If you're 870 barrel is smoothbore and uses screw in choke tubes, try a modified with Remington Sluggers, either 2-3/4 oz or 3". These patterned the best with my 1187 with a mod choke tube.<br>
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As for 2-3/4" or 3", there isn't much difference between the two. The 3" recoils more, but not because of more powder or slug weight. It has more wadding to fill up the space between the powder and the base of the slug, and thick wads do add some mass and thus increase recoil. Try both slugs and see which pattern best in your gun.<br>
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If your 870 has synthetic stocks, try the Remington R3 pad, which is made by Limbsaver. It's worth the $25 it costs.