I use my 880 for indoor use on 1 pump as it allows me to get Rifle practice on wet days or when it's dark without risking too much damage but remains...
My back packet clone is pretty good to buy it's only 4ftlb. Don't know enough to go grinding the valve. The real backpacker with the. .177cal barrel...
I've owned a mk1 but no idea how the barrel is fixed. Do you want to remove the choke or simply shorten it ? Is it possible to remove a bit of barrel...