I have both an encore 12 inch and contender 10 inch 45-410.
you do not have to use the choke, in fact you should not use it with buck shot or slugs pet the instructions manual. but for shot shells it will open the shot pattern way up to be almost useless. the choke is to stop the rotation of the shot cup. if you look inside the choke you will see straight rifling and the rest of the barrel has normal twist rifling.
do your self a favor and do not send that barrel out to put a brake made to screw into barrel to make it over 16in it will not do you any good and will may even hurt more. a 45-410 barrel doesn't shoot 45 long colt well enough to justify it.