Re: Uppers and lowers
Your standard mil spec AR15 uppers will work fine and dandy. Differences will come down to your gas block placement on your current upper and how it correlates to your current lower.
If you have a carbine rifle, then you'll have a carbine gas system on your upper and a carbine buffer/spring/tube on your lower.
Which means that you'll want to run a carbine upper on that lower.
Also, if you have a rifle with a 12" gas system, you'll want to stick to using rifle length uppers for that lower
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