X2. Don't even bother doing otherwise, way too much hassle. I tried officially changing caliber on a 5.56 AR, they wanted to start a new FRT entry and get my rifle verified. Probably would have taken 3 years.
Tried to register my IUR from a SA20 to a 15.7" barrel - same thing, they wanted to start a new FRT table entry for a SA20 with a 15.7" barrel.
They were asking me on the phone if I had a gunsmith convert the AR from 5.56 to 7.62. I had to try and explain that the upper gets changed with 2 push pins and I convert the caliber in 20 seconds. They had absolutely no clue about firearms.
Totally bonkers, I abandoned both enterprises.