Just different types of AR's
For example:
The M4 and MK18 are like the AK74 and AK105
In airsoft, same gun on the inside, slightly different look.
The HK416/417 are a bit different, not quite as interchangeable on the outside, but still the exact same gun on the inside.
In the world of M4s, you buy whatever the heck you think is cool and run with it. Cause they're all the same on the inside
Here's mine:
SR15E3 and a MK18 (except mine ARE different on the inside, cause they're PTWs)
