Right now, the cost for AR15 builds are at an all time historic low.
I say this based on the fact that I have kept my receipts for every firearm purchase I've made since I was 18 years old.
Aside from my old Norinco SKS that I bought back in 1995 for roughly 100 bucks, my current Bullet Button builds are coming in at quite a value.
Back in 1998-99 I built my first Bushmasters. Assembled lowers were 200-225 dollars, that took an FFL with their fees plus shipping, all told lower assembly roughly 275-300 bucks. Add to that an upper, my receipts show 500-550 bucks for the uppers before shipping. Total for my ARs back in 1998-1999, roughly 800 dollars.
Current prices for ARs are nearing the 800 dollar point again but back around last November it would have been EASY to build a decent AR15 for around 600-650 dollars.
They are just now starting to head back up due to election year hysteria.
There are numerous shops in the Kern County area familiar with the bullet button AR. The most reputable shop to go through will be Wes Morris at 10% Firearms out in Taft. He's been around since the bullet button/Off List Lower thing got kicked off around 2004-2005. He knows his crap.
With a lower receiver from Palmetto State Armory, at 79 dollars regularly or 50 dollars during one of their many sales, you can start off on a very inexpensive bullet button build. Right now it's possible to build a bullet button AR for the range of 750-850 dollars. A price I couldn't beat way back in 1998 when I first started out with this stuff. ARs are extremely easy to build from a lower receiver, it's like an erector set or tinker toys only it fires bullets.
And as for "getting around any laws", the bullet button is perfectly legal if you use it as intended with 10 round magazines. It will allow you to retain all the "evil features" on the rifle including the pistol grip provided you keep to the bullet button AND 10 round magazines. In California if you were to use a 10+ round magazine on a bullet button build you just created an "assault weapon" with the fixed magazine.
I've been doing the AR thing for the better part of 15 years. I have both pre-2000 registered AR15s that are full functioning as well as 10 round bullet button builds.
You simply cannot beat the modularity of the AR15.
As for the facts regarding the ammunition specs. Yes, 5.56x45mm is loaded to a slightly higher pressure than 223Remington but it's not necessarily the ammo that is the problem, it's the chambers of the rifles. 556 Nato chambers are cut to a different spec with a different throat than that of the average 223Remington chamber, firing 556 rounds out of a 223Rem chamber can lead to pressure problems. There are however 223Rem chambers that will work fine with both types of ammo.
As for 7.62x51mm and 308Winchester, the inverse is true of the ammo. 7.62Nato is the lower pressure round with 308Winchester being loaded slightly hotter. Shooting certain 308Winchester loads in a 7.62 Nato chambered rifle can be problematic if the rifle is over gassed.
This information comes from 15+ years of playing with ARs of all kinds including the AR10, every bit as long a time period on AR15.com forums(think of what this place is to fish, AR15.com is 10x that with regards to firearms), as well as nearly 20 years of reloading experience.
I am a self admitted whore for the AR15 and AR10. But then it has a lot to do with respects to the adaptability of the firearm. I have a 24inch bull barreled AR15 that is squirrel rifle and holds roughly 3/4 inch for 10 shots at 100 yards. Then go the opposite direction and I have a 14.5in barreled rifle for home defense. Truth is I could have one lower and switch between configurations on a whim by changing out upper assemblies. I can change calibers between 5-6 rifle chamberings, including 22lr. Or I can go to pistol chamberings if I so desire. There are even the options to go to 50BMG or 510 DTC for a 40 pound 5 foot long bolt action single shot.
