No legal, new-production lowers are "Mil-Spec", as Mil-Spec requires M16/full-auto features. However, the specs do specify a number of important things, such as parts sizes and finishes that are relevant for civilian semi-auto ARs.
Colt is the only manufacturer who makes non-standard, non-Mil-Spec-sized parts. On their old "pre-ban" guns, the pivot pin was oversize to make using a standard M16 upper difficult. The lugs on the receivers were larger and the larger hole was drilled off-center compared to a standard-size hole. Adapters are available for these (both large-to-small and small-to-large). Many of these older Colts also had a pinned-in steel "auto-sear block" to prevent the installation of a USGI auto-sear, or even a Drop-In Auto Sear. The block can legally be removed, but it is difficult to do, and is usually a task for a professional.
"Post-ban" Colts went back to the standard-size pivot pin, but switched the fire control parts to larger, non-standard pins. This obviously affect all of the fire control parts.
Virtually all ARs have some type of auto-sear block. This is commonly done by simply leaving a portion of the lower unmachined, compared to an M16 lower. There are three styles:
- "Unmachined Side" (many DIAS are actually compatible with this type, and it does not interfere with any aftermarket trigger). Manufacturers: Colt (SP1s), Bushmaster, Armalite, Olympic, Essential Arms, DPMS.
- "High Rear Shelf" (the "shelf" area just behind the selector is left about .5" higher than normal. This can interfere with some aftermarket triggers, and requires AccuWedges to be altered). Manufacturers: Rock River, ASA, PWA
- "Full Bridge" The latest Colts have a full "bridge" of material left unmachined in the lower, instead of just one side.
Finishes are another issue. Mil-Spec requires hard-coat anodizing, followed by a black dye. This is what most manufacturers do on their aluminum parts, but there are/have been exceptions:
- Many older Olys used some kind of bake-on paint finish.
- DPMS uses a teflon coating over the top of the HCA
- Hesse, Vulcan, and a few other parts vendors offer receivers with no HCA at all, with various finishes.
Last, it may be worth noting that Rock River purposely changes a couple of dimensions on their receivers. They leave the lower narrower where the takedown lug goes into the lower, and they machine the take-down lug on their uppers narrower to fit this. Using any other upper will usually require forcing the receivers together, sometimes with a mallet, until the lower has been pried apart enough for the lug to fit. This shouldn't be an issue with an RRA upper.