Thank you for your reply! I’ve known of 556’s for a while, but they’re definitely nowhere near as popular as they could have been. I’ve also noticed that parts are nearly impossible to come by. Do you have any suspicions as to why they never caught on?
I bought my 556 back in 2008, because I really wanted a 550-series rifle but will never have any desire to spend $6000+ on a 5.56mm carbine (imported Swiss or not).
The short answer to your question is that Sig USA has a long history of shooting itself in the foot with poor decisions and then calling "do-over!"
The long answer is a history from my recollection (likely not in precise chronological order).
Initially, Sig USA badly misread the market, assuming that nobody would make the jump from the burgeoning AR market to a 550-series that seemed just too different. So they instead made a bastardized hybrid: the internals of a 550-series, but taking AR magazines (which is a huge plus), and an AR-style T6 collapsible stock. But they wanted to make it stand out, so they came up with the weird "fishgill" handguard, and put similar fishgill grooves on the stock. The stock makes me angry to look at. Some people hate the fishgills: I don't exactly hate the handguard, but I replaced it with a classic style because it doesn't look like what I want. It also lacked the iconic diopter sights, instead going with a flip-up front blade and a very fragile-looking flip-up "nail file" rear backup sight, evidently assuming everyone wanted to put optics on it instead. Overall, I was disappointed and ended up spending a lot of effort and money making it look closer to a 550.
They further tried to tap into the AR market with the SWAT model, which had a monolithic quad-rail handguard. This wasn't a terrible idea because quad rails were pretty popular at the time, but it was even more front-heavy, and looked even more like an AR and even less like a 550. Excitement wasn't as great as they had hoped. They had still failed to recognize that people who want an AR want an AR, and people who want a 550 don't want something that just looks like an AR. So they discontinued the original 556, and then eventually discontinued the SWAT.
They tried a few short-lived variants around the same time. There is a variant with no sights at all, called the "optics-ready" model. There's the 556 Holo, which was the same rifle but with a Sig-branded red dot sight included. There was a Commando model with a green 550 handguard and a Magpul CTR stock. There was a Patrol model with a shorter 550-style handguard, and a SWAT Patrol with a shorter quadrail handguard. There was a pistol model, but these pretty much only sold to guys who wanted to add a stock and register them as SBRs because there wasn't a brace. There was even a DMR model with a 24in heavy barrel, better trigger, a bipod, and a precision stock. There was a .22LR model, and a .22LR pistol. These weird line extensions didn't do much to generate big sales, either. All of them were discontinued before they could build any organic popularity.
As was already mentioned, at some point during all this the quality control dropped off. The system is solid, but they couldn't even be bothered to install the rails straight. That's a HUGE and inexcusable failure for a $1600+ rifle, and it created a lot of customer dissatisfaction.
They at some point finally listened to customers and came out with a 556 Classic, which had a proper 550 handguard, and a folding collapsible stock that looked kind of like a 550 stock. Except it didn't look ENOUGH like a 550 stock, plus the plastic stock was chintzy and there were reports of them falling apart. People were pissed. It was discontinued.
There was an improved 556 Classic that had a proper side-folding 550 stock. The sights were inexplicably higher than the original 550, and apparently not a comfortable sight picture with the stock. Since the hooded front sight was so high, it blocked the view of optics and there was nothing available to replace it with. People were pissed. It was discontinued. That being said, if I had it to do over knowing everything I know now, I would probably buy this model and replace the sights with an MFI diopter and hooded front sight (which are the same height as the original 550s), and find a cheek riser for the stock.
They released the 556 Russian with much fanfare, a 556 that fired 7.62x39 and took AK mags! Quality control was terrible, and a lot of people reported problems with ejection. They released a 2nd gen that tried to correct the problems, but the damage was done. People were pissed. It was discontinued.
They released the 556xi, which had a reversible charging handle, and a cheek riser on the stock to address the previous complaints, and a few other changes I can't remember. You could swap uppers to change calibers: it was available in 5.56 and 7.62x39, but they never came through on their promise of a .300 Blackout upper, or anything else. Accuracy was reportedly poor. People were pissed. It was discontinued.
They finally listened to the remaining hardcore customers and came out with the 556A1, which was much more true to the 550 roots, with a forged receiver instead of stamped, even using imported Swiss 550 magazines instead of AR magazines, and imported Swiss stocks. They were all supposed to have 550-style flash suppressors but there were further quality control problems where some shipped with AR-style flash suppressors. There was also some drama surrounding this because CDNN convinced them to do it and they were supposed to be exclusive to CDNN, but then Sig USA just went ahead and started selling them to other distributors in violation of this agreement. CDNN was rightfully pissed and stopped carrying them. By the time they reached this point, though, people had enough of Sig USA's repeated blunders. Most of us just didn't trust them enough to drop more money on their products, and it was very expensive for a mere 5.56 carbine. Overall, people didn't even care enough to be pissed anymore. It was discontinued.
The people who answer the phone at Sig USA generally aren't familiar enough with this tangled history to answer questions about specific models, and have a general reputation for being incompetent. Additionally, nobody trusts spending $2000+ on a rifle they probably can't get spare parts for.
I'm sure I missed a few minor variants. It would be nice to be able to look on the manufacturer's site and list all these by year, but they've been deleted as if they never were. You'll not even find them on their page of "discontinued products". Sig USA now only sells the AR variants they clearly wanted to make all along.
