For what it's worth - I have yet to see a Mosin packed with arsenal cosmoline that had a sewer pipe bore. However, I have seen quite a few that had bad crowns, or had been counterbored at some point in its life. The sewer pipe Mosins I've seen have all been in pawn shops, or rifles offered for sale by private owners.
After cleaning the cosmoline from my 1933 91/30, I found that it had a somewhat dark bore, but no rust pitting. The rifling isn't all that strong in it, but I think it's because the rifle probably saw lots of use from the war - a gunsmith suggested that the wear on my rifle may have been from a cleaning rod. He gave my rifle the OK to shoot, though...it headspaced just fine. It tends to shoot high and left if I don't have the bayonet attached to it...and tends to hold groups around 2-4MOA.
Another observation - the postwar Mosins I've looked at tended to have less overall noticeable wear. So you might want to look at finding one of those if you want a shooter.