I am a bit late to this thread, but since unlike most other posters, I own both a DDM4 and a SIG516, I think my opinion is relevant:
The Daniel Defense in a great rifle, and the lack of good finish or workmanship of the SIG 556 Classic rifles have given SIG a black eye as of late...I had a SIG 556 before and thought it quite crude in many ways..... so I am not a SIG fanboy.
But the SIG 516 is NOT the same as teh SIG 556.. it is MUCH better!!!!......
the SIG 516 Gives up NOTHING to the Daniel defense in workmanship, looks, accuracy and feel.
The fit between upper and lower on my SIG 516 is even tigther than on my DDM4V5 (and tighter than on the LMT CQB MRP I just picked up yesterday!)
As for accuracy I have not seen any difference between the 2 (though in fairness to both I have done only short range shooting with either)
Both have CHF CL barrels in 1 in 7.
Both shoot very smooth, the urban legend that a piston rifle has to shoot less smooth than a DI is just tha,t an urban legend.
My SIG shoots buttery smooth, doesnt feel front heavy either.
The piston design in the SIG 516 is different and distinct from that of the SIG 556 ( the latter, but not the former, is an americanized version of the Swiss army rifle).
There is a growing school of thought that maintains the high end pistons like the 516 doent really need to be cleaned after every outing or maybe evenb hardly at all...more like when the barrell starts seeing some chrome desposits..so maybe after 5000+ rounds.
As an expirement i might stop cleaning my SIG 516 until 5000 rds and i am confident it will not harm the rifle or significantly affect its reliability.
I could not say that about any DI gun no matter what the brand.
As a life-long DI guy ( 23 yrs in army) i am really warming up to the piston solution especially nice pistons that have solved carrier tilt like the SIG 516 has.
Among the real high end rifles I own I place my SIG 516 slightly above my DDM4V5.
When I got my DDM4 I loved it and wanted to buy more of them..
but then during my search for more DD's, I stumbled on a SIG 516 and gave it a try and now i am in love.
I am sold on the 516 and had been looking for second and maybe even third 516 for myself when the LMT fell in my lap this weekend.
I am on travel so cannot post pics of the SIG.
The lower and upper are in black but the stock, grip, and rail protectors are in FDE.
My SIG 516 is one of only 2 guns (the other is the LMT) I would not sell.