Measure berries into large saucepan. Mash with potato masher as they heat and bring to a full tumble boil. Boil 2 minutes. Add warmed sugar and vring to a full rolling boil for 1 minute. Remove from heat; beat with a rotary hand beater for 4 minutes (stick beater through a piece of heavy brown paper to protect hands from spattering and steam). Ladle jam into hot sterilized jars, leaving 1/4-inch headspace. Seal immediately with two-piece lids according to manufacturer's instructions and process in boiling water bath for 5 minutes.
