My Bushmaster has seen approx. 10 - 12 thousand rounds. While shooting today I got 100% FTE (fail to extract).  Casings were not stuck, and I could easily extract them by hand cycling. Is this symptomatic of short stroking or a weak extractor spring??? Ammo was xm193 and some m855, both of which has run flawlessly for me in the past.  I'm leaning towards short stroking since if the bolt was coming all the way back I'd have been picking up another round.  How many rounds can you typically fire before needing to rebuild the bolt?
Thanks as always.

Easy enough to fix. An extractor spring and insert are cheap. The military says MTBF for an extractor spring is 4000 rounds. At least thats what they say is acceptable. The new M4 bolt has a newly designed extractor and spring assembly that is designed for 25k MTBF. HTH and YMMV

Replacing the extractor spring and insert would be a good idea considering the military considers 4000 rds to be MTBF. Don't forget to check and see if your gas key has become loose or unstakes as that too would cause a failure to extract on a consistent basis. That happened to me last week and i too thought it was the spring at first....

Yes, the military considers 4000 rounds the MTBF for the extractor spring--but that is under full auto/burst fire and extreme conditions.  A civilian semi-auto AR will probably go twice that--so it is time to replace both your spring and plug.  You might even want to replace the extractor, too.  Good insurance.

Don't forget the ejector and spring either--might as well replace those items too, considering the mileage you have on your AR.  An ejector and spring is only a couple of dollars.

As for when you need to rebuild the bolt--that is a judgement call.  Remove the cam pin, cotter pin, firing pin, but leave the bolt in the carrier.  Over a soft surface, attempt to shake the bolt out of the carrier.  If it comes out after one or two shakes, then replace your gas rings.  If it stays put, or takes a LOT of shaking to get it to fall out, then you are OK.  

You won't believe it.  The cause of my problem was a blown primer out of a Maine Cartridge Co. M855 round.  Believe it or not the primer got in the gas key just as straight and square as if I had put it in their on purpose.  It took a bit of work to get it out, but there it was.  Compare the OD of a primer to the ID of the gas key, it's an exact fit.

I do appreciate the advice you all gave me on replacing extractors / springs etc.  Should I use the black insert on all my AR bolts?  What do you guys think of the one piece gas rings I've heard of?  After 10k rounds should I just completely rebuild the bolt?

Thanks.

-Craig