Here is a more through explanation from Vic's " Mosin-Nagant.Net web site
One of the most puzzling markings commonly found on the m/27 is the capitol letter "F" found over the date of the rifle on the barrel shank. This letter was erroneously thought by others to be the designation of Swedish Fägestra steel used in the fabrication of the barrel production. This is not the correct meaning of the "F" marking. In actuality the "F" marking is one of three that were used. The trial rifles of the m/27 series were marked with an "A" to denote the shape and contuor of the neck area of the chamber. The Finnish army had been experimenting with different bullet loads and shell casing measurements to determine the optimum cartridge to achieve the greatest accuracy out of the m/27. The first shell designation with a modified casing measurements in the neck area-that was made with a much steeper shoulder than the normal Finnish 7.62x53R cartridge, was the "A" cartridge designation. This shell was not adopted for wide spread use and the marking was dropped. No known m/27 that I am aware of are found with this "A" chamber shape marking. (The author would be glad to offer a great trade should one pop up!)
The other marking that was used on the barrel was the capitol letter "F" which identified the chamber measurements of the "F" designated cartridge. This cartridge used a shallower neck incline as found on early Finnish 7.62x53R ammunition. The final marking of "D" is as outlined above. The final letter code of "D" indicated an alteration of the chamber diameter and shape to accept the Finnish D-166 cartridge that differed very slightly from the "F" chamber. So in review the barrel markings of "A", "F" and "D" were marked on the barrel of the m/27 just above the date and between the serial number to indicate the barrels chamber characteristic and what type of ammunition was capable of being fired through the weapon.
