The Gen 1 Bushmasters have no tear-drop milling on the side and earlier ones experienced some problems with the first shot being too low. Also was available in black and silver only.

The Gen 2 Bushmasters added the tear-drop milling, the upper body at least on the silver one has a shinier finish to it than the silver Gen 1's. The 1st shot problem was also fixed. Also offered in many more colors.

The Gen 3 Bushmasters have slightly different teardrop milling, has a coin slot LPR adjustment knob instead of that 2 holed thing and it comes with a highriser.

The Gen 4 Bushmasters have the choice of teardrop or fish gill milling, battery door on the tray and a new jewel logo.

The Gen5 Same as the Gen4 but with new regulators. The HPR has a bigger plunger and heavier duty spring designed to take a preset tank (800psi). The HPR also has 5 finger grooves instead of 6 for higher comfort. The LPR has also been made shorter internally to allow for more air storage and faster recharge rates. Serial Number 7200-7299 were a combo of gen4 and gen5, anything over 7300 is a 5 i'm told.

The Bushmaster 2000 LCD comes in Gen4, 5 and 6 style depending on the serial number plus it comes with an LCD on the back of the tray that displays cps rate and fire mode as well as a count up timer and shot counter. It has 8 fire modes instead of 4 and has the programming to utilize an anti-chop eye although ICD doesn't sell one just yet (after market ones will work though).
The Gen6 Bushmaster brings C&C Machining to the mix with a new body X milling design that looks very sweet. The gun also sports a re-designed barrel as well as new micro gauges. They look like macdev gauges but I'm not 100% sure that they are actually by MacDev.

The Gen7 Bushmaster is essentially a Gen6 with 1 upgrade. It has the same X milling but with a flat Blade Trigger as well as the new 1 piece barrel.

The Gen8 Bushmaster is a Gen6 with new swoosh milling, Blade trigger and the brand new 2 piece CP barrel. Also there have been some changes in the HPR, the plunger is now derlin with a stainless steel pin pressed into it.