RE: Marlin Model 60 .22lr Semi-Auto.
Portage, how is your gun jamming? If it's stove-piping or not fully ejecting rounds, check out the ejector and extractors.
Typically, if it's stove-piping, the extractor claw is hanging on too much, it might be because of wear or imperfect machining...more likely it's because the claw travel is restricted some way to prevent it fully releasing the rim. Usually this is because of gunk build up under the extractor that limits it's movement. An overly stiff bolt return spring can also cause this problem.
If it's failing to eject, running rounds nearly straight back into the chamber, it's likely the ejector (although it could ALSO be the extractor, the ejector is likely playing part). This would suggest that the ejector is either bent, broken, or worn, or simply just too short to pop the rounds out. If the main bolt spring is too stiff or the loads too light, it'll also give this result because the action isn't opening up enough.
If it's not picking the rounds up properly, or not feeding properly, check out the feed lips and the bolt face. If your extractor is worn or bent, or if there is gunk build up under the front side of the extractor, it may not be controlling the round properly, so you'll get rounds missing the chamber, jamming up into the receiver or bbl face.
Does it jam with High velocity rounds as well? If your gun is still new, your bolt return mainspring might be too stiff, so it's possible that it isn't recoiling/opening far enough to give positive extraction...High velocity rounds might have enough stank to kick it open far enough.
Do you or have you ever dropped rounds directly into the action or into the chamber? If you dropped them into the action, you likely just crushed it, but I know a lot of people that have bent their extractors (or at least increased the wear on them) by putting a round into the chamber and slamming the bolt home. Marlins CAN do this, but it's not exactly good for them, and can damage them. They're kind of like 1911's, just because you CAN chamber a round and close it doesn't mean it's SUPPOSED to do that. I've heard a lot of guys complaining at the range that their 1911's started jamming, the same guys that I've watched drop the slide on chambered rounds for a long time. Try that with a stock mauser sometime.
It never hurts to polish the bearing surfaces of the bolt and receiver, just don't take off too much. I like to do it manually, wet/dry paper of 400-600grit with G-96 gun treatment as lubricant, but buddies of mine have had good luck wth polishing compounds and just cycling the action. I like to be able to control what parts I smooth, and how much I'm taking off. It'll make the gun function more smoothly, which almost always increases reliability.
Typically, if there's any probem, Marlin will either send you the parts for free, or do the repairs themselves for free, you'd just have to send them the gun.
A good gunsmith should be able to easily fix any of the above problems if you're not willing to try it yourself.