Check out this review done by member full_auto
https://www.indianagunowners.com/forums/long_guns/140219-review_sig_556r_not_what_i_expected.html
Don't pay attention to that above thread. That is for the SIG chambered in 7.62x39. Different gun all together in terms of craftsmanship. I lucked and got one of the first SIG 556s. Mine is one of the first 400 with the 4-screw top rail w/ all Swiss internals. Replaced the M4 style stock with an ACE folder. If you have the classic style, replace the crappy Chinese stock with a Swiss one. Go to here:https://www.cogunsales.com/index.php/vmchk/SIG-551-556-Parts-Accessories/View-all-products.html Mine eats everything and is just as accurate as my AR or Mini-14. There were some with bolt-carrier peening issues so keep an eye on that. Other than that enjoy as they are a pleasure to shoot and so SMOOTH!!
Can you post pics of your 556?