AAFTA has no overall pistol length restrictions. Currently, my pistol/carbine with buttstock removed, and no rear extension, is about 18.5" long overall. Unlimited silhouette pistols, including air pistols, must be under 25" long per IHMSA rules. In the USA, pistols are required to be under 26" overall length, though it does not apply to airguns.
I want to extend the rear shelf of my pistol and add balance weight to the rear.
Example of pistol w/extended rear shelf:

How long can I extend it? I'd like to make it 25" overall length. I don't want to break any unwritten rules. If no one can point out anything wrong with this idea, I'll proceed.