Hornady TAP or Federal TRU
Ok, if it was made in 1976 that means it's 1 in 10" twist. You are going to be looking for something in a 55 grain. I would suggest a decent brand softpoint or a defensve bullet like Hornady TAP. Go to Academy and look for a name brand 55 grain softpoint or defensive bullet. I think Academy sells both. Also, pick up a copper solvent. M-Pro 7 is sold at Academy, works well and is like $6 or $7. First thing I'd do is give the gun a good cleaning and do some copper removal. If you're having issues, I'd imagine your rifle is fouled to hell. Follow the directions on the M-Pro 7. Make sure you're using Ruger factory magazines and double check that your scope is mounted tight. Go out with your ammo and zero it for your ammo at 100 yards. If everything is done like I said, it should shoot good enough to hit the vitals on a deer out to a couple hundred yards. Buy a couple boxes of ammo and stick with the same brand and bullet that you zeroed with.
You are really crippled hunting with a .223. You can take down anything with proper shot placement, but that's going to take knowing where to hit on what you're hunting. Head shots on pigs are your only option. I've heart shot pigs with 77 grain 5.56 and had them run off and not found them for days. Head shots on doe, and lung shots on buck. You're going to lose some buck with 55 grain .223, that's just how it goes with small bullets.
I hope that helps.
Save a bunch of money and buy Academy's store brand Monarch 55gr Soft Point at $9.99 + tax.
Shoots dirty, but gets the job done just as good as all the others.
Having said that, I'll never shoot game, other than varmints with a .223.
If you need to defend yourself from two-legged critters with a .223, just double or triple the dosage.
.223 soft points of any flavor will do you well.
At close range, rate of twist is of no concern.