BCG's in original Colt AR15/M16?

[Armati]

Original Colt's had chrome BGC's. Why did Colt move away from this? Did this actually improve reliability?

IIRC some Colts had jeweled bolt carriers. Has anyone else seen this? I believe the idea was it would better hold an LSA type lubricant. Can anyone shed any light on this?

At some point there was a run of aluminum bolt carriers (during the 80's?). Was this ever a Colt part? What ever came of these?

[ZRH]

I don't know for sure, and the M16 review documents arent available online but generally: Hard chrome is wear resistant but parkerizing holds lubricant better (it is porous).

An aluminum bolt sounds... retarded, to me. I have read that the very early AR-15 prototypes were tested with aluminum barrels but have never seen a picture of one.

[scottryan]

Colt rifles have never came with a jeweled moving parts assembly.

There could have been some engineering prototype made like this but I have never seen one.

A chrome moving parts assembly was used due to the slickness of the chrome and was though to provide some of the benifits of lubrication.

Parkerizing holds lube better and it was discovered the chrome was not needed.

It has no effect on reliability either way.

[BSmith]

My friend says that it was blackened for a similar reason the M1 Garand gas block was: shiny stuff generally isn't a good idea.

[ZRH]

It's not blackened, it's parkerized, it's a phosphate conversion coating that reduces wear/holds lube. Your friend apparently forgot about the dustcover, and that lot of model 603s had parkerized bolts and chrome carriers.

[BSmith]

Is blackened an actual type of finishing (for metal, not fish)? Maybe I should have used "darkened" instead. The point being that it went from shiny to not shiny.

I'm no history buff on this stuff, and I know he's *generally* right if not very close to right. Is he also incorrect about the M1 Garand?

[Der Waffenkollektor]

It has no effect on reliability either way.

Not so. Chrome bolt carriers suffered reliability problems from peeling. In the Army technical manual, it states any chrome bolt carriers are for training only and not to be sent into combat:

From: ARMY TM 9-1005-319-23&P - AIR FORCE TO 11 W3-5-5-42 - MAY 1991:

NOTE:
There are bolts and bolt carriers on fielded rifles, some with chrome-plated exterior surface finishes and some with phosphate coating Both finishes are acceptable under certain operational requirements and or restrictions Phosphate-coated bolt carriers are required for divisional combat units Chrome plated bolt carriers are acceptable for divisional noncombat units and training center units. Chrome- plated and phosphate-coated bolt assemblies, bolt carrier assemblies, and repair parts for these assemblies may be intermixed In any combination, with the following exception:
Phosphate-coated bolt carriers are required for all deployable and deploying units Chrome-plated bolt carriers are acceptable for nondeployable and training center units.

That isn't in the context that I was refering to.