Okay, I am restoring an original Armalite / Colt Model 01.  It will see both semi and full auto fire.
I am going to try to keep everything as original as possible, but change out a few things (such as the buffer
and fire control group) for reliability and to reduce wear and tear.  Very minor updates such as that.

The BCG is what is concerning me now.  The bolt carrier looks good.  Gas key staking, etc.  Everything looks smooth
and in good shape.  I see no problems from just a visual inspection.  But is that enough?
I am going to replace the large head firing pin with a modern firing pin.  Most importantly will be the bolt.
I will probably buy an aftermarket chrome bolt such as from Young Mfg.

Is there any reason from a reliability standpoint why I would not use the original bolt carrier?  I am assuming if it
works, it works, and a bolt / firing pin upgrade will keep me safe and reliable in full auto (I won't be doing much of that
but, well, you know.  That is the magic of the machinegun).  Thanks for any advice.

I have two older and well-used bolt carriers that have seen a lot of F/A fire with no real issue. Had to restake a gas key once, but that's it. You'll be fine.

I don't know if they were unitizing the carrier keys back then with aviation-grade Permatex Form-a-Seal compound, but the main thing you have to worry about is the key coming loose.

If I were going to use it as a beater like you plan to, I would just put the original Colt stuff aside, build a reproduction upper, and go to town with a modern Chromed Carrier assembled to Colt's standards, which included the Permatex, or use Black Loc-tite after checking for fit of the gas tube to the correct carrier key.

The problem is that most carrier keys on the market don't meet the Mil-spec, which has very tight ID tolerances that can't deviate above or below the ID.  Additionally, the steel needs to be of a certain grade, and a certain harndess.  When I have staked my carrier keys with my compression-staking tool I have physically felt a huge difference in surface hardness in an after-market chromed key that I have, and it didn't give me warm and fuzzy at all.

There are many bolt carriers and keys on the market that meet the mil-spec. And there is a crap-ton of surplus (that obviously meet the surplus) out there, too. All that said, again, yours will almost certainly be fine, OP. Restake your key if you start to develop gas issues.

It seems these days the term " mil spec" gets thrown around a lot,Like its
More for marketing tool than anything,
Wouldn't military specs on rifle parts be something of a secretive nature?
How do you know who does and doesn't meet spec?

If it has an early FP retainer, I would replace it at least temporarily with a current cotter pin type for range use. I also favor leaving Edgewaters on the shelf. There's a reason some of these parts were redesigned. Make sure the rings are in good shape and check the tension on the ejector.

I also vote for building a second upper for it. I would make one that looks close, and put the original stuff away for later. There's investment value there with a transferable.

However:

How do you know who does and doesn't meet spec?

Most are purchased from same sub contractors.  Obviously the Colt and FN FA parts are definately true milspec and not many secrets out there when it comes to military rifles especially after one is captured.  The secret would be vendor X and where they actually obtained their product that they are selling for less.  If it's truly made of the spec steel  / aluminum,  spec heat treated,  spec dimension, spec coated  and spec tested then it is milspec.  

OP,  as stated why take a chance on possibly damaging the rare expensive pieces when modern improved pieces are readily available.  What these pieces are really depends on you.  I wouldn't really call an 01 BCG overly expensive but small parts like original FP retainer and firing pin are nearly impossible to replace at times regardless of budget and edgewaters can be replaced for pennies on the dollar with something less prone to breakage.  Run it wet and most internals will be fine.  FCG's don't wear out like talking about it but the earlier pieces are also hard to come by if looking for particular stampings not to mention carbon steel gas tubes which are more prone to problems over time than stainless.  Keep in mind your not gonna be running it in a combat situation.

I almost like the dedicated upper idea for regular FA use but I also will pin most any of my upper builds on a registered lower when opportunity knocks with the exception of target barrels and internals.  Would they work sure but that's not there intended use and I'm kinda particular about how they are used and maintained.  Same thing really in comparison of an 01 barrel being worth as much or more than a Compass Lake but neither has the shelf life of a modern machine gun barrel IMO.  Both can be hard to replace in a timely manor with an 01 being much harder to locate.  Not to mention the condition of the 01 barrel that may become available.  Most of that stuff was trashed long ago and only few pieces were actually saved before being deemed junk by those in that decision making capacity.  

