Using a shim with dropper post?
 

My old (ancient?) GT has an odd 29.6 seatpost. I'm not the youngest riding trails, so don't plan on getting another bike. Would shimming to 27.2 so I could fit a dropper post work, or would I be dealing with the post slipping?

No way it's 29.6...maybe 28.6, that was a popular size for aluminum frames. Should be no problem shimming it. I've done it.

Know it's hard to believe, I've never seen nor heard of a 29.6 other than this one. It's a 4130 chromo frame with a Reynolds 525 seat tube, and for certain is a 29.6 post. Was just digging around in an old bike parts storage container--spare post is in there, stamped 29.6. Yep, it's an odd one! Glad to hear it can be shimmed. Found a 27.2 to 29.6. . Guess I'd better grab it!

You’d want the shim to extend below the bottom of the top tube junction. Down to there it’s helping the frame properly transfer the forces around. I don’t know your bike, but some MTB’s have a brace from the top to the seat tube and/or an oblong top tube right there to spread out the force.

Shims are just as OK kn a dropper as on a rigid post.

I ran a 27.2 dropper in many different frames for many years, always with shims. Just get one of the longer shims.