I do not think there is a most accurate semi-automatic .22 rifle for under $250.00. I think accuracy would mean, what each gun owner subjectively thinks .22 rifle accuracy is...
If I only had $250.00 to spend, I would get a Ruger 10/22 and have fun accessorizing it over time. You CAN spend a bunch of money doing that. The Ruger 10/22 is probably the most reliable, and most widely purchased, .22 semi-automatic to be bought at this time.
A lot of people like the Marlin semi-automatic .22 rifles. I don't...! However, I love Marlin's lever guns to death.
I have had cheap inexpensive .22 rifles in the past, and all have been a great disappointment to me. I have enough .22 rifles now, to fill a large sized safe. They are single shot rifles, bolt action rifles, pump action rifles, lever guns and semi-automatics.
The best, and most accurate, semi-automatics I have, are my two Winchester Model 63's that were put out by Winchester/Miroku a while back. I promise can you this, they cost a bunch more than $250.00...!
I would surmise, in the long run, that what a person buys, is probably what level of quality they are lookng for. I spend a bunch of money on firearms. However, I am looking for the best quality I can get.
Bill