Here's something that I've always wondered and I have been unable to find an answer on the BATF site ...

I understand what a 0 percent lower is, and I also understand that an 80 percent lower means that more machining operations are performed, but what EXACTLY defines when you have, say, an 80 percent lower versus an 81 percent lower (if there were such a thing) ???

What does the BATF use as a definition determining which or how many machining operations are counted towards that first 80 percent ? and when have you gone over the line ?

Is it based on the time it takes to do the machining ? The number of machining operations ? The type of machining done ? What ???? .... because it certainly seems that not all 80 percent lowers have the same number or type of machining operations completed on them.

Anyone willing to cure my ignorance, I'd certainly appreciate it

I don't really think there's a standard, I mean look at all the 80% AR receivers out on the market today, some are finished more than others.  Since there's probably no real way of measuring the amount of completeness, I think the standard is at the liberal of the ATF.

There is no set standard, but it seems to me that everyone keeps the 80%'s so that it takes the average person more than 8 hours to finish.

Millsusaf is correct.  ATF doesn't use percentages to determine the amount of 'completeness'. The percentages are 'marketing' hype.  One companies 80% can be different from another's. All ATF does is determine if the hunk of aluminum is considered a firearm or not.

At one time the basis for determining whether a block of aluminum was considered a firearm was that if it took the average person less than 8 hrs to convert the block so that it could fire a single round, it was considered a firearm. Appearance/finish does not enter into this 8 hr determination, nor does the gun have to operate 100% or as designed, it only has to fire a single shot. If it took the average person more than 8 hrs, it was still considered a block of metal.

I don't know if ATF still uses this rule, as they don't seem to want to publish what they use to make these determinations. But, as far as AR15 are concerned, they seem to concentrate on 3 areas of completion....the magwell, the fire control area, and the buffer tube tower/hole.  If no fire control holes are drilled, any two of these items can be completed and it will not be considered a firearm. This is why you'll see various combinations of completion of these areas, which usually vary by manufacturer. It depends on what the manufacturer sees as the most marketable solution.

Selling an unfinished firearm (which really isn't techincally a firearm) requires a sample to be submitted to the ATF for an evaluation.

The ATF really has no standard.  For example if two companies submit the same sample, one might get a cease and dissist and the other might get a green light.  I have seen it happen with 1911s several times.  Sometimes the ATF says the absense of rails is enough, other times they want the fire control holes left out too.  In each case you will get a letter and need to operate within those guide lines.  

They also consider wether the item is to be sold to the public or another manufacurer.  A 1911 frame minus only rails are often OK from a machine shop to a firearm manufacturer but not OK for the manufacturer to sell to the public.  

Also the ATF considers wether the item is to be exported, imported, or used domestically.

Finally there is a 'feel' in the gun industry right now that the ATF is clamping down abit because of the proliferation of the DIY crowd and the ease afforded to them by the web.  This is not based on fact, just a reaction to the increasing difficulty getting things approved.

Back to the AR15.  In the end anyone who intends to sell a 80% in any number should send in a sample, even if they have seen the exact same thing sold elsewhere.  I would guess that every source you have seen on the web have gone through this and have their own individual letter.