Hard shot at a reasonable veocity kills birds. Excess speed and soft shot ruins pattens which doesn't help bag birds. Federal shot used to be soft but I haven't shot it for years. I do pattern my loads and know this for a fact.
Hockeybob, I have killed 95% of my roosters with the 16 gauge and 1 1/8 #6. Always excellent results as long as I did my part. I also prefered to use mod choke instead of I/C for a little more pattern density. Interesting our results are so varied.
I bought a Benelli Ultra Light 12 gauge this year and love the gun. Haven't had a chance to hunt with it but I have shot some sporting clays with it. It tips the scales at 6 pounds 2 ounces. I used to shoot only 16 gauges but the BUL is filling their niche nicely.
I like the old pigeon load myself in #6 shot. 1 1/4 ounce at 1220 fps. Kent offers it in their all purpose load.
http://huntstuff.com/catalog_vendors/kent_cartridge/allpurposediamond.html