Not sure what you mean by SBP? short barrel pistol? there is no such thing as a short barrel pistol if it is classified as a pistol then it's a pistol if it has a rifle stock then if the barrel is under 16" then its an SBR and needs a stamp or a pinned and welded muzzle device that makes the barrel exceed 16" which then makes it a regular rifle.
For example, I have an AR9 with a 5in barrel if I have a pistol brace on it then it is classified as a pistol. Or you can do like I did and put a stock on it and get a stamp to make it an SBR.
I think he means a pistol caliber SBR…but I’m guessing. Caliber of the round doesn’t matter. .22, 9mm, 5.56, .300 BLK, 7.62x51 etc. all SBR. A rifle with a short barrel.
If it’s an AR with a barrel under 16” and a rifle or carbine stock then it’s SBR, regardless of caliber, (unless it’s a shotgun but that’s a completely different animal) if it’s an AR with a barrel under 16” and no stock, or with a pistol brace, then it’s a pistol. For now……
@shrxfn spelled it out perfectly earlier above. There is no SBP (or short barreled pistol) in the NFA. Never heard anyone refer to a "SBP" acronym in 15 years. SBR or SBS, yes.
Interesting side note, before the GCA of 1968, the original NFA of 1934 specified a "firearm" as a rifle or shotgun with a barrel length < 18 inches. GCA 1968 now defines a "short barreled rifle" as with one or more barrels < 16 inches. And any weapon made from a rifle with an OAL < 26 inches.