I have spent time with all 3 choices. With that said I would without a doubt go for the M16. The Ruger is a pretty nice gun and they work well, but you are limited to shooting 5.56/223 or 22 long rifle with the conversion kits. I do like the fact they are a factory machine gun and Ruger has always been great to deal with for repairs. The down side is there are several parts including the original folding stocks they do not sell. The Smith & Wesson Model 76 is fun to shoot as well. The original magazines and other parts are hard to find and usually expensive. The one thing I did not like about the Smith is that the stock is not very solid when extended. But no options to convert to other calibers. As others have stated, the options and parts for the M16s are endless. Easy to convert to other calibers such as 22 and 9mm. I prefer the original Colt M16A1s for general use. But the conversion guns seem to dropping in price recently. The source listed in the links always has nice guns, but are known for their premium prices they charge also. If I was looking for something inline with the S&W 76 I would also consider the Uzi. They work well, plenty of parts and magazines available and you can find the 22 kits for them. The Uzi has a similar rate of fire to 76.