Stock valve
Turbo-valve
In this modification of the stock valve, the cup-seal of the pin is shaved narrower near the seal, and the valve opening to the bolt is widened. The end result of this is reduced resistance to gas flow to the bolt, and thus increased efficiency. This was originally designed for and used effectively on the old Piranha LB's back in the pump gun days and improved shot efficiency by 20-25%. Obviously this will be much less on an autococking gun due to gas use by the autococking system. By the way, KAPP now offers a stainless cup seal that you can drop into your stock valve guide to get the same thing for less money and less trouble than having it done as a mod.
RAT valve
The Rapid Air Transit valve is a drop-in valve made by Bad Boyz Toyz. This valve includes the improvements of the Turbo-Valve modification in addition to increasing efficiency by eliminating blow-back around the stem of the cup seal into the 'gun body (normally uselessly lost). BBT claims a 30% increse in efficiency. My experiences with the valve tend to agree with that figure. I'm getting many more shots, especially with nitrogen, than I ever did before with an equivalent sized tank.
RAT valve II
The RAT valve II is the sequal valve to the original RAT. It offers several significant improvements over the original valve design. First, the new valve is of all stainless steel construction. Second, the valve orifices are opened even wider to provide even more free flow through the valve. Third, the valve seal is now incorporated into the valve body rather than in the cup-seal. The result is that the seal is much easier and much less expensive to replace (~$2 as opposed to $20 with other valves) in the case of a leak. It is also designed to run at 400 psi - lower than the original RAT. Efficiency so far is a bit better than the RAT valve. More impressive is a significant improvement in velocity consistency (averaging +/- 3fps). This is also the lest expensive stainless valve on the market.
RAT valve 3:16
The latest iteration of the RAT valve now has an aluminum valve guide (the main body of the valve) to reduce cost, making it about $10 cheaper than the RAT II. The wide gash in the top of the valve has been replaced with a hole that matches the channel up to the bolt. Finally, an extra seal is included with the valve and is meant to be mashed in between the jam screw (the valve retaining screw behind the valve) and the back of the valve for a better seal and to help keep the jam screw from backing out. Works every bit as well as the RAT valve II, but less expensive.
ANS valve
ANS' valve is a pretty standard turbo-style valve design, but knocks off the RAT II cup-seal design of placing the seal in the valve body. I haven't had a chance to test it yet except in the ANS Gen-X Autococker where it gave approximately stock valve performance.
Tornado valve
The Tornado was designed as a super-low pressure, super-high efficiency valve. In my testing (article to come), I've found their claims to be true, allowing me to get 1300 shots from a 68 ci 3000 psi nitrogen system, running just 200 psi into the ´gun. It has some strict requirements to get it working properly, though. You need a large valve chamber - Autocockers older than 1997 models need to be drilled out and all Minicockers require an air reservoir to increase the valve chamber size (see Miscellaneous for more information). You also need a high-flow bolt, a tight-bore barrel, a Nelson-stye spring kit, and an extremely consistent low pressure air or CO2 system. Performance is superb, but the price of entry is steep.
No picture yet. Bob Long valve
The Bob Long valve is designed to allow you to run your Autococker at low pressure - in the 300-350 psi range, though I have heard of people running it much lower. This brass valve shares some striking design similarities to the Tornado valve that preceded it. The big difference is an angled channel rather than a wide-open chamber. I don't have much experience with the Bob Long valve, but I've heard that it provides decent performance. Like the Tornado valve, it does require a ´97 or newer Autococker, or a drilled out valve chamber in an older Autococker to work properly.

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