Doc, Shinnlinger kind of nailed it.
There are big differences between a gravity feed gun and a HVLP (high volume low pressure) gun. Both can be gravity fed which mean (simply) the cup is on the top of the gun body.
I apologize in advance for all the silly abbreviations and acronyms that developed in California where this two part history started.
GUNS:
(newer HVLP guns have a transfer efficiency to 80% vs. **siphon fed guns at 30-35%)
I’ll try and make this brief IF I can. In the late 70’s and early 80’s OSHA and CARB (California Air Resources Board) mandated that all paint guns produce less VOC’s (Volatile Organic Compounds) which was roughly 65% of unusable over-spray and 35% of actual paint chemistry applied to the subject surface. In the process new guns were designed and manufactured as a cure all to counter the high levels of the VOC’s and only a few actually worked…and many others were dismal failures. Most painters did not reform and kept (hidden) using their trusty siphon guns. But alas the war ended soon after OSHA teams were selected and visited plants, industries and body shops (like my own). Paint storage areas, mixing rooms, paint equipment (guns) and spray booths were inspected and you either passed or failed. If you failed you had 180 days to meet conformity or you lost your business license. I passed in 91 days at a great and unexpected expense.
PPE:
(personal protective equipment)
Painters were dieing, some from long term systemic effects from isocyanides poisining and some actually died in their spray booths from respiratory failure. Soon after(but not soon enough), Gloves, full faced respirators, Tyvec suits and fresh air supplied masks-hoods were introduced to meet and combat all the deadly toxins associated with the newer paints.
**As Shinnlinger stated…siphon guns are illegal to use for painting or have on your inventory of shop tools. In some instances businesses like upholstery and furniture shops can use siphon guns for spraying low VOC’s adhesives…but you still can’t legally buy one in the state of CA.