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Bought a Mosquito about a month ago for fun and cheap practice. Standard model, very accurate. I had some initial trouble with FTEs and FTFs. Then, I found this on the Sig Sauer site. I don't think this information is in the manual and it makes all the difference in the world. I had thought the high velocity required the heaver spring, but it's for hyper velocity, which I have not yet tried. I also bought two Sig Sauer magazines that are polymer but without springs. The one that came with the gun is polymer but has a spring and it functions flawlessly. I have noted that the front lip on the two without springs are not the same (not cut as low) and will be taking the Dremel to them. Also, Academy now has replacement magazines (with springs) for $24.95 ea.

From Sig Sauer:
Please try these basic trouble shooting tips before requesting service: *Please make sure the recoil spring is installed correctly. The narrow end of the spring MUST be on the guide rod first...pointing towards the rear of the gun. If it is not, this will cause feeding, extraction and firing issues. *The longer (light power – some are painted white) spring should be used with Standard or High Velocity (1000+ fps) ammo. Use the shorter (heavy) spring for Hyper-Velocity (1400+ fps) ammo. *Like many .22 LR’s, Mosquito’s can be sensitive to ammunition used and to cleanliness and lubrication. Be sure your Mosquito is cleaned and lubricated regularly and before the first use. *There is a lubrication port on the right side of the slide for the extractor, be sure you add a drop of lube in this hole every hundred rounds or so. Make sure the gun is very clean. *Some Mosquito's benefit from a break-in period. *Be sure to use factory recommend ammunition - CCI Mini-Mags. Winchester Super-X, Remington Golden Bullet, Federal High Velocity etc...also usually work well.