I got a new scope so I put it on my deer rifle. I put my old scope on my muzzle loader for the time being. It's a Redfield 3-9 30 mm, about 15 years old. Question is, it shoots about 18 inches to the left. I've walked it over to the right but now the adjustment screw is getting tighter. I'm still about 5 inches off @ 50 yds. Do I keep adjusting? Why did the adjuster tighten up?

Because your at your end of windage adjustment range. What type of bases and rings are you using? If you have the adjustable windage rear base and ring you need to adjust the windage on the base in the opposite direction. I.E. if you want to move your poi to the right you need to move it left.

No adjustment there, it's fixed. Beginning to think that scope is ready for the trash bin!

Maybe switch bases to one that offers windage adjustment.

Try lapping the rings, or use windage adj bases. Standard bases have windage adj screws on the back, or Millett makes windage adj weaver style rings.

I'd try windage bases and send those to me....