Doesn't mean you can't shoot the original 01 in FA I would just personally suggest limited use in original configuration.  I really like Young products and recommend them even over Colt in certain situations but I believe if I personally was gonna buy a FA exclusive bolt I'd use something else as they seem to run a little on the tighter side of clearances IMO and they aren't recommended or warranteed with staking.  

A new Colt MPC or MPF should last a long time at ammo costs today but a spare P or MP chrome Colt wouldn't break the piggy if chrome is desired and a rebuild kit is cheap enough.  All I listed have been tested and were built for extreme use by our military.

Face it most any authentic retro clone is built with the correct pieces for extreme use and if one can afford a registered lower they can surely afford a dedicated upper although I am aware that in many instances,  especially with retro,  having and using the correct pieces is of more importance than other variables.

The military specifications are public information and well defined. Any company that advertises components as meeting that spec almost always does. Read their claims carefully, however.

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Regardless, there is a world of difference IMO between a DPMS 'mil-spec' BCG and a Colt mil-spec BCG. Mil-spec means very little these days. I don't even drink Colt Koolaid, but I do buy Colt BCG components and other parts whenever possible.

DPMS (the current manufacturer) does not offer a "mil-spec" bolt.

Please go find a bolt advertised as "mil-spec" that does not actually meet the mil-spec.

You guys can trust me when I say that no violence has been done to any rare or fragile 601 items.  In fact, here
is the rifle as I have it currently assembled.

The only original 601 parts are the lower and the LPK.  Everything else is either
new, like the BCG and the FCG, or is garden variety 603 retro.  That is an H&R upper in black with a
gold slip ring and a C MP Chrome Bore barrel.  Improved 3 prong, late 60s black furniture.  Nothing high dollar
or rare.

Next to the rifle is my 601 upper.  What I am doing is going through the original 601 stuff piece by piece and
trying to determine if it needs to remain locked up at home packed safely away, or if I can still shoot with it.
For instance, the 601 upper and ejection port door, hey, no problem.  Good to go.  I am going to use a modern charging
handle.  But looking inside to the BCG, is where I came up with the question that started this thread: can I at least use an
original 601 bolt carrier, or should I just use an entirely new BCG.  I think the responses have lead me in the direction of just
buying a new chrome, slick side BCG from Young Mfg.  I like their stuff.  I have one in there now with my H&R upper and it shoots
without fail.

Thanks for all the responses.  Also, do not worry.  I have a few 601 barrels, but they are packed safely away.  I will likely be using
a Green Mountain barrel, or maybe keep the C MP Chrome Bore barrel on there.  That Colt barrel on there right now shoots pretty
straight.  It also does not get too hot.  I think Mr. Stoner was on to something way back when
I know it isn't period correct for the 601, but I can't help but to wonder if an H&R upper ever really did end up on a 601 lower.  The H&R upper
has a rebate in the front lug.  So, by my way of thinking, it was there for a reason ......

I know it isn't period correct for the 601, but I can't help but to wonder if an H&R upper ever really did end up on a 601 lower. The H&R upper
has a rebate in the front lug. So, by my way of thinking, it was there for a reason ......

You can bet your nads it did.  Same as we find H&R parts and pieces on other mfgs models.  When armorers were taking two and making one they didn't have three different mfg piles.  That's a couple of the real positives of the M-16 platform,  basically a moron can repair one and parts interchange with little to no additional modification.  Also violence or not,  anything mechanical will have parts wear and breakage no matter what the costs to manufacture.

I know member Ol Gunner uses his or has used his model 01 in as originally assembled condition.  When they were $3500 on the high end most everyone did.

Almost sounds like you didn't need to really post but most here enjoy reading about your 01 purchase with envy if nothing else.  Maintenance and recommended spare parts are the same for any M-16.  Model 01 parts are just harder to find and more expensive.  Most every 01 I see today for sale or on display has had some of it's original parts replaced.  

One of the reason the early parts are so expensive is those restoring an 01 can increase the value in $Ks with all authentic build parts and without serialed small parts who will ever know.

I shoot the original parts on both the real one and the clone, full-auto or semi.  I inspect them like I'd inspect any other bolt, and I'll probably run them till they break...then replace them with authentic spares.
But that's me, and I ain't about to say it's the smartest approach.
If I were planning to use the 01 a lot––especially on auto––I would probably swap out several pieces, including the BCG.  I do have a Young Manufacturing smoothside unit with no markings.  Runs great.