Its just like any other gas gun, save for the fact that it uses all of its gas in one instant. But even still the weather hot/cold will have an adverse effect on the fps.
At a game in avra valley in january, it was maybe about ~50 degres out, I popped out of a bush and pepered reaver from about 15 ft away. I can recall more of a puff than a boom, and a signficant amount of liquid gas coming out of the barrel (cloud). In turn the bbs didnt hit him as hard.
A couple of months ago at florence, it was maybe 95 degrees out, when I shot mugen with one. My grenades were literally hot to the touch, much as if one were to leave a soda can out in the sun. In this instance it was a loud BOOM, and almost no cloud of gas was visible. He very loudly told me that he was hit when this happened.
The best time that I like to use my grenades is during the hot weather time. But not too hot as in the case of florence, because that day did result in a couple of blown orings. For areas that will be close quaters, I prerfer to use the max limit that each grenade can hold. But in regular skirmishes, I never pack more than about 30 per shell.
Probably the best grenades that I use are the MM 24 rounders. They get good range, and are eaier to maintain than the others I have. Coming in second are the deep fire 180s. These are much better made than the MM, but they do have more of a problem than the MMs with blowing orings. The kings arms (i have 5 of them) work about as well as the deep fire, but are a bitch to maintan, and their build quality is almost satisfactory. The old or first production run of the MM grenades really leaves somthing to be desired. I have one of their earliest 165 rounders, and its a pain to reload, and maintain. It was really a first attempt, but finally they wised up and started making em better about two years ago.
the new mad bulls soud like the real ticket though. Ive heard nothing but good reps from them, but I havent tried them out first hand, so I cant really speak for them. It seems that they have a much better design than the 8 2mm bearings, and have 4 6mm ones. I think that these would be able to hande the heat much more satisfactorily and I look forward to purchasing some in the near future. They are actually american desinged! Yahoo!
Also: MM actually did have somthing like this rifle grenade. It just wasnt as popular, and didnd have as big of a market as the mad bull one.