I've been trying to find out more about options positions which are both delta neutral and gamma neutral--created with some kind of calendar spread. Supposedly, such a trade will be perfectly hedged with relation to the underlying so that the value of the position will change only through a change in vega and possibly a small amount of theta. This would seem to be a good way to trade reversions of implied volatility...creating a positive vega position when betting on increasing implied volatility and creating a negative vega position when betting on decreasing implied volatility.
It sounds like this would be a superior way to trade volatility since being gamma neutral would remove the need for continuous delta hedging. However, I don't read much about people doing this. What am I missing?