This question rears its ugly head several times a year. Well, you "can" of course leave your muzzleloader loaded for as long as you want, and you are the one that is responsible if you do so. Shooting it out at the end of the day is always best, but those that plead "but do I have to?" likely will not be satisfied. Of course you don't have to. You don't ever "have" to change the oil in your car, either, and you don't have to add salt to your water softener, either. You are far better off if you do, of course, for many reasons.

Blackpowder is hygroscopic even in its unfired state. That is not speculation, it is well-established: Black powder absorbs about 1.5 weight percent moisture under 75 percent relative humidity at a temperature of 21.1.degrees C. (70.degrees F.) over a period of 24 hours. If black powder picks up sufficient moisture, there is a possibility that the black powder will not burn as fast. High relative humidity may cause erratic behavior. Water may cause the potassium nitrate to migrate out of the black powder and cause corrosion of metallic parts.

That refers only to black powder, a mixture of three components, not a compound. Synthetic substitutes marketed as "black powder" substitutes are generally worse. If it cleans up with "regular tap water" it is naturally water-soluble. Anything that uses a sugar-base (American Pioneer, Pinnacle, Black Mag3 in the ascorbic acid department, and Triple Se7en in the gluconic acid department) will soak up moisture. The resultant erratic velocities or misfires are contingent on a host of variables: humidity, temperature, ignition type, specific rifle, etc. A loaded muzzleloader well might go bang the next day, or the next year for that matter. Or it may not.

Open packs of pellets, or opened jugs of powder lose their potency over time. On a very humid day, you can see the loose powder start to clump in particularly in the case of American Pioneer, and even Triple Se7en if you give it enough time. No one would think of storing powder with the cap off; at least I hope not. American Pioneer / Shockey's Gold are so bad at sucking moisture that desiccant packets are included in the jug. The diminishing performance is something you can readily see if you shoot through a chronograph. Most don't bother; back to the ignorance is bliss department. There is no benefit to leaving a muzzleloader loaded, except for those that are too lazy to handle their firearms properly.

It is basic, fundamental gun handling to understand that a muzzleloader is considered unloaded when the ignition source is removed AND the powder and projectile are removed from the barrel. Though you might think it common sense, it is still loudly recommended by every muzzleloading manufacturer and powder manufacturer.

It is more difficult to double-load a muzzleloader than is not already loaded. Leaving a muzzleloader loaded makes you responsible for that condition, and for whatever transpires as a result. There is no guarantee that your gun will misfire, nor is their any guarantee that an idle powder charge will rot your barrel. Nor can any specific velocity loss be guaranteed; there are too many variables.

What we are trying to guarantee is that the next shot out of the barrel at a game animal, the only one that counts, will be the most consistent, reliable, and effective load there can be. That's what you are betting your hunt on. There is only one option to give you the best chance of success in the field, and that is a completely fresh powder charge at the start of every day's hunt. Everything else is second best, or worse. No muzzleloading propellant improves residing in a rifle. It cannot possibly get better, it can only cause problems.

No one makes you drain the gas out of your lawnmower or motorcycle at the end of a season, or makes you hit that idle snowblower with a trickle charger. Yet, the number of lawnmowers, motorcycles, and snowblowers that fail to start the next time are significant. "Jim Bob" has a snowblower that never has failed to start for him though, and "Jim Bob" has a muzzleloader that he thinks will be okay that he has kept loaded for a month. Heck, Jim Bob never has checked the air in his tires, or his engine oil. Everything "works for me," Jim Bob likes to say.

It is the feeble-minded like Jim Bob that give Murphy's Law a fabulous head-start. And that is an advantage that Murphy has never really needed.