I got my first AR a few months ago and really enjoying it.   I've been modifying and upgrading.

Question is what spares should I have on hand in case of a failure?  What parts fail most often?

Thanks

mm.

I'd start with a backup BCG or at least a spare bolt and cam pin.

Be careful as this is how complete rifles start.

What he said.

But, you should consider having replacements for parts that wear and break with use.  You can never have enough springs, pins, rings, and screws because something always goes wrong over time.

Look for a high quality set sold by a reputable dealer.

Any suggestions for a good pack/kit? Been looking into picking a couple up. I know I know... late to the game.

Depends on your wallet...

Bravo Company and Daniel Defense kits run around $100, CMMG kits run around $70, Anderson MFG kits run around $50, and somewhere in between you have other companies like DPMS, Wilson Combat, etc.  Most of the above companies also sell upper parts as well.  I've never seen what I'd consider to be a comprehensive upper parts kit.

I like the Bravo Company kits I've purchased. I also like the CMMG "gunbuilders kit" as it comes with some standard parts that are handy to have and their "springs and pins" kit is fairly inexpensive and it contains some frequently lost parts as well as springs.

Check out Brownells.com for a start then go price shopping.

Now, my opinion.  If you bought a Colt rifle, I'd stick with Colt parts.  That is just my opinion, everyone has their own.

I would start with:
All parts for a BCG rebuild.
Action spring.
Then build from there until you could fit up a stripped lower and upper.

I am a hoarder. I buy spares for my spares.

Complete bolt, lpk, receiver extension, and action spring at a minimum. Imo. Think anything that has potential to shut your weapon down at the worst time.

All of the springs.

Here is not a bad place to start.

I inventory these:

- bolt assembly (probable service life 5~10K rounds; can break lugs unexpectedly)
- extractor/spring/guide
- cam pin
- firing pin/retainer pin
- gas rings
- action spring
- selector detent & spring (can be lost changing a grip)
- hammer spring
- buffer detent & spring (can be lost if you have to change a receiver extension)
- take-down pin detent and spring (can be lost or damaged if you have to change a receiver extension or end plate)

Another good question to ask is if yours is a garage build or a name-brand, factory-built gun. No-name and budget parts may not last as long as true mil-spec stuff, so be honest with yourself. If you bought everything in your gun at gun shows, beware and stock all the critical parts early. Also ask yourself if rapid fire is common on your rifle; if you tend to run budget ammo with bi-metal bullet and steel case; if either, figure on more rapid barrel wear, and consider a spare barrel (though those have a way of "attracting" other parts with which to build another upper or complete gun). And acquire a full set of head space gauges: as round counts go up, the possibility of a head space issue increases, and you *need* to know if your barrel and bolt pass or fail. YMMV.