If you have the 14,5“ outer barrel, buy one 10,5 and install it. Now find someone who can cut, drill and tap your 14,5" outer barrel to create the extension like this: https://www.taiwangun.com/fr/canon-interne/external-barrel-for-m4-14-5-10-5-cyma
I did it on my PTW. It’s really nice to switch...