Not exactly what you’re asking for but maybe this helps:
I have the 5.56 Jakl upper, it has a 10.5” barrel and the length from flash hider to picatiny rail is 20.5”.
The .300 BO has an 8.5” barrel so I’m assuming you would be at 18.5”.
Hope this helps and hopefully someone with a .300 upper will come along and either confirm or fix my measurement.
Good Luck