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(Just moving this response to it's own thread.)

I noticed that the 1/55 dated No.4 Mk2 that I just unwrapped on Saturday had a little paper sticker on the knox form of it's F52 dated barrel that read "O" (unlapped barrel)? We're barrels routinely lapped. What is the practical significance of having a non-lapped barrel? Thanks.
 

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During WWII to speed up production the finial lapping and polishing of barrels was discontinued, tests were conducted between lapped and un-lapped barrels and excessive nickelling did not occur (copper jacket build up in the bore) and lapping/polishing was discontinued. (See attached photo from E.G.B. Reynolds “The lee Enfield Rifle”)

A lapped barrel will have a very smooth bore surface with machining marks removed and have less jacketed material build up in the barrel (easer to clean) after firing. Shooting and proper cleaning of an un-lapped bore is in effect polishing the bore and after shooting several hundred rounds your bore will be much smoother.

The manufacturing tolerances on a military rifle barrel would not be the same as on a well known rifle maker’s match grade barrels, which strive to be dimensionally perfect but these final production Mk.2 barrels are noted for being very accurate, lapped or unlapped.


"One of the last steps a custom barrel-maker does before shipping a tube, or installing it on a rifle, is to hand-lap the bore. Lapping uses a lead slug cast to the bore, impregnated with abrasive and worked back and forth through the newly crafted barrel. The lapping removes miniscule tool marks from the bore with the intent of reducing fouling and improving accuracy.


High-volume production barrels do not get lapped. Most production barrels are more than good enough for their intended use, and the time and cost of hand-lapping is something most deer hunters wouldn't pay for."

http://www.rifleshootermag.com/gunsmithing/finish_0723/
 

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RGL01, My UF55 that I unwrapped didn't have a sticker similar to yours, but I don't think the barrel was lapped either. I've only had one No.4 with a lapped barrel and it was marked on the knox form with a D within a circle. I can only imagine that the barrel wasn't up to production standards and needed further attention.

Regards, Brad
 
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