Thanks to the forum reviews about the 516. I can't tell you how much it helped in deciding whether or not to purchase the 516. It was great to go out with the gun knowing of the potential issues. Sight in with a Leatherwood CMR4 1-4X24 scope. New 516s sport a B5 Bravo collapsible stock, ambi safety, etc. Objectives today were sight in, accuracy check and watch to see if there was lock back(LB) on the last round. Beginning shots all fired with piston in normal position and only one round in the mag. RESULTS: (1) WW 55g FMJ .223, Fail to LB (FTLB), accuracy very good. Re-fire in adverse mode and LB. (2) PMC 55g FMJ NATO, FTLB, in normal mode, accuracy was very good. Refire in adverse mode and LB. (3) Black Hills 52g. Match HP .223, LB in normal mode and extremely accurate. (4) WW 55g. BST Frag Poly Tip, LB in normal mode and extremely accurate. (5) Fed 55g. FMJ-BT, FTLB in normal mode, extremely accurate. Re-fire in adverse mode and LB. SIG 10 shot magazine used WITHOUT any problems LB with any ammo  Sig Polymer 30 rd. mags so tight the slide wouldn't close under normal hit of slide release. Mags used for testing were Lancer 20rd. and Magpul Gen 3 30 rd. All mags brand new. Weapon run wet with little powder residue after firing the various ammo. I suspect the mag selection and ammo is part of the problem with a new gun during break in period. It's interesting that the Black Hills 52g .223 and the WW 55g. .223 fired without problems in normal mode. I'm wondering if anybody else has played around with different ammos/magazines and found a workable solution from the get-go. Installed a Hiperfire 243G trigger using the gray (2.8 lb.) spring setup. You're at the wall right from the start with the trigger and it breaks very nicely. Not a trigger for entry/sweep work unless you go with the 3.5/3.8 trigger springs.

Try  breaking long posts into paragraphs or something similar.

Thanks!

^ this!

Unless you don't want people reading your post

A sub 4# trigger isn't a good choice for a duty carbine at all.

Agree on sub 4 lb trigger. T carry a Wilson Howe or Baer M4 DI gun with heavier triggers for real world. Piston gun is experimental right now.  BTW, local gun builder advised he has been opening up gas port a tad with 100% success. Others say cut buffer spring or stress the spring by locking action open til spring loosens up.

The real question is ? When will Sig ever wake up to 2016 and update the 516 ? Longer handgaurd for one.

Update-Sigs position is the gun is manufactured to NATO specs which means 62g. ammo only!  Pictures of the inside of the ejection port area show unusual wear down to bare metal on the backside of the receiver at the entrance to lockup and to the right of the half moon area. Sig has issued a RMA for the gun. The gun doesn't even have 140 rounds through it and doesn't function with 62g NATO.

They did. Google 516 carbon.

It's a great rifle, one of my favorites.

Sig Factory Update-Gun found to be have a defective gas valve, etc. Shipped gun Monday PM on their dime and they received Wednesday AM. Gun fixed and shipped out Thursday after test firing several mags through the gun. It appears somebody at the factory has a handle on the problem. Never heard of 62g only in a NATO weapon. Customer Service was very nice and helpful. I think the particular individual she was dealing with re the problems was out of the loop on existing problems with the 516s. Hope this helps somebody else with a 516 FTLB/FTL problem.

BTW, advised by FEDEX Dangerous Goods Hotline, Memphis, TN., that a firearm does not come under their title and so the gun can be shipped by any FEDEX facility to a FFL. Many local FEDEX offices around here wouldn't ship. IF you have a problem, tell the FEDEX office to call their Dangerous Goods Hotline and they will resolve ASAP.

Good to know. Thank you for the update. How should I inspect my 516 for the problem you experienced?

I purchased my Sig 516 about two weeks ago and have made one range trip with it so far.  The only modifications I've done was install a Surefire 556 muzzle brake to mount my Socom 762 Mini 2.  Tried several different types of ammo and observed the following this past Saturday:

Wolf Gold 55 gr. .223: No issues running the gun unsuppressed or suppressed with the gas system in their respective settings.

PPU .223: No issues running the gun unsuppressed but with the suppressor on, I experienced multiple failures to feed and the bolt didn't lock open on the last round fired (tried both metal GI magazines and Gen 2 Pmags).  

American Eagle AE223: No issues running the gun unsuppressed or suppressed with the gas system in their respective settings.  

Tried one other type of ammo but I can't remember what it is.  Overall, I'm very happy with the performance of the rifle so far.  I still need to try running it with a Gen 3 Pmag but I'm not expecting any issues.