Looking at buying a factory Daniel defense mk18 sbr. I googled the hell out of having multiple uppers of various calibers for one registered sbr lower. I thought I figured it out that it was ok as long as you have the original upper and can reassemble the gun as it was registered. So I posed that question to the dealer in an email, and his response that since it was a factory sbr it could not have multiple uppers. Any input is appreciated. Two nights of google search has me confused by my dealers response.