Because the U.S. 1934 NFA regulations set the minimum rifle barrel length at 16 inches, Charter Arms made the barrels of the Explorer I rifle and Explorer II pistol non-interchangeable to prevent installing the pistol barrel on the rifle.
The AR-7 barrel has an alignment lug that mates a notch in the receiver.
The rifle receiver notch and barrel lug are on top; the pistol notch and lug are broader and on the bottom.
If a Charter Arms factory-made pistol barrel were installed on a rifle, the extractor on the bolt would be opposite the extractor slot in the barrel, preventing the bolt from closing (plus the front sight would be upside down).
Modifying the pistol barrel to fit the rifle, or modifying the rifle receiver to accept the pistol barrel, would be "making a short barrel rifle" legally requiring federal registration on an ATF Form 1 with payment of a $200.00 tax.

Conversely, after the Supreme Court ruling in United States v. Thompson/Center Arms Co., modifying the pistol to accept the rifle barrel and/or stock, or modifying the rifle barrel or stock to fit the pistol is legal so long as you do not have the rifle stock attached at the same time as the pistol barrel.
It is legal (as a pistol) to have the rifle barrel attached with the pistol grip; there is no federal maximum pistol barrel length.

In the United Kingdom this rifle is a section 1 firearm requiring a FAC
Because the U.S. 1934 NFA regulations set the minimum rifle barrel length at 16 inches, Charter Arms made the barrels of the Explorer I rifle and Explorer II pistol non-interchangeable to prevent installing the pistol barrel on the rifle. The AR-7 barrel has an alignment lug that mates a notch in the receiver. The rifle receiver notch and barrel lug are on top; the pistol notch and lug are broader and on the bottom. If a Charter Arms factory-made pistol barrel were installed on a rifle, the extractor on the bolt would be opposite the extractor slot in the barrel, preventing the bolt from closing (plus the front sight would be upside down). Modifying the pistol barrel to fit the rifle, or modifying the rifle receiver to accept the pistol barrel, would be "making a short barrel rifle" legally requiring federal registration on an ATF Form 1 with payment of a $200.00 tax.
Conversely, after the Supreme Court ruling in United States v. Thompson/Center Arms Co., modifying the pistol to accept the rifle barrel and/or stock, or modifying the rifle barrel or stock to fit the pistol is legal so long as you do not have the rifle stock attached at the same time as the pistol barrel. It is legal (as a pistol) to have the rifle barrel attached with the pistol grip; there is no federal maximum pistol barrel length.
In the United Kingdom this rifle is a section 1 firearm requiring a FAC
ArmaLite AR-7