Like all have said, you need to think, then start with a configuration and stay with components that work for that configuration. Carbine buffer tube w/carbine stocks, but either commercial or mil-spec will need to be considered, even though they're both carbine components.
As to grips, you'll probably be changing out a few to find what you want or like as a style/comfort/usage(A2, Ergo, more vertical, A1, Hogue over-molded, and many others). Then there's all the colors for what is called furniture(grip, stock, handguard).
One thing I've seen done that is a little different is going with barrels in either Carbine, Pistol length, or Mid-length w/corresponding gas tubes and putting on a "Rifle" buffer tube/buffer and spring w/corresponding A1 rifle stock.
So, there are some odd changes you can use, you just need to remember what parts are needed to make that combination work properly.
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