Depends on what the OP is wanting a big bore semi-auto for. If the main concern is to get one now while the getting is good, I'd say you really can't go wrong with the usual suspects in 308, all of which I think have already been mentioned (M14 clones, FAL clones, and G3 clones, plus some honorable mentions like the FNAR and Saiga). They're not the only show in town, but if you're buying in an era of uncertainty as to just how damn-fool crazy our government, with its known history of damn fool craziness, is going to get, I'd want something in a common caliber with a big pool of magazines and spare parts floating around just in case things go profoundly sideways.
If I was going to go for a less common caliber (458 SOCOM, 50 Beowulf, whatever) I'd also invest in a lot of brass for them in case the present insanity starts including ammo restrictions or considering certain calibers to be an "evil feature" in and of themselves.
(For the record, I don't think will go that sideways, and am actually guessing restrictions end up as a no go, but if I were worried about making purchases before the government went off the rails, the above are the sorts of things I'd be thinking.)
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