Generally barrels got shorter over the years.
I have doubles with 28,26 and 24" bbls. All are perfect for the frame size and calibre.
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24" bbls on big bore doubles seem "stubby" to me, but that is just because I am used to my 28" 450 No2.

As long as the double feels good to you I do not think bbl length matters that much.

PS those 21" bbled 9,3x74R Merkels are SWEET. I shot one at the last DRSS hunt/shoot.
I like them.