I will state first and foremost that I am far more comfortable in my role as student..but since you asked..I'll give it a go.
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Chrome lined bores are just that..chrome lined bores. I could only guess that the chrome plating occurs straight after polishing and prior to barrel join in the case of a sxs..I dunno’ but that would almost seem the best manufacturing approach.
I cannot proclaim to know the specific process nor the specific spec (what type & what class) but typically an allowance of .00001 is all that is necessary unless a drawing specifies a thicker application.
That .00001 is designated as the dimensional allowance for buildup per surface so on a bore or any other round surface - an allowance multiple of 2 would apply…in other words a bore that requires an accurate size will have to be over-bored/polished by .00002 to allow for the plating which would then bring the item into spec.
What does that mean in this particular application from a dimensional/manufacturing perspective?..exactly nadda’ since shotgun bores do not require precision nor do manufacturers bother to focus upon that due to lack of necessity relative to functionality…manufacturing money (labor) is better served elsewhere.
One of the key considerations when making preparation for chrome, is the material finish prior to plating b/c chrome will bring out every flaw upon the surface, but this is an aesthetic concern.
That said, and from a practical view relative to bores in firearms..bad surface finish = harder cleaning b/c the residues tend to occupy the space below the surface (metal imperfections)…so smooth bore = easier cleansing.
A common misnomer is that a “good finish” must shine…not true at all as many a dull appearance possesses a very fine surface finish throughout all of industry.
The application of chrome in shotgun barrels is a fantastic marriage of benefit..it’s hard, it resists corrosion, it’s slick, and it cleans up very easily.
I think Greg and others have stated that Regeant purposely left the choke areas void of plating so that choke modifications would be simple using standard HSS tooling (High Speed Steel).
HSS would cut through it no problem provided it is sharp, however tool life will suffer.
If the chrome plating is desirable to a Parker Reproduction owner..any modification to chambers, bores, or forcing cones is a no-no because even with simple polishing..it is gone, with exceptions.
A wadded up pc of Nevr-Dull or other non abrasive polish at high speed won’t touch it…it’ll just polish the surface making it just that much easier to clean provided the regimen is maintained periodically.
The only exception to chrome lined barrels amongst the Parker Reproductions are the 16ga barrels as supplied by Krieghoff which do not “appear” to be lined at quick glance..maybe I’m wrong??
Chrome is nasty stuff for the folks who work with it day in/out.
*Edited to add
Everything I have described is what makes the SSS unique..special attention was paid to bore size etc in order to achieve the best patterns for a specific application..or so the story goes..
The only other feature which would come into play with respect to patterns would be the relationship between forcing cone, bore, and choke..the geometry of these features and the relationship between the three have much more influence on the quality of pattern than that of simple bore size.
As I stated, I haven't any idea whether the forcing cones or the geometry of choke differ in the SSS.
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