So it just crossed my mind that I might have left my ML loaded (just power and projectile no cap) the last time I hunted with it which was like last November. Sure enough I just checked it and it is. Did I mess up big? Will it be all rusty in there? It’s a stainless CVA Accura loaded with Blackhorn 209 loose powder and a sabot.

Also, should I just throw a cap in it and discharge it at this point or take the breech plug out and knock it all out the back?

Yes. Shoot it and run a wire brush through it. I had this happen before. Solvent and a wire brush on your ramrod with an electric drill will help.

What he said! Maybe some fouling, but should be fine.

It may not shoot at all
Alot of times when a ML is stored standing upright the oils in the bbl will creep downward towards the breech and it will ruin the powder
You might be looking at a disassemble the empty the contents and push the bullet out with your cleaning rod
Id wrap oooo steel wool around a bore brush and swab the bbl to clear it of any fowling and corrosion

I leave mine like that all the time. I haven’t found it to be a big deal. Shoot it or pull the breech plug, your choice. Just clean it like you normally would and you should be good to go.

you'll be fine. I've left many ML's loaded for a year with a range of propellants with no issue. Blackhorn is one of the most stable and is likely to fire just fine if you have a convenient place to discharge it. otherwise, you can use a ball puller or remove the breech and just push the poweder/ projo out.

Many accounts of muzzleloaders that have been loaded for many years with no corrosion or side affects and yes most of them did shoot. Just pull the plug and dump the load you are fine

I am surprised to hear this. I have read that powders will ruin a barrel if left loaded.

Fired powder residue will absolutely rust a barrel if left for an extended period of time, but I've never had a problem from unfired powder. Even if it did rust the barrel, in theory it wouldnt matter, since it would be rusting the section where your powder charge sits and the bullet would never even touch that area.

I can imagine some people might get themselves in trouble by firing at something, reloading their rifle for a follow-up shot that isn't needed, and then leaving their rifle loaded for the rest of the year.

I left an Optima loaded 2-3yrs. It was a mess. Had to do a really good cleaning/oiling to get it back running properly, but i did get it there

Left mine loaded before - shot it cleaned it - was ok

Dang it - hope i did not leave dang thang loaded again

You'll be fine. Whole different story if you had shot it and never cleaned it.

👍

Up until about 15/20 years ago, blackpowder was sold in metal containers. I still have some from the 1960s. The metal containers still look new on the inside. As long as it wasn't shot and not cleaned before it was loaded again you will be fine. I always clean a fired gun as soon as possible. So you should be fine. Shot it out and clean. Just make sure it is not double loaded......