I've owned three DPMS LR-308s and they are a good entry system. Each of them had 18" barrels and were sub-MOA shooters. Two of them had aftermarket custom barrels but my current one is a stock SASS configuration.

With the AP4, you can expect battle rifle accuracy 1.5-2" accuracy at 100 yards, but it is a good platform to build from. Selection of aftermarket barrels seem to be better for the LR-308 platform vs the AR-10.