Quoted: First of all, how important/beneficial is to have a carrier with a shrouded area for the firing pin? That isn't mil-spec, is it?
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At best it just keeps the firing pin retaining pin from getting beaten up. It's not critical.
What do you keep for spares for your rifles?
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1) Complete identical spare rifle
2) Spare complete bolt
3) spare firing pin, several spare firing pin retaining pins, spare cam pin x2, spare rings, spare extractors with spring & insert (several of these), spare hammer & trigger springs, spare disconnector & spring (x2), spare buffer springs, several spare hammer/trigger pins.
I've got more, but that is what I always try to have on hand.
that any failure is rare?
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Failures are rare - it's mostly springs that wear out. But a spare bolt (heck a bolt-carrier group is even better) is the fastest way to get your carbine back to function should something go south during a class. Hammer/Trigger pins have been known to break (rare but it happens) the same with cam pins.
How important is a chrome-lined carrier
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Very important.
? I guess I'm confused by the bolt's movement as the gun is fired. Can anybody explain when/how it rotates?
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This gets conda complex. I'd suggest you download FM3-23.9 from the Information button above and read it. There is a very detailed (with diagrams) explaination on how the bolt is pressurized from the gas and causes the bolt carrier to move to the rear (while the cam pin forces the bolt to rotate to unlock).
Also, any downsides to installing an A-1 site? They seem to be a great deal at $13, and are very simple. I figure if i'm not shooting out past 300-yards, I don't even need an A-2.
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?? You can't install an A1 sight on an A2 upper.
A1 uppers run $70-$140.
If you are talking apertures, then the A2 aperture is better than the A1 (many of us with A1 uppers use A2 or Ashley apertures in the A1 uppers).