You might have some residual solvent or oil in your bore. I run a boresnake through my bore before shooting to get that residual out. It is usually close after that.

I use to leave my bore on my rifle fouled some during hunting season. I don't see a problem with it for the short term. I learned that little trick before a rifle qualification in IOBC when I was a young Infantry 2LT from our Platoon Trainer.