AIA No4 Mk4

[Bruce_in_Oz]

Whilst the "National Match" M-14s were specifically fitted with "un-chromed" barrels of extremely fine tolerance, the (X)M-21 snipers were, apparently, mostly built on TRW rifles with TRW CHROME lined barrels. The un-chromed tubes were supposed to deliver tighter groups at "polite" venues like Camp Perry.

However, on the "two-way rifle range", especially in soggy, tropical places, chrome-lining was (and still is), a "good thing".

A major part of the trick for a "good" chromed barrel is to have the rifling formed without sharp internal or external corners, so that the chrome deposits on the steel as evenly as possible, AND provides no "lumpy bits" to be torn off by the first bullet and thus compromise the coating. H&Ks take the "no sharp edges" idea to extremes, but the rifling form is ideal for plating.

I once had a TRW specimen that got the AMTU treatment and wore a nice bit of Redfield glass. It retained its early-60's chrome-lined barrel and shot VERY well with Oz F4 ball ammo, and astoundingly well with a couple of different 150gn "hunting" bulleted handloads.