"George E. Cawthon" writes:
Why don't you use the standard metal shelf support rails (the ones
with slots that use metal V support clips) that are dadoed into the
shelf sides sides?

I personally don't like the rails because they are more obtrusive.

Sure 1/4" will support a heavier load than 5mm. 1/4" is about 6.4 mm.
But you are talking about shear strength. I can't imagine how much
weight a shelf would need to shear off four 1/4" pins (dowels).
Probably more than one could put on a shelf using 12" high books.

Sounds like I will be OK with the 1/4" pins (though I have some sets
of books that as big as 14"x12"_

My question was not so much about breaking the pins (which I imagine
would be hard to do) as about tearing out the hole.
What I really wanted to know is whether the 1/4" pin system is
significantly stronger than the 5mm system to justify the bigger and
more obtrusive 1/4" holes.

No, don't use any kind of sleeve. just make sure the pins are snug in
holes that are accurately at right angles to the shelf side and are
drilled to a depth of 1/2" and the pins extend about 1/4" from the
surface.

Do the sleeves add no strength advantages then?

Thanks