[HuntingWolf]

What would be the best spare parts to have on hand for a M1A?

 

[Cornbread2]

Extractor
Extractor spring and plunger
Ejector and spring
Firing pin
Trigger pin
Hammer pin

You can keep these in your stock ready for replacement in the field.

 

[Bravo762]

While I'm not disagreing with CB2, I will say this:

I've found the most convenient way to carry and install the bolt parts is by carrying a spare complete headspaced bolt.

The same thing applies to the trigger group, instead of loose pins.

I also keep a spare gas piston around, so that I can take my time cleaning the dirty one without having the stick down while doing so.

Odds are though that you'll spend FAR more time carrying those spare parts than using them....... I hose mine down hard in CLP, store in a double-ziploc, outer ruck pocket. They kind of get a film to 'em, but I doubt it's anything other than cosmetic.

 

[Cornbread2]

Keeping an extra bolt to fit your rifle is the best way to go but it is expensive.