Perhaps an enterprising gunsmith counterbored
the barrel to repair damaged rifling at the muzzle.
The Model 851 has a 6-shot cylinder, and the rear of the chambers are not counterbored
for the rims of the cartridges.
The breechface of the barrel is counterbored
for about .
The muzzle is counterbored
and the metal has been re-Parkerized.
44 Flattop has the chamber counterbored
to enclose the rim of the cartridge--which also hides the rim from view, making it difficult to ascertain which is the empty chamber.
Both these holes will be drilled from the front at one size and then counterbored
from the rear to another size.
22 rimfire--excessive dry firing may damage the counterbored
Up on the head a counterbored
bolt face was incorporated along with the plunger-type ejector that has become so common-place today.
As such, the annular rim surrounding the counterbored
bolt face is interrupted to accommodate the 1/4" wide extractor As to its strength and reliability, this extractor is the same as that used on the M-16 rifle; as such, it takes about as big a bite of the case rim as does the '98 Mauser.
The bolt is counterbored
to enclose the head of the rimmed case and uses a plunger-type ejector that throws empty cases well clear of the shooter.
However, the entire rear face is counterbored
, creating a rim around the outside of the cylinder.