The military is responsible for the location of the serial number on the AR15. For the military the serial number is for inventory purposes so the location of serial number is for ease in reading it. When the AR-15 was approved there was no legal reason to change the location of the serial number. Also no one back then could have anticipated how popular the AR-15 would become.
That was my point.
However in 1968 GCA ‘68 created a reason to address the serial number location, either by requiring it to be moved or duplicated on the upper, or by writing the statute and regulations in a manner that clarifies the definition - along the lines of what the ATF did, 54 years after the fact, and 50 years after it came into existence.
More to the point, even in 1968 there were numerous rifles for which the definition of “receiver” was fatally flawed when the Treasury department wrote the implementing regulations.
Why did it take them 50 years to change the regs? Because most fed attorneys believe that being consistent is far more important than being accurate or fixing a mistake. The ATF only moved to fix this 54 year old mistake when it became clear that courts were very rigidly applying that 50 plus year old incorrect definition.
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I disagree with your thoughts on AR-15 popularity.
Yes, no one envisioned it being the most prolific civilian rifle in US history. That’s mostly an artifact of the 1994 AWB and subsequent efforts to ban the AR-15 and other modern semi auto sporting rifles.
But prior to 1994 Colt still sold on average about 10,000 of them per year, and that’s a significant number, and roughly 300,000 since 1964, not counting the small number of SGW copies that started showing up after the Colt Patents expired.
More to the point, Colt has to jump through a lot of hoops and make a number of changes to AR-15 parts to satisfy the ATF that it was not readily convertible to a select fire or full auto configuration. Colt would not have bothered if it didn’t think there would be a substantial market for it.
You have to consider that surplus, rebuilt and even new manufacture M1 Carbines were very popular in the 1960s. Consequently, for the same reasons the USAF was interested in replacing the M1 carbine with the M16, it was reasonable for Colt to assume the semi auto only AR-15 would find a viable and even lucrative commercial market.