I'm using my Lyman Great Plains .50 for the first time to hunt deer with this year. My question is, how long can a muzzle loader be left loaded?
I did everything I could to make sure the bore was completely dry before loading with 70gr. of Black powder and a patched ball seven days ago. Today I fired the gun, but has a hang fire. The ball hit the target where it was supposed to, but the hang fire worrys me. I've never had a hang fire using Black powder.... So how long can a rifle be left loaded and still be sure of ignition?

There are a lot of variables. Moisture exposure is the critical one. I've experimented by leaving a cap'n'ball revolver loaded for 6 years without problems. Every charge went off the first time and every round flew as true as is possible. The key was preparation that is sealing it from moisture.

Some people don't like to leave them loaded and there's safety and legal reasons to unload (pull the charge) or shoot them out (or not...). If you had a hang fire (and don't normally) moisture got in there somehow. A homemade greased leather nipple cover (as simple as a 1/2 inch dia piece of leather) will, with the hammer let down on the nipple with the leather between the hammer and nipple, keep the wet out pretty well. If it doesn't, you'd better get good with a worm or use the opportunity to practice.;)

Although I've never tried one, there are a couple of compressed air kits for removing the charged ball without shooting or using a worm.

I've left my T/C Renegade loaded for several days with no problems on firing by simply doing as at the range when sighting in. Mainly, dry the barrel and nipple to the extent possible, then fire several caps to assure everything is open and dry. Then, load as normal. The advice on the greased leather patch sounds like a good one even when not in wet weather.

Sealing both ends of the system will keep most moiature out (may have a bit of condensation if moved through different temperatures). Need a patch lube that is pretty thick and not likely to dry out (commercial mini-ball grease works well). Kind of amazing how much rain a rifle barrel can accumulate.

IF you don't mind a bit of modern mixed in whith your traditional, a cover of scotch tape works fine.

The back end...and now that I re-read the first post, realize you don't mention if it's a percussion or a rock-lock...lets go with percussion. Sealing the nipple end of things can be done with a little bead of bees wax...roll it in your fingers until it's a string, warp it around the BASE of the nipple. Wnat it to firmly contact the cap's mouth, but not cover the vent or contact the cap's compound.

IF a flinter, things get more complicated. Really haven't seen a good way to 100% waterproof the pan...there is usually some leakage from the hinge between frizzen and pan. Of course you can't block the open vent, most have a bit of space between vent and lock, and most frizzens don't fit all that well on the pan.
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Better to pull the load or fire the load at the end of a day. Yes..you'll have to clean it and start over the next day, but it's the most sure fire way (pulling the ball and load is easier to clean up, but a lot less fun at the end of the day).

...double dip...sorry

Spent a good amount of time in duck blinds with a SXS 10ga. percussion. Can count of a well loaded/sealed charge going off...always got my first two shots. But reloading in foul weather is another matter.

Got into the habit of tossing the tail of my rain gear over the locks...got a wet rear, but at least kept the rain from beating on the percussion caps.

I had a friend who always left his ML loaded during the season. Personally, I dump my charge before leaving the field, law and paranoia make me do so. One year when the season was over, we were unloading, me with a CO2 discharger (works great on my TC 58 and 50, not at all on my 700 50) and him finally shooting his out. Pop. Pop, Pop, five times the cap went pop, but the charge never went off. At home he wormed out the bullet and dumped the charge, or rather dug out the charge with a worm. Moisture had gotten in and rusted up his bore and charge into a terrible mess. For the sake of a few lousy bullets, he pretty much ruined his rifle.

Friend gave me a .54 cal CVA inline. He told me it was loaded. I called a friend who is a gunsmith and told him about it. He said he has bought guns that have been loaded for 50 yrs and no corrision. Apparently no moisture, no problem.
Anyway, I achored the rifle down and tied a string to the trigger, pulled and BAM-blew to kingdom come. He he he-just kidding- rifle shoots fine.

Lot of time the hang fire can be caused from the powder allowing to shake around and fill up the area up to the nipple, then it takes a bit to burn to the main, charge, same reason a flinter puts a quill in the flash hole while loading. Get yourself a piece of 3/8 brass rod make a ram rod and use your powder measure to apply a good solid push against your powder charge to hold it in place. Just one more variable. Take care and have fun. Swany

Just a small caution to anyone leaving a cap & ball revolver loaded: you CAN unload them by removing the percussion caps, unscrewing the nipples, dumping the powder, and tapping out the ball with a small dowell or metal rod BUT PLEASE consider the following: when I was a young tom-fool instead of the old fool I am now, I had a misfire in a Remington repro. I fired off all the chambers but one, assumed the cheap Italian made cap was a dud, and proceeded to try to remove it with a small pliers while holding my left hand around the cylinder. Only by the grace of the good Lord above did I not permanently remove a finger or two, but darn did it hurt! Removing stuck caps can be quite risky.

Well...I guess I'm just going to have to shoot them.

Got to thinking about this, so I won't be shooting my cap-and-ball revlovers for a few weeks. Loaded three revolvers, one with pyrodex, one with tripple Seven, and the other with FFFg. Have protected the outside with protectant (but not a penetrating protectant), plastic-tie to keep inadvertant fingers from cocking (gun show ties), and will trot them out sometime near Thanksgiving.

Front end of the system shouldn't be the problem...felt wad, tight ball, and lube over the ball. Rear end of the system may be...just a tight fitting #10 cap that may or may not allow mositure in.

Are in a locked shed...non-climate controled...and will be cycling though the day's heat and the night's cool...high humidity. May all crap out..could be none of them will give me any trouble...but with three powders, may just be that one of them sucks in more moisture than the others.

Now, this sounds like a test we all will be interested in! Be sure to post results when available, ribbonstone.

Well...checked on the loaded percussions again to day. Experiment will be cut short...by not using a penetrating oil to protect them, there are some rust spots showing. Weren't "perfect" guns before, all three have been well used...but I can't set by and watch them rust. and won't beable to get them apart for a good cleaning/oiling until I shoot them.

So...will be off to the range for a quick visit. Just about a month now, so we'll see if FFFg, Pyro, or tripple seven can stand a month.

About 2 1/2 hours later.

Called a friend at an indoor range..he wasn't busy and allowed he could step up the volume on the vent. system for just a few BP shots.

1851 Navy (Uberti): 22gr. FFFG (old GOEX)..#10 Rem. cap
1851 Navy (Pietta): Tripple Seven at 22gr. volume ..#10 Rem. Cap
1858 Rem. (no name brass frame repro.): 30gr. volume Pyrodex-P ..#11 CCI cap

All were loaded with a dry lube felt wad on top of the powder, ball, and Navy Arms mini-ball lube over the ball. Caps were tight fits to the nipples, but unsealed with wax.

Short of it is simple: they all went BANG and shot decently. Knew the front end was well sealed, but though the rear would allow at least a bit of moisture in...caps are tighter than I thought.

Wasn't going to test the owner's good mood by asking to set up the chronograph indoors (have the light sourse)...but as far as I can tell from "normal" sessions, there wasn't any widly low vel. shots or unusual happenings. Do miss the stale-gas-passing smell with Tripple Seven....will have to just eat Taco Bell and pretend the oder comes from the powder.

Was going to try an old can of Clear Shot...un-opened...but when I opened it, the powder was in a big ball...evidently this stuff sucks up moisture like a sponge. Will get used this New Year's as a sparkler.

Got to finish cleaning them...and getting after those rust spots. Doesn't really make a differnce in cleaning...have to take them apart anyway to get to the insides (rust on the outside isn't a good sign for the insides, sot ehy get a detailed cleaning).