Hi Everyone,

I'd just pick up 2 of the Sig Sauer 516 Stripped Upper Reciever with Gas Impingement Adaptor.

I am wondering, How difficult would it be to build a piston upper?  
In the past 2 years, I have built 4 gas impingement AR upper and none of these 4 uppers has any cycling, feeding and ejecting issues.

This will be my 1st piston upper. any suggestion and tips?

Thanks

If you can gunsmith up a DI upper, than you can handle a GP build. Fundamentally the build process is no different.

Hints:
Buy quality parts from a reputable GP kit manufacturer. Ideally from one that has been around >2 years. Fly-by-night co.'s can go out of business at any time and make finding parts should something go wrong a chore.
Make sure whatever handguard you select has enough height to clear the op rod.
Remember that only all-metal front sights can be used on the front gas block if its railed. I've seen at least two people melt their  BUIS.