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OK, brief overview. Disclaimer: I am presuming that your saw was NOT set up with 3/8" pitch chain, which is not an appropriate choice for a stock 346xp anyway.
There are two kinds of chain that most likely came on your 346xp. The first would be standard-kerf .325" pitch chain. The second would be narrow-kerf .325" pitch chain, which (big surprise!) is somewhat narrower in the cutter and utilizes a slightly thinner bar, the net result being that it makes a narrower cut and uses engine power more efficiently. You need to figure which of these chain types you have. The numbers stamped on the bar or the chain will answer this question for us, so check it out and post that info on here. Once this is figured out you'll know whether you're running regular .325" pitch chain or narrow-kerf .325" chain ("NK" chain). The numbers will also indicate what gauge the bar/chain is (.050", .058", .063"; NK will only be .050, but be aware that not all .050 is NK.
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After you figure out which type of chain you have - pitch, gauge, NK or not - your next challenge is to figure out how many drive links there are on the loop. So count 'em. The numbers on your bar will likely answer this question for us, but count the number of drivers on your chain just in case.
Then, armed with this information - pitch (.325"), gauge (to be determined), NarrowKerf or not (to be determined) and # of drive links (also to be determined), you will know exactly what you have and what you need. Then, you can either pick some up at your local dealer or place an order from one of the better online sources for saw parts and accessories, like Bailey's or Amick's (both Arboristsite sponsors).