A friend has a Starr cartridge carbine, low serial number. About all the info I can find says they were chambered in 56-50, however this one has a .535 groove diameter barrel and the chamber is shorter than my Spencer 56-50 and to me looks more like 56-56.
Any info on them or a source of info would be appreciated.

According to John McAulay in "Carbines of the Civil War," the Starr was initially produced as a percussion breechloader with a linen cartridge featuring a 444-grain .555 bullet over 62 grains of powder. Toward the end of the war, the carbines were converted to rimfire and modified to accept the .56-52 Spencer cartridge.

The Starr Carbine was chambered in a short rimfire cartridge almost the same as the Spencer but not exact .the Spencer round would chamber and fire though.
The original round is quite scarce and sought after by cartridge collectors.

The 56-52 would fit the chamber but the 50 cal bullet would just rattle down the barrel.

One story I read (can't find it again) said the initial Starr cartridge carbines were chambered for 54 Starr cartridge but production for the army contract was for the 50 cal 56-52 or 56-50.

Because the barrel is a bit large for the 56-52 I have been wondering if it is for the Starr cartridge, the barrel is also 5 groove.

The cartridge Starr serial numbers were said to start at 30,000 this one is 30,057.