Never expect much from surplus ammo. The best groups I have obtained were from match grade ammo and that was from handloads.
Like anything in life you get what you pay for usually and it is no different with the purchase of ammo.
The .308 will generally enable one to shoot about 6,000 rounds before the accuracy really starts to fall off assuming that 1. You never overheat the barrel in prolonged rapid fire and 2. You remove all of the powder and copper fouling after you shoot the weapon at the end of the day. This is often easier said than done.
I have found that the .223 is the more economical round and with it one can often get 10,000 to 12,000 rounds of shooting out of a barrel when using this caliber. Shooter fatigue and therefore overal consistant accuracy is often way better with the .223.
Again using surplus ammo in the .223 often results in as large as 3 to 4 inch groups even out of a custom built match gun but using match grade ammo will often provide the shooter with one whole groups at 100 yards. This is a tremendous difference in accuracy and once again it proves you generally get what you pay for.
If you want to do a lot of shooting with quality ammo you will still go broke shooting .308 ammo as compared to shooting .223 ammo. Carefully put together handloads with match grade bullets will cost you less than half what you will spend on bullets and powder for the .308 and with todays new miracle bullets shots at 1,000 yards with the .223 are being done at some of the major competitions in this country.