I believe the Grendel is based on the at-15 platform.
I believe you will also have to determine what you want the gun to do or how you will apply it. as another poster referenced there are deals to be had, the at market is soft. everyone who had a wrench, a vise and a set of punches became a builder. the at has become the tinker toy for big boys.
for just plinking and learning to shoot one I would tell you to get one in 5.56 nato or a wylde chamdered 223, ammo would be cheap. a basic gun to me for general use is a standard lower with a six position collapsible stock, a flat top upper but with the sights, a sixteen inch medium heavy barrel, 7 or 8 twist, with one of the slimmer forends that have the options to attach accessories. maybe I'm totally off as to what you want to use the gun for but like I said you have to determine what you want to accomplish.
