The problem could also stem from excessive rod wear at the muzzle. Also I have seen 1/4" of the barrel nipped off at the muzzle by someone that suspects rod wear. If the resulting muzzle isn't perfectly perpendicular with the bore, horrible accuracy can result. Also the muzzle can have a ding that causes awful accuracy. That makes me wonder why some Texas Ranger pics show a group of men with muzzles down (on the ground). Oh yes, it was safer than muzzles up!
I was once invited to a backyard cookout, barbecue and pelon were served. After eating, I got to talking about old Winchesters etc. The host said "I have an old Winchester 1892 carbine in 38-40 that won't hit the side of a barn". I asked if it was for sale. He said "Hell no, I will give it to you"! He did. I took it home, and a few days later simply re-crowned the damaged muzzle in a lathe. That SRC had been shortened about 3/16" with a hacksaw. And no it wouldn't hit squat! But after re-crowing, I got 2" groups at 50 yards with those (crude) original SRC sights!
So you might want to check out your gun for anything like I mentioned above. If it is determined that your bore is hopelessly bad, one option would be to re-line the bore and chamber. Re-lining jobs cost about $400. Eventually it comes down to "Is the gun worth all of this additional expense"?