well you have been quite well informed but you have also been informed in such generalities as to be misleading.
1. AR15s are not available in 308. such cartridges like 308, 243 and 22-250 require a more robust design and are only available in AR10 variants. probably the most versatile caliber available for the AR15 is the 6.5 grendel which is a very good round out to 200 yards for hogs, deer and paper targets. it is not difficult to get ahold oh but if you reload then this would be a very good move to invest in dies for it as it is similar in cost to 30-06(which out of a semi is not a lot of fun).
2. it is true that Colt has begun cutting corners in their production as many other companies have which has lowered the quality of their current stock of rifles and they do charge an arm and a leg for the Colt brand name however they are still a good AR manufacturer with a respectable track record.
3. even though I despise the company and fanboys that tote them, you may look at a smith and wesson MP15 as very few that buy them have a bad experience and their basic 5.56 sport model is very competitively priced(around $600 usually).
4. it is not always cheaper to buy the uppers and lowers separate and in many cases this is false. it used to be cheapest to buy the stripped lower receiver and then buy a rifle kit which was everything that you need to make the rifle functional except the receiver so it could be shipped right to your door without a federal firearms license and paperwork. however a few years ago it became cheaper to buy a stripped lower and lower parts kit and then buy the complete upper but now it seems that many companies are offering complete rifles for close to the same price as a DIY rifle. it all depends where you buy from.
I hope that my rambling has proven educational or at the very least entertaining.
ignore my complete lack of capitalization. I still have no problem correcting your grammar.
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Bean counters told me I couldn't fire a man for being in a wheelchair, did it anyway. Ramps are expensive.-Cave Johnson.