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The Mossberg looks like great bang for the buck.
The lower does not appear to be an AR-15 standard lower.
For example, the safety is a button above the trigger guard, as opposed to the lever below the ejection port.
That might not be important to you, especially since the price of this complete rifle is about what a low-end complete lower (only) would cost.
Note that Ruger also has an AR15ish .22 based on their 10-22, and it's more than double what this Mossberg costs.

You can assemble a basic M4 in .223 for less than $500 if you REALLY shop the deals.
A .22LR dedicated upper alone will usually cost you close to your $500 mark.

Sure, you could buy a standard M4 upper for $299, sell the BCG for $110 - $125 (unfired) and put the proceeds for a Ciener .22LR convertor ($175)
But the 1:9 twist rate of the .223 barrels does not work well with .22LR, and accuracy suffers.
.22LR likes 1:16 twist rate.

That would put you at $350 for the upper complete. The best I've ever done for a complete lower is $150.
So that best-case $500 AT15 in .22LR is $500, and it's not very accurate.

You can buy a Colt Umarex .22LR AR15 for $500

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