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Unless you are planning on shooting full auto, or more then 300 rds per range trip I'd stick with DI. My DI LMT will go few houndred rounds on mediocre ammo before I break it up, spray some balistol and wipe it down, then go again.

I've never owned a piston ar so I can't speak to their performance or reliability.

I thought that Olympic barrels were not chrome lined, Colt barrels are. If so, I can imagine how the first few rounds irrespective of distance would be close, and then as they both heat up, non-chrome lined barrel would heat up faster, get messy faster and be drastically less accurate.

20" barreled AR, assuming it is chrome lined and quality, should be very close in accuracy when compared to a 16" 1/7 counterpart. In 16" 1/7 you get 2.29 turns roughly, compared to 2.22 turns from a 20" 1/9. Now, is it better to have 1/9 20" bull barrel, chrome lined, then 16" 1/7....?

But yes, I'd spend few extra dollars and get a good quality chrome lined barrel for sure.

1/7 twist is great for heavier bullets, as stated previously. I do think that you really need a longer barrel still (20") to see a substantial difference though over 1/9.

If possible and budget permits, I'd suggest a chrome lined barrel.
It will be easier to clean and won't get hot as quickly, so your accuracy will be improved. You will also have a longer barrel life.

If I was building, the right barrel would be my biggest investment next to properly staked bcg.

Remember that when you build it, you will lose some $$$ when or if you try to sell as most people look for factory setups. You'd be better off parting your build out and selling it like that, rather then whole rifle.

Don't ask me how I know