There are basically two types of gun. The HVLP uses what the name says, a hgh volume of fluid and shot at low pressure. Because of this it doesn't atomize as well as the other type, the traditional high pressure gun. The drawback to this type is that you get a lot of overspray. Also your transfer rate, how much stuff actually goes on the guitar rather than in the air is crappy with a traditional.
To minimise the orange peel make sure that you have thinned out your finish a lot. This is different for evry finish and weather condition. Temp and humidity have a larger influence on HVLP than conventional. Also, I use retarder,
wWhich slows down the drying time and allows for better flow out.
I have been a professional finisher for 25 years and it took me about six months to get the hang of HVLP when I first converted, but now I wouldn't use anything else.
American Turbine makes good basic guns for a realistic price, and they show up from the manufacturer, new on eBay all the time.