Ruger puts out a couple different variants of their AR-556. The big difference is the front handguard and whether it's a free float or fixed at the front.  An AR with a fixed front handguard can cause a minor distortion of the barrel if you run a lot of rounds through it and as the barrel heats up.  This will cause it to be less accurate when the barrel is hot.  If you have the fixed handguard, I'd consider swapping it out for a free float handguard.  This would also involve removing the front sight (which also acts as the gas block) and replacing the gas block with a low-profile gas block.

For reference, here's the standard Ruger AR-556 with the fixed handguard: https://www.sportsmansoutdoorsuperstore.com/products2.cfm/ID/148056/8500/ruger-ar-556-5.56-nato-m4-flat-top-autoloading-rifle

And the Ruger AR-556 with a free-float handguard: https://www.cabelas.com/product/RUGER-AR-RIFLE/1932150.uts