I've taken a liking to the ICS line in the last couple of years. They have 2 real big advantage - 1 gun platform can suit multiple roles, and the split mechbox makes altering velocity quick and easy with no tools required.
For example, you can buy a full length M16 with a 400 fps velocity for full outdoor use.
You could have another upper / front end of M4 length for mixed style engagements where an outdoor field may have structures and some CQB aspects where a full length M16 would be awkward to use.
You could have another upper with a short CQB-length front end and a separate upper mechbox set for 350 fps for straight indoor / CQB games.
Basically, you have 1 base platform that you can use in all situations. Economically speaking, it makes good sense, as you only have one base gun, 1 type of battery, etc, 2 upper mechboxes for the different velocities, and the different upper receiver / front ends for specific purposes.
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