Can anyone here enlighten me as to the difference of these 2 types of lowers? I have ordered a low shelf billet lower, and now I'm hoping it will work with my RRA Predator Pursuit upper. The lower is available at Denny's Guns, AKA Global Tactical, but it's pricey.
I'm certainly not an expert, but my impression is that one type of lower can be made into a selective-fire (full automatic) M-16/M-4 clone with the right M-16/M-4 parts (and the right license/permit to do it legally) and that the other type could not be made into an M-16 clone (but could be rigged to be made into a selective-fire rifle with a certain product, again with the proper license/permit for legal purposes). My impression is that either type should work fine for a standard semi-auto AR-15 build.
You might post in the AR-15 forum for a more definitive answer.
I have no need or use for full auto...I much prefer one shot, one kill to spray and pray, as I don't foresee the need for squad tactics in my AO. If I wanted for oppositional forces to duck down and cease firing in order for someone in my squad to be able to advance or flank, then it might be handy. But I live in a quiet neighborhood, thank God.
An old ARFCOM thread that lasted for 4 weeks and I don't even know haw many pages was "What if the Zombies Come??" There's still replies to this day that speak of Zombies... Literally what they meant was that the walking dead uprise from graves... Not the mindless masses of idiots.
So you may live in a nice quiet neighborhood, but I'd recommend at least a 3K Zombie stash of ammo, just in case
A truly mil spec lower receiver has the lower rear shelf you speak of which provides clearance to accomodate the auto sear/spring assembly in a M16 select fire lower receiver or a drop in auto sear unit (dias) used in conjunction with a non-drilled AR15 lower receiver to achieve a select fire weapon.
Either of these devices will also require the other 5 additional M16 parts to reliably operate as a select fire automatic weapon:
The other method of achieving a select fire platform with the AR15 rifle is to install a registered Lightning Link (sp?) formerly manufactured by SWD and still available on the civilian market. This device still requires the "mil spec" lower rear shelf in the lower receiver and a Colt SP1 style bolt carrier, but does not require changing any of the semi auto AR15 parts to full auto/select fire M16 parts to operate reliably.
Your varmint/target build will work just fine with either one, makes no difference in the case you mention.
BTW, billet lower receivers/upper receivers are the cat's ass when in comes to AR's.
Have fun with the build and please post results/accuracy details.