Ok, what you really need to do is clean it, and in a proper way.
This is what your breech area looks like,
The area mark with "D" is a problem area with these types of rifle.
You'll need a "cleaning jag" to hold a patch inside the barrel. It'll screw into the end of your rod, I'll let you look for one, they are caliber specific and available all over the place.
* Get a bucket, an ice cream pail will work. And find the dish soap.
* Remove the barrel from the stock.
* Remove the nipple and clean out screw from the "Drum"
*Put Luke warm water in the bucket and a squirt of Dish Soap (I use Dawn),
Submerge the breech end of the barrel in the soapy water, now place a wet cleaning patch on the jag and slide the rod/jag/patch down the barrel while the breech is submerged.
Begin moving the rod up and down in a pumping motion,, this will draw the water and detergent up into the barrel through the open holes in the breech and force it back out with each stroke.
Change the patch a few times and continue until the patch looks clean.
Rinse the barrel with fresh warm water and dry it completely with a few more patches.
You gotta get that breech clean of all oils and debris, it can be tough with a second hand gun because you don't know how the last guy cared for it.
If it was fired right out of the box without cleaning the factory packing grease from the barrel that breech can be plugged with some nasty fouling. If that's the case,, repeat the above process with Gasoline!!
Any Questions?