I have shot both .308 and .310 bullets from Ranch 762x39. In mine, the 308's have been more accurate, but they were good Hornady bullets.
This is an older article, but states that Ruger now use 'nominal 0.311' grooved barrels versus older 0.308.
They shot 310 and 308 bullets and had best luck with a 308.
https://www.shootingtimes.com/editorial/review-ruger-american-rifle-ranch-model-in-7-62x39mm/99212
To know for sure in your rifle, you need to 'slug' the barrel.
The SAAMI Spec for 7.62x39 is 0.311 -0.002. That puts .308 barely out of spec on the small (safer) side.