Your groups are closing up as you're shooting because, in my experience, 303's like fouled barrels. That's why I no longer fully clean mine after a shooting session, just three passes with a boresnake style pull through and the third pass with a bit of oil on the end, then one pass of the pull though at the start of the next shoot session to clean out excess oil. Only fully clean your barrel if you've shot corrosive military ammo. Micky Duck is right about a cheek riser, makes a world of difference. Also be a good idea to find the lowest scope rings you can find.
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