Good Rifle/Carbine to Buy?

My dad and I recently took an interest in collecting guns, and we started by buying a DPMS AR-15 which we're proud of right now. What we're looking for right now is a another semi- auto rifle/carbine for possibly$500- 600 with preferably an AR-15 build, or basically something with a magazine and pistol... show more My dad and I recently took an interest in collecting guns, and we started by buying a DPMS AR-15 which we're proud of right now. What we're looking for right now is a another semi- auto rifle/carbine for possibly$500- 600 with preferably an AR-15 build, or basically something with a magazine and pistol grip about between .223 and 7.62(any) or anything close. No AK-47's please, my dad has personal experiences in Vietnam(many buddies killed) with it, and he doesn't like anything that looks like it, and I'm pretty sure he wouldn't like an SKS either. Thanks for your help.
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