Fun fact re: replacement buffer retainer spring
This might be old news to some.. but otherwise, a fun fact.
I recently had to cannibalize an AR to get my Magpul ASAP plate off. Looks like when storing the lower, I managed to lose the spring to the buffer retainer. So in lieu of the 'right tool for the job', I further cannibalized a Bic-style clicky pen and stole the spring from it. Turns out it's a perfect fit for the retainer pin, with very strong pressure.
Haven't fired it yet, but a function check showed no obvious issues. Resilience and resistance to heat might be factors, I'd imagine, that would limit longevity, but of course I'll get a proper replacement soon.
Hopefully this is a useful substitute in case you ever find yourself in a jam.