Need Help Deciding On Stock AR-15

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Again, forgive my ignorance, but what happens when I want to swap over and shoot Grendel out of that 6940? Wouldn't there be more choices on a conventional AR-15 platform? I see me buying a stock gun, mixing and matching components on it, starting with a Grendel upper and learning as I go, until eventually I have a stand alone Grendel build and the old original parts back on the original gun. By then I'm thinking I'm off to the races on the knowledge of the AR-15. While I don't question the value of the 6940, won't I be limiting my options on my swap outs as I go along? Are the lowers and recievers interchangeable?

Aaahhhh.... My bad. I'm failing reading comprehension this morning (haven't been to bed yet!). Nevermind. If you are going to switch it over, skip the Colt. A 6940 is too nice to give it the frankengun treatment.

Disregard me and pick a cheaper regular AR. Unless you want to just build a seperate upper. In that case you could enjoy the 6940 unmolested, and then just drop your separately built Grendel upper on it when you want to shoot Grendel. The 6940 is a regular lower, and thus will work with Grendel uppers.