I already have a papered (SBR) AR lower and would like to get a 9mm upper for it. Does anyone have any recommendations for a magazine block that doesn’t require modification of the lower?

The type of magazines (colt vs Glock) don’t really matter so much as I don’t own any glocks, nor do I plan on it.

I’m guessing I’ll also have to swap out at least the buffer, and maybe the spring, when using the 9mm upper unless there are other suggestions to get around that. It currently has a standard carbine buffer and basic mil-spec spring.

Any input would be appreciated, thanks.