As stated above, in the Stingray there are three dynamic seals - the hammer, the valve stem, and the bolt. The hammer and the bolt both act as pistons, with the o-rings in a groove on the moving part. (See drawing to the left.) The o-ring on the hammer prevents the loss of the blow back pressure between the hammer and the inside of the hammer tube, forcing the hammer to move backwards to recock. If the hammer o-ring is worn or damaged, it won't recock consistently, or not at all. If the bolt o-ring is worn or damaged, gas will escape around it, causing lower velocity.
The valve stem o-ring, on the other hand, fits in a groove inside the valve housing. The o-ring remains stationary, with the valve stem moving in and out of the o-ring as the hammer strikes it.
If the valve stem o-ring is worn or damaged, gas will escape around the valve stem, lowering velocity, and in extreme cases, reducing the amount of blowback gas. As stated above, in the Stingray there are three dynamic seals - the hammer, the valve . While o-rings are made of many different compounds, your Stingray's o-rings are generally of two different elastomers - Nitrile (buna N) and polyurethane.
The majority of the o-rings are nitrile. Nitrile, aka buna N, is a common rubber compound with two qualities important to paintballers - resistance to carbon dioxide (both liquid and gaseous) and automatic transmission fluid (ATF). Nitrile o-rings have a medium to high abrasion resistance, and are resistant to most petroleum based lubricants. Nitrile o-rings also have a very broad temperature range: -65F to +275F. The nitrile o-rings in your 'Ray are the black ones. They are softer than the polyurethane. The nitrile o-rings in your 'Ray are on the hammer, bolt, valve housing, and valve retainer.
The milky-white o-rings are polyurethane. They have a higher abrasion resistance than nitrile, and are usually of a higher durometer. The polyurethane o-rings in your 'Ray are on the transfer tube and inside the valve housing (the valve stem passes through it).
Whenever you disassemble your Stingray for cleaning and lubrication, you should closely examine all your o-rings. Look for cuts, nicks, rough spots, and worn areas. Any defective o-rings should be replaced immediately. Always carry spares with you to the paintball field. A complete set of o-rings for your Stingray costs about $3 from Brass Eagle. Cheap insurance!
You should lubricate your o-rings well before reassembly. I soak mine in ATF (or whatever lubricant I'm using) for a while before I put things back together.
A few words about alternative sources for o-rings are in order. I realize that you don't always have the luxury of waiting a week or two for an order to arrive from Brass Eagle. O-rings can be bought at hardware stores, auto parts stores, bearings suppliers, hydraulics and rubber suppliers, and heavy equipment (forklifts, etc.) dealers. They can also be found at most plumbing supply stores.
Hardware store o-rings are generally of two different materials: ethylene propylene (EP) or nitrile (buna N). They are both black, and unless it states on the package, you can't tell which is which. The problem is this - EP o-rings are NOT compatible with CO2 or ATF. EP o-rings used anywhere in a paintball gun will swell horribly and become unusable very quickly. If your o-rings are swelling like this, they're probably EP.
Nitrile (buna N) o-rings, on the other hand, ARE compatible with both CO2 and ATF, and other lubricants. Since EP and nitrile o-rings look identical, you may have trouble getting the right ones from a hardware store. If you ask the guy whether they're ethylene propylene or nitrile (or buna N), you'll most likely get a blank stare and an answer like "Uh, they're rubber."
You may be better off getting o-rings from a rubber supplier, a hydraulics supplier (like a forklift or diesel truck place), or maybe even a Napa auto parts store. They will probably know or be able to find out what compound their o-rings are made from.
You may have a tough time finding polyurethane o-rings, though. Nitrile o-rings can usually be used on the transfer tube and in the valve housing with no problem, although they may not last as long. But they should get you through a day's play. Just be sure to lubricate them very well.
An o-ring can also be identified by its dimensions. The two dimensions used most commonly are the inside diameter (I.D.) and cross section (thickness). Sometimes, though, an o-ring will be identified by its inside diameter (I.D.) and outside diameter (O.D.). All three dimensions are listed in the chart below.
The most recurring o-ring problem I've experienced in my 'Ray is the hammer. The hammer and top valve retainer o-rings are the key to the Stingray's blow-back action. The blow-back action is very simple - every time the hammer hits the valve, a small amount of gas is released inside the hammer tube, which pushes the hammer back for the sear to catch.
If the o-rings on your hammer and/or valve housing are worn or damaged (or the wrong size), some of the gas will leak around the o-rings, and the full force of the gas won't push on the hammer. Replacing damaged o-rings can make a world of difference.
During normal operation, the hammer slams back and forth with some force. This can cause burrs to form around the pin holes in the soft aluminum of the tube. (See photo above.) These burrs will damage your hammer o-ring whenever you remove the hammer. Before you replace the o-rings, you should deburr the inside of the hammer tube. I use a fine grinding stone in my Dremel to smooth the burrs, then I repolish the inside of the tube. (See photo to the left.) Be sure to deburr both ends of the tube, as burrs can form at the donkey pins and valve pins.
Deburring the hammer tube and replacing the hammer and valve o-rings often have made a huge difference in the consistent recocking of my Stingray.
Finally, I'd like to discuss CO2 tank o-rings. These are the o-rings that fit on your tank valve that seal it when you put the tank on your gun. It fits in a groove at the top of the valve. The tank o-ring is usually polyurethane, but some are made from PTFE. PTFE o-rings are white, and as they are of a higher durometer (100+), they are stiffer than polyurethane. They seem to be more abrasion resistant than poly, but may not seal as well in some situations. One very common problem with the tank o-ring is the threads in the donkey or bottomline (duckbill) cutting it.
When you unscrew your CO2 tank from your gun while it's pressurized, the pressure will tend to push out the o-ring, and it'll get caught in the threads. To reduce this problem, when you remove the tank, only unscrew it a couple of turns. Fire the gun until the pressure is released, then finish removing the tank. This will dramatically increase your o-ring survival rate!
In a pinch, you can use nitrile o-rings on your tank, but they don't usually last very long. One trick I've done, especially with nitrile o-rings, is to not put the o-ring in the groove on the valve. Instead, I'll put it inside the donkey so that it will seal against the face of the tank. (This is presupposing your tank face is undamaged, ie. it hasn't been dropped.) The only problem I've encountered with this method is losing o-rings. There's a chance they'll drop out when you remove the tank.
If the threads inside your donkey and/or on your tank valve are VERY sharp, you can smooth them a little bit with some 400 grit wet-or-dry sandpaper. Be careful you don't get overzealous, though, and ruin the threads.