Flattest would probably be some throat burner wildcat like a .17 x .460 Weatherby or .22 x .50BMG. It would not fit in a standard AR and generally be a terrible mess. For an AR type rifle I think NEMO still makes a 300 win mag AR so you could have one custom barreled in .17 x 300 win mag get...
I attended an expensive class there while the range was still owned by the founder and had a really bad experience. I notified the management never heard back from them other than a slew of sales calls. It may be better now.
I have used one, it was a great help with slugs at longer ranges but unless you are a LEO it would be very unusual to need to make a long (75 plus yard) defensive shot and it would probably be hard to justify as well. For me at close range with buckshot it didn't help much more than a good...
It is a great chambering, a real deer slayer and very comfortable to shoot. It is just a 308 with a lighter flatter shooting bullet. Less drop and less recoil than a 308 with a great selection of match and hunting bullets. It is one of my favourites but I want to try 280 ackley next.
If you are even modest mechanically inclined ARs are super easy to work on. The only "special" tool you really need is an armourers wrench, the others make it slightly easier or a lot faster if you are building them for a living. Watch some free videos if you are still worried buy a DVD and...
If you are planning on building one, a great gunsmith told me years ago that the barrel is the soul of the rifle. I agree with him. If funds are limited spend extra on the barrel trigger and scope, for the rest just get functional parts.
Be selective, don't assume a smith is capable just because he or she has a store front or a vocational certificate. I've been charged a lot to have nice guns butchered a few times over the years. It really hurts when it happens.
Try the light springs, if they give you trouble just put the originals back in. My experience is that the spring strength needed can vary a lot depending on the specific components being used.