Hello everyone.  The post on Remington rolling blocks by frnkeore is very interesting.
   Years ago, while restoring a Borchardt, as action was being polished, kept running into inclusions in surface...some of these actually became larger the more it was polished.  I stopped trying to get them out..and chalked it up to 19th century  archaic steel manufacture.
   Not too long ago, I was building a detail out of 1018 CRS.  Part was about 2"X 3", and in some areas, 
1 1/4" had to be milled off.  I was taking fine finish cut, when I began to see inclusions..scattered in various places and varying in depth.
   So even modern steels can have these defects hidden deep inside.

Many of the early breechloading rifles had actions that were roughly formed by drop forging. The cut to size billets of steel or iron were heated to a yellow heat, placed in the female mould whereupon the male mould was tripped and dropped onto the ingot forging it to shape. Now anyone who has heated steel in a forge knows it gets a scaled surface due to oxydation of the surface and under the impact of the drop forge the metal is deformed and bent and the thin surface scale gets twisted and bent into the metal as it deforms. Usually actions were blued and the inclusions are virtually invisible. With CC Hardened actions they are more visible. Other inclusions can be slag which was accidentally left on the surface of the molten metal before it was cast into ingots prior to rolling and forming to shape after manufacture. All steels and irons are molten when smelted and are cast initially before other processes took place so there is always a chance that some metal will have inclusions in it. They may be scarcer today with modern casting techniques but errors will occur now and then.

Harry

Cold shuts (maybe caused by inclusions) are also a concern with any wrought or forged metal. I have seen one cold shut in aerospace quality aluminum bar which was dye penetrant inspected after machining. It was not discovered until installation of the machined part (fasteners caused it to split) and would have caused a catastrophic failure. It is my excuse for moderate proof testing every new barrel or action.
Chuck