Around five years ago , When I was experimenting with reduced diameter comp tube tunes I had a go at a 20 mm, I lashed up a comp tube securing the end plug with dutch dowels (grub screws in the joint) not ideal, and unsurprisingly it blew the end out on the first shot. I found it in a draw recently and had another go at it , this time I cross drilled the end plug and used two 4 mm pins , so it shouldn't move, and sealed it with loctite . Its using a 30 mm long bronze skirtless piston with an o ring ,and a very light 2.6 mm wire spring( with 30 mm of pre load) it has a 4 mm long x 3.6 mm trans port , and after a lot of fiddling with lube , and maxing the stroke out ( its in a TX with a full length barrel) its doing 776 fps with 8.4 exacts ( 4.52), that's 11.2 fpe, any thing else i have tried pellet wise makes less power , not tried express yet as i don't have any , If i increase the piston weight or the preload it diesels so its on the limit, The shot cycle however is fantastic. Going to shoot it at the range today. I think another 10mm of stroke will put the volume where my 21mm is at and make the power easily, I have an idea that will achieve this in the TX, and if all that works as i want it to I will build a bespoke action making the most of the reduction in diameter meaning a compact rifle , and include some other changes i have found to be useful, ie short port, extra spring room, & solid comp tube lock up in the breech, it will be aluminium , so very lightweight. Not really sure if this is a hobby now , or an obsession !