Seems to be more on the order of 20.00 to 30.00 complete BCG versus bolt assembly.
I dont have a set number of spares I think I need but anytime I see good small parts at decent prices or even randomly add them onto my orders. As for BCGs I keep 2 spare bolts and 1 spare carrier. I have several sets of gas rings, cam pins, firing pins w/retaining pins and spare gas key and screws. Id also try to have a complete lower parts kit set aside just as spares, not for building another gun (have another set for that event). These are probably the most important to have on hand, IMHO.
I dont throw out any parts I replace that are not actually broken, I keep those as loner parts for the unprepared. I also dont dip into my spares if I can quickly get a part via normal channels, also good excuse to buy a few more small parts to make shipping worthwhile.
If you have some hi zoot trigger Id try to obtain at least a spare spring set for it or hopefully have a spare stock trigger. Geissele sells spare parts kits for their triggers.
Slowly aquire other spares as money and opportunity allow, things like gas tubes and mil spec receiver extensions etc dont often shit the bed but during a panic or in a ban situation is not the time to try to buy one.
If you use QD swivels Id have one of those in your spares box too.
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I just blew out an LMT enhanced Bolt with ~2000rds on it. Shit happens. If I didn’t have a spare bolt around, my MRP Upper would’ve been out of commission.
BCM bolts are about $70-80 while the complete BCGs are about $170. Colt carriers are $110 IIRC. That’s what I based it off of.
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Where did it break? Lugs or cam pin hole?
Same question. I thought those were bullet proof…
For the average guy a spring kit (all springs), all roll pins, all detents, takedown and pivot pins, a bolt + cam pin or bolt rebuild kit + cam pin, firing pin, should be more than sufficient. A quality bolt carrier is probably not going to fail although I guess anything is possible. I agree with 26Infantry about getting a spare rifle. 
I agree with 556BlackRifle. I bought a spare bolt 18 years ago, and I’ve still yet to need or use it, though it is a good thing to have around. The parts I found myself needing but never having are various springs, pins and detents, not the bigger parts of which I seem to always have in abundance.
We (Forward Controls Design) created couple of kits based on my experience shooting and building ARs, one has just springs, but four take down and pivot pin springs instead of two (the take down pin spring is easy to bend). The other is a detent and pins kit, 4 take down and pivot detents (since they lost easily), 2 selector detents (considered wear items), a buffer retaining pin, bolt catch pin and plunger, and trigger guard pin.
Thought they were too. Bottom lug had a huge chunk break off.
This topic and variations of it have been covered ad nauseum here before. Assuming one wants to actually search for it.
I’ve not won the powerball yet, but I have seen ,while overseas, a forward assist fail and lock the gun up. Rare, but made me question the value of one.
Well, for what it is worth, Stoner never wanted the forward assist. The Army insisted on it. Remember the M1, M1 carbine, and M14 had the operating rod attached to the bolt and you could manually shove the bolt home so they wanted that on the AR15. Stoner felt that the forward assist could turn a minor problem into a major one by jamming a round so thoroughly in the chamber you would need a rod or other tool to get it out.
I want to thank everyone for their suggestions and I will take your suggestions into account as I build up my spare part supply.
Anything can fail and jam up the weapon, FA is but one of the components. My buffer retaining pin broke after 20 years, you can agree it’s no reason to not have one. Also, the tool’s being misused and abused by users is also no reason for its demise. Anything can be misused and abused to cause failure. During testing a few years back, I was able to over rotate any semi auto safety with sufficient torque, out of its SAFE and FIRE detent holes, onto what would normally be BURST or AUTO. This again illustrates anything can fail if one tries hard enough, intentionally or otherwise, but it isn’t justification for its omission given its usefulness.
We put serrations forward of the gas vents on our SBCG to make it into an impromptu FA. A real FA will be able to exert more force with its pawl onto the carrier than a finger actuated/pushed action, we put the serrations there because it’s otherwise wasted space, and it’s a good location to close the out of battery bolt gently.
This is a pre-production SBCG and no longer made, production SBCGs are based on M16 carriers, using Ned’s OCKS, torqued and staked by the experts at Sionics, and NP3 coated.
Any thing can fail, true. But, at the risk of hijacking my own thread, Stoner’s reasoning was that if the bolt failed to go into battery you simply extracted the round with the charging handle and loaded the next round. If that failed to chamber you investigated the reason for the stoppage. What you didn’t want to do was force the round into the chamber and then have the round jammed in the chamber. And to really make things worse the case fails to extract and the rim is torn off. Now you have a real problem. Let’s be clear the only purpose of the forward assist is to force the bolt into battery. The use of the forward assist to “silently close the bolt” is an after the fact justification. The Air Force agreed with Stoner and their rifles didn’t have the forward assist.
The FA’s purpose is to force a round into battery. Whether it’s a good idea or not, while your argument is valid, so is the opposing view. Stoner might not have thought it necessary in the design, the Army did, from what I assume combat experience and what its users demanded. Many combat weapons designed around that time and came after also had a similar function designed into the charging handle, and some, like HK roller locked weapons, either had an indentation with finger grooves, or added the FA (HK G41), so I dare say it isn’t a bad idea shunned by major manufacturers.
ETA: sorry about the hijack 
This is ARFcom subject matter from the late nineties… but… what the hell? It’s a slow day.
Who would know better than the stoner company, knights armament.
(1x) Ejector
(1x) Ejector Roll Pin
(1x) Ejector Spring
(1x) Extractor
(1x) Extractor Pivot Pin
(2x) Extractor Springs, Heavy
(1x) Cam Pin
(1x) Firing Pin
(1x) Firing Pin Retainer Pin
(3x) Gas Rings
That’s the whole BCG minus the bolt and carrier.
Thats a good looking BCG. Thanks for bringing it to my attention.
I confess, I’ve not read much of this thread so if someone has already stated this I apologize. I keep all my spare parts together inside a Colt 6720. I’d like to shoot 1000’s of rounds per year but I don’t. So if my BCM goes down the Colt steps up. Highly unlikely either one would go TU on me.