So I have a stock LE6920 with Magpul accessories and what I plan on doing in the future is to own an 18 or 20 inch rifle since this is my second rifle. My question, is it cheaper/better to use the 6920 as a base and upgrade what I need to upgrade to make it a 18 or 20 inch barreled rifle? Sell the 16 inch upper that came with the Colt to fund this project? Or just start from scratch and build a 18 or 20 inch rifle? Thanks for the inputs! I want a 18 or 20 inch rifle because I feel that I am a great shot already and would just want that extra edge at long distances for hunting, etc.

I have no idea what this has to do with pistons.
Be that as it may, I would keep the Colt hardware and do the barrel, longer gas tube, spring, and buffer swap.

On second thought, the gas port diameter may be different which would require a new gas block.

May be better to purchase an entire dedicated 20" upper.

Not piston unless you're going to do a conversion along the way.  None the less build a second upper, might as well try a new caliber while you're at it.  Variety is the spice blah blah blah...

Whoa sorry about that, meant to post in AR Technical Discussions. Needs to be moved please.

It seems to me changing the 6920 barrel to 18 or 20 inches defeats the purpose of having a handy light carbine. If you want an 18 or 20 inch rifle either buy or build one and keep the 6920 as is.