It's not C&R unless the conversion took place at least 50 years ago. It's the exact reason why the Spanish 1916 Mausers that were converted to .308 for the Gaurdia Civil didn't become C&R until fairly recently despite their much earlier original date of manufacture. C&R only covers the date for the entire weapon, not the receiver, that's why you can't C&R actions or barreled actions. It has to be a "complete" weapon per BATFE regulations. It doesn't have to work, but all the parts must be there. The only weapons that are exempt from the modification rules are weapons built on "Antique" actions/receivers that are dated in or before 1898. A good example of this are pre-1899 Mosin-Nagant actions that have been re-arsenaled as recently as the 1970's by Finland. They qualify because they are considered Antiques and theoretically don't even require a FFL for transfer. The reason you can get a .308 Garand from the CMP is that the CMP has different rules that allow them to sell via interstate commerce to non-FFLs.
So the short answer is: If the conversion to .308 is more recent than 50 years, and you aren't purchasing it directly from the CMP, it's not eligible for C&R transfers.
The workaround is if the seller has all the original parts, they can re-assemble the rifle to .30-06 and make it re-eligible for C&R and then just ship you the .308 parts along with it. Stupid law, but what can you do.
P.S. The workaround was specifically suggested by the BATFE's FTB to me about two years ago when I ran into a similar situation.
So the short answer is: If the conversion to .308 is more recent than 50 years, and you aren't purchasing it directly from the CMP, it's not eligible for C&R transfers.
The workaround is if the seller has all the original parts, they can re-assemble the rifle to .30-06 and make it re-eligible for C&R and then just ship you the .308 parts along with it. Stupid law, but what can you do.
P.S. The workaround was specifically suggested by the BATFE's FTB to me about two years ago when I ran into a similar situation.