The way I understand it you can buy replacement subassemblies or individual parts to repair your gun but the lower receiver is considered to be the legal base of the gun hence the higher cost of lowers.
“The way I understand it you can buy replacement subassemblies or individual parts to repair your gun but the lower receiver is considered to be the legal base of the gun hence the higher cost of lowers.”
BATFE requires that one part of any given firearm must be the serial-numbered part. Legally, this becomes “the gun” and sale is subject to Federal and local law. In most cases, the largest, most complex, difficult-to-make part is the frame or receiver and bears the serial number, model, caliber, manufacturer’s name and other information
Theoretically, no other part can be “the gun” and no Federal law restricts domestic sale of these other parts. There are commercial and practical constraints:
Not every manufacturer will sell every other part to a gun owner: concerns over liability and local restrictions limit availability of parts like cylinders, bolts, carriers, etc. All manufacturers restrict the sale of some parts to FFL holders, certified repair technicians, or other persons according to preference.
Practical and technical constraints involve aspects like installation and fitting. The average owner has neither the specialized tools nor the know-how to install barrels, adjust triggers, perform machining operations like chamber reaming, or understand the concepts involved in setting headspace. The fitting of revolver cylinders and adjustment of timing is another area requiring specialized experience and judgment.
Many parts cannot simply be dropped in, nor replaced in a one for-one swap.
In the case of AR-15-style arms, the lower receiver is the single largest, heaviest, most complex part. It must be manufactured to closer tolerances because every other part is attached to it, and the rest of the gun owes its functionality to the precision and accuracy of the lower receiver. So higher costs ought to come as no surprise. Administrative/regulatory aspects account for only a minor fraction of the cost.