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:
M16 hammer
M16 trigger
M16 disconnector
M16 selector
M16 bolt carrier
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.
