I have the opportunity to purchase a Drilling of an unknown caliber. I was able to inspect it today and found that it is marked 8,7 m/m over 72.

What is puzzling though, is that the grooves measure .360 (9.14) and the lands measure .345(8.76). If my math is correct, an 8,7 should measure .342?

Can anyone point me in the right direction? My cartridges of the world does not list any 8,7.

Quite likely that it's a 9.3x72 R for lead bullets. Do you have a picture from this drilling? Year of manufacture?

There are no makers markings on the gun, only proof marks (Nitro so post 1912 pre 1939?) and these caliber markings.

After some searching I found this drilling:


The markings are almost identical for the shotgun and rifle barrels.

That Crown over an U, resembles an East German Suhl inspection mark since 1950 according to a list I have.

9,3x72R CIP data:
Pmax: 2000bar
Twist: 420mm (1-16,5")
lands: 8,75mm
grooves: 9,25mm
bullet: 9,57mm (.364")

Martin

lands: 8,75mm (.345)
groves: 9,25mm (.366)
bullet: 9,57mm (.377)

Maybe? Maybe it just has really deep grooves?

Have you slugged the bore?
.360 grooves is a bit smaller than a 9,3 maybee its smaller than normal. Think of it as a .355-.358 for bullets.

Well, it is a non issue as of now. I lost the auction It was a steal for whoever won it, but it went above my limit.

I did some checking on a relatives JP Sauer drilling in 9.3x72r, its barrel is marked 8,8mm over 72.

So I assume the 8,7 is just a small bore, still 9.3, but on the tight side. Probably meant for lead bullets as someone suggested.