Bolt cocking piece loose

[Bruce_in_Oz]

Chrome plating critical gun parts is potentially fraught with "excitement".

There are two sorts of "chrome plating".

Commercial "decorative" chroming, like what one used to see before car bumpers became plastic, is generally applied after a preliminary "undercoat" of copper is laid down. It tends to be applied fairly thickly, about 2 to 4 thou. The problem is that it comes out "lumpy" and thus usually has to be polished to achieve the nice smooth surface that covered so much of those old Cadillacs and the like.

"Hard chrome" is the satin-finished, slightly greyer looking stuff you find on gas pistons, barrel bores and other interesting bits in firearms. It is generally applied without the "undercoat". It has a texture all of its own and polishing is not recommended. It is often used to build up worn parts. It is INCREDIBLY hard and as a consequence brittle.

Furthermore, the process of plating, which is done in an acid bath, intordices an nasty phenomenon called "Hydrogen empbrittlement". The "loose" hydrogen from the chromic acid brew cheerfully invades the steel substrate during the process. This "loose" hydrogen seriously degrades the steel's ability to withstand shock; hence "Hydrogen EMBRITTLEMENT".

The solution, so to speak, is to bake the plated part for several hours to "cook off" this potentially dangerous Hydrogen. Then you can grind and polish to your heart's content and place the component into service.

Finally, even hard chrome can create interesting mechanical problems. A classic is inside barrels. The current that does the work flows between the bore and a chrome wire running (with any luck) up the centre-line of the bore. The problem is that the key to nice, even coverage, is nice, even current density.

Sharp corners on lands and grooves and nice, square-cut muzzles play havoc with this plan. Sharp "external" corners, like the tops of lands and the bore/crown junction, lead to high current density and greater deposition. "Inside" corners, like the junction of the groove and the flanks of the lands, result in lower current density and thus lower deposition.

That is the BIG reason that chromed barrels on MOST military hardware has rifling with lands having angled flanks and radiused roots and crests. Ditto a nice radius on the runout of the rifling at the muzzle. H&K took it to extremes with their polygonal rifling that looks for all the world like Metford rifling but with fewer "grooves".