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.22 rimfire ammo is the dirtiest ammunition made especially the stuff loaded for the bulk boxes.

Now as for the chunks of lead you are speaking of - - - the bullets should have been pushing it out ahead of them each time you were firing. Inspect your bore carefully and make sure there are no obstructions and then run a DRY patch through the bore slowly feeling for "catchy" places. Inspect the patch after you remove it for any flakes or chunks of metal. If all is OK proceed to clean as usual paying special attention to the bore. If you do find a "catchy" place have it looked at by a competent gunsmith and a trip back to S&W may be in order.

I have never heard of leading accumulating like you have described it's usually streaks inside the bore.