I have come across an upper receiver marked 'Sig Sauer 516'. Barrel is stainless and marked '.223 Wylde 1:7 SS USA'. Handguard is 10". Has no gas piston assembly but has a gas block and tube installed. So did Sig ever make or convert 516's to Non Piston or was this done by person or persons unknown?

Sounds like a Bubba build. I don’t think SIG ever made a non-piston 516.

^^^ Yup.

For the AR-style SIGs, the 516 family is piston driven, and the M400 family is DGI.

I have several 516 uppers....not all are marked 516. It's conceivable that Sig had some extras sitting around and built them as M400's instead.

The rifle Duster described had a .223 Wylde chamber. SIG produced .556 NATO barrels. It’s a Bubba build.