whoa. Just stop right there. No 18" pistol barrels. That's illegal. I'm not sure where to take this so I'l leave it to others to comment.
Is that specific to your state or the OP's state? If not, please show me a USC or CFR reference that validates your claim. Here's how USC Title 18 Part 1 Chapter 44 Section 921 defines "handgun:"
(29) The term �handgun� means�
(A) a firearm which has a short stock and is designed to be held and fired by the use of a single hand; and
(B) any combination of parts from which a firearm described in subparagraph (A) can be assembled.CFR Title 27 Chapter II Subchapter B Part 478 Subpart B Section 478.11 defines handgun the same way. The CFR also defines "pistol," "revolver," and "semiautomatic pistol," but there's no reference to maximum barrel length in the USC or CFR to my knowledge:
Pistol
A weapon originally designed, made, and intended to fire a projectile (bullet) from one or more barrels when held in one hand, and having (a) a chamber(s) as an integral part(s) of, or permanently aligned with, the bore(s); and (b) a short stock designed to be gripped by one hand and at an angle to and extending below the line of the bore(s).
Revolver
A projectile weapon, of the pistol type, having a breechloading chambered cylinder so arranged that the cocking of the hammer or movement of the trigger rotates it and brings the next cartridge in line with the barrel for firing.
Semiautomatic pistol.
Any repeating pistol which utilizes a portion of the energy of a firing cartridge to extract the fired cartridge case and chamber the next round, and which requires a separate pull of the trigger to fire each cartridge.Again, I'm unaware of any USC or CFR guidelines related to maximum barrel length of a "pistol" as you referenced. Please show me the language here or provide me a link.
