Can an AR-15 receiver be converted to an AR-pistol by removing the buttstock and installing an arm brace?
That makes it a rifle. It was sold as a rifle. It cannot be converted by the simple addition of the particular parts to reclassify it to be a pistol. It can be made into an SBR, which will cost $200 and take about a year to get the tax stamp. You cannot configure the lower as an SBR (attach to it an upper with a barrel of less than sixteen inches) until you get the tax stamp from the ATF. The law is clear on all of this stuff. Just abide the laws and stay cool with ATF. They can get really ugly if you cross them...The complete lower comes with a mil-spec collapsable stock on it...
It may not take an entire year. There was a backlog but ATF put extra examiners on the payroll, and I've heard it can get down to less than nine months. Next time you're in a place that deals in Class III stuff, ask them what's the turnaround on the tax stamps. You might be surprised.... . . a year-wait for the tax stamp is better than attorney fees or years in prison.