen 1 upper extrusion is different than the gen 2. The gen 1 extrusion has the rail that the bolt cam rides and pivots on to rotate the bolt out of the breech block built into the extrusion. This is the part of the lower at the lower left corner of the picture. The Gen 2 upper does not have this, it has a hardened metal rail that is held in by the 4 screws opposite the ejection port. The 4 screws is the easiest way to tell if it is a gen 2 upper. If it has the screws it is a gen 2.

2. The second difference is the extrusion on the top inside the upper. The channel is where the charging handle return spring and bracket ride. This was a feature that was not needed and and was eliminated with the gen 2.

3. The third difference are the tangs that attache the upper to the lower. On the gen 1 upper they are not solid. They have a split to allow the push pin from the lower to engage with about 1/4" of pushing engagement.

Lets now talk Lowers: There were several different changes to the lowers and I will only address 2 of them.

1. If you look at the pictures below the gen 1 lower is on the top and the gen 2 is on the bottom. The gen 1 lower take down push pins do not need to be pushed out all the way to separate the upper from the lower, where the gen 2 lower take down pins need to be pushed out all the way to separate the upper from the lower.

2. The mag well was also changed to add an extension to give it more surface area. Not all gen 2 lowers have the extended mag well. There was a transition period so you could have a gen 2 lower with the short mag well.

3. The gen 2 lower is on the left and you can see that the take down pin is solid, where the gen 1 lower on the right has a step pin.

Another thing to note. There was a time when Bushmaster was not officially recalling the gen 1 rifles for safety malfunctions. They did not call it a recall but if you had a gen 1 rifle they would exchange the lower to a gen 2 which would fix the safety issue. The issue was the rifle would sometimes slam fire. I have a letter from Bushmaster on my article page that shows the letter to the ATF because this issue was being fixed during the assault rifle ban.

Issues with my modifications to the gen 1

1. Because of the upper having the 2 rails in the extrusion for the bolt rotator to ride on, if I do one of the standard slot mods it will leave a shelf that you can see because the top rail of the extrusion is just below the center line. So if you do not want to see the shelf you need to choose the small slots as these are not down that low. The picture to the left shows the shelf and the one on the right shows no shelf as the right of the extrusion has no rail. You will also not be able to have holes on the bottom. Now I guess I shouldn't say cant but rather should say that it will probably look weird.

2. Since there are no 4 screws opposite the ejection port you can not just attach the cheek rest to the upper. You will need to drill and tap (4) 8-32 threads 1.00 apart so you can attach the cheek rest.