Plinking .22...
If you're looking for a 'plinking/targeting' .22, I would say your Ruger Mark III is an excellent choice! The Rugers are a quality gun and are very reliable and extremely durable (have had a Mark II 20+ years, NO problems!!) and is a great plinker that you'll have for many years... and for about $250!!
The SIG Mosquito is really another story. The gun has had a bad history of FTF, FTE, jam-up problems using different types of .22 ammo (shot one many months ago, TERRIBLE... bought a .22 CZ Kadet instead, flawless)... alot of SIG'ers say the newer models have been corrected and are not as tempermental. Retailers in my area have had them marked down to $235 and can't move 'em!
SIG historically makes a GREAT gun (have a 9mm P-229 and I love it... flawless) but I wouldn't recommend the Mosquito unless you can rent/shoot a newer model WITHOUT hiccups!...
Remember, the purpose of a .22 is to be a FUN shoot, not a tempermental pain-in-the-ass!
Browning BUCK is another classic .22 that you might want to look at, another excellent .22 but alittle more $$.
For me, the 'no-brainer' is the Ruger Mark!!... Good Luck
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