A semi-auto rifle for a trunk gun while spending $500 huh?
Well for that you have quite a few choices.
AK with side folding or under folding stock (the bolt handle is on the right side so it would work great for a lefty). You can find one at any gun show. Yugo's and the Polish ones would be the best for that money IMO.
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EAA Saiga AK in 7.62X39. Basically it's an AK with a sporting stock.
Everything that is said about the AKM you can say about this as well (stone reliable/you can drag it behind a truck and it would still probably work etc). For $270 this would be the one I'd pick to stick in my trunk as it's cheap, it works well, it's semi-auto and you can always fit it out with a scary folding stock and a pistol grip later if you want to.
http://www.militarygunsupply.com/shop/item.asp?itemid=FIEAA762
If it gets knocked around a bit in your trunk and it picks up a few dings or scratch's it isn't that big of a deal as it's an AK and it was $270 plus tax.
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Ruger Mini-14 (I personally like some of the others better, but the controls would work for a lefty as the bolt is on the right side). You can find one of these at just about any sporting goods store that sells guns.
Ruger Mini-14/The price here is more that $500, but you can easily find them for cheaper.
http://www.impactguns.com/store/736676058013.html
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Daewoo DR-200 in .223 Rem (also has the bolt handle on the right so it would work for a lefty), they seem to be going for between $600 to $800 though. I've seen some over at
www.ar15.com and
http://www.thefiringline.com/forums/forumdisplay.php?f=16 for $600 to $800.
They don't import them anymore, so if you got one you'd have to get one used. I saw one at a gunshop for $550 ($600 out the door with tax) a couple months ago and it had sat there for 8 or 9 months. Finally the owner just took it back and decided that he didn't want to sell it. So you can find them cheap sometimes.
Daewoo DR-200
http://world.guns.ru/assault/as32-e.htm
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The Kel Tec SU-16 .223 Rem carbines are going for about that amount as well (and it just so happens that they have their bolt handle on the right side as well).
Kel Tec SU-16 .223 Rem carbine.
http://www.impactguns.com/store/kel_tec_su16.html
Anyway due to price and the semi-auto requirement these are basically your options.