If its dried enough that the core comes out an inch or two, just use some kind of HOOK- bend a coat hanger, fish it into the core, and pull it out. There will connecting tissue still in the horn, as well as some of the core, possibly.
But, it will smell better when you boil out the remainer with a bit of soap in the water if you aren't boiling the entire core!
Expect to be doing some scraping inside the horn with a long handled spoon, one way or another. If the horn has a severe curve, even that won't get all the tissues out.
I put some sand- about a handful will be enough- in a horn and swirled and shoot the horn back and forth, to abrade away tissues in the top of the horn around the curve where I could not see.[I covered the mouth of the horn with the palm of my one hand, to keep the sand in the horn during the shaking.] Small birdshot would do the same. A vacuum machine sucked the last of the sand and dead tissue out when I felt it was done.
I hope this helps. :thumbsup: