The twist of the barrel's rifling will determan the maximum weight/length bullet it will stabilize. You can go lighter, and all gun are individuals. As a rule of thumb, 155 Palma, 168, & 175 grain bullets are the weights, that work for long range accuracy in 308's. Many of the polymer tipped bullets are performing as well as or better than the BTHP Match bullets. Sierra's loading manual has a whole section dedicated to semi-auto 308 loadings. I don't currently have a semi-auto 308, but for long range look at Sierra's 155 gr Palma (product# 3256) and their new 155 grain poly tipped match. Hornady also has VERY GOOD poly tipped bullets at 155, 165,& 168 grains in the A-Max line up. They have been pushing the new 185 grain A-Max for a couple of years. It is quite accurate but, just too slow in normal 308's.
I have one friend that really likes the 125 gr A-Max for 600 yard 3 position targets ON CALM DAYS!
I have had two guns in 308, that liked/stabilized 110 grain V-Max bullets. If you gun will shoot them well, it will be lazer beam flat (compared to heavier bullets) and vaporize varmints out to 400-450 yards.
I still have an older Bushmaster AR-15 in 223, from when they were at Windham, and it has always preferred lighter bullets for the twist rate than other brands of AR's and shoots them extremely well.
Keep us posted on what works in your rifle.
Ivan