So, your grandpappy’s old .22 rifle is a tack-driver?
Here's another entry in what should be called the "How to drop a grand before you know it" blog.
If you think you have a .22lr rifle that will shoot the proverbial "dime-sized groups" all day at 100 yds, go to the USBR site and order some targets. Or download a practice version at CZ452.com and print out the practice target. The official targets are printed on heavy stock and are much easier to score though. Read up on how to score the target at USBR or RimfireCentral.com, get some sandbags or a rolled up sweatshirt (for true bubba style action), and see what the old squirrel killer can do.
I would consider a score over 240 @ 25yds, or 220 @ 50 yds, excellent for a sporter type rimfire rifle. Ammo will make a huge difference, I would suggest buying several brands of premium target ammo to see what your gun likes best. A good starting point would be the economical Wolf Match Target or Match Extra if you can find it. The Wolf will give good to excellent results in most rifles, much better than the 500 round bulk packaged stuff from Wally World.
DISCLAIMER: This site will not be responsible for max'd out credit cards or spousal neglect due to the onset of ERBD – Escalated Rimfire Benchrest Disorder. Symptons include spending ridiculous amounts of money on little 22lr ammo, frequent all day sessions at the range and a rimfire rifle that costs more than any other centerfire gun you have ever owned.