I thought I posted a reply earlier today, but I do not see it. Apologies if I have served up a duplicate.
First, thank you all for your helpful replies. I had read the thread referenced by malco, but wanted to see if anybody had any new ideas.
I went back and slugged my barrel again and again got a .2694/.2695 max groove diameter. I also re-read Ray Ordorica's article in Handloaders Digest 15 and saw that when he slugged his bore he found, "that it is .269-inch at its tightest groove diameter."
I shot three three shot groups at 100 yards with a bolt peep and some old Norma 156 gr. soft point ammunition the previous owner of the rifle was kind enough to sell me. Groups were 2.875", .915" and 4.25" The front sight bead was larger than the 8" black bull, so I had to hold at 6 o'clock. The groups were around 20" high with the Lyman screwed down all the way, so I will have to put in a higher front sight. Function was perfect, but all the fired cases seemed to have a stretch mark about one half inch up from the head. I cleaned the cases in a sonic cleaner and used a piece of coathanger with a 90 degree bend and a sharpened tip to check for a seam inside the case at the stretch mark. It was definitely there so I will not be re-using these cases.
I had already received some new Norma cases, which were about 5 grains each heavier than the old ones, so hopefully they will tolerate the large chamber somewhat better. A problem with the new cases, however is that the primer pocket is either too shallow or tapers toward the bottom. I had to use two primer pocket uniformers, one .206 dia. and one .208 diameter to open them up enough to seat the Fed. 210 Match primers. I have used them for years in other large rifle cases without a hint of trouble. I am going shopping at my reloading supplier with my micrometer in my pocket. Anybody have a suggestion for a large rifle primer that will fit the new Norma cases?
As Kiwi bloke pointed out, the Carcano bullets definitely will have to be seated quite deep in my rifle. I measured at least one third inch push back when a Carcano bullet was loosely seated in a fired Norma case and chambered.
I will next try loads using IMR 4350 and H4831 and the Lapua Mega 155 in the new Norma cases. At OAL of 3.010 or less they feed and chamber perfectly. Looking for Hornady .264 160 gr. round nose to try after that. If nothing else works I guess I could have the throat opened up to use the Carcano bullets.
Not a normal load development process. But fun!