I have shot a fair number of grouse using a 6lb 25" barrel 2&1/2" 12 bore, worked great for me and I know quite a few others who agree. I shoot longer barreled guns at prairie birds but short tubes in a grouse thicket work out just fine.
Twenty five inch barrels will work just fine on pheasants as well. All one has to do is learn to shoot shorter barrel guns. They're no harder to hit with than long-barrel guns, but you do need to shoot them a few times at least before heading off to the field. Getting used to the swing is all it takes.