I used to be a firm believer in Venturi bolts. Long before the first venturi bolt came onto the market for autocockers I was trying to come up with a way to make my own. Much of the reason I want a Lathe/Mill so badly today is due to the frustration I encountered while trying to come up with a venturi bolt all those years ago. My obsession began years ago when Air Power released the Apex Elite pump gun. One of the main features it touted was a Venturi bolt. Accuracy of the Apex was legendary. Many, myself included, attributed this to the venturi bolt. When Smart Parts finally released a Venturi for the autococker, I bought one of the first. Clapping it into my Mini-Cocker I made an interesting discovery.
I could not believe it. With the venturi bolt installed, my long range accuracy fell to nothing. The only noticable improvement was less of a tendency to break paint. This set me to thinking. It all comes down to gas flow. Gas flow has to be the single most critical, but yet unappreciated factor in paintgun accuracy. With the exception of Air Power, I don't think any company puts a moments thought into gas flow. Yet this factor is so critical to accuracy. When an uneven flow of gas hits the paintball it imparts a spin to the ball. Spin in a paintball is not a good thing. Here's why. First, a paintball is not round. The seam tends to be slightly greater in diameter than the highest point on either half. I won't go into the aerodynamics of this but they are not good. Secondly paintballs are not solid like a musketball. They are a liquid filled capsule. Even if the shell was perfectly round and you fired it from a rifled bore it would not fly straight. The reason is that the liquid inside the shell would not spin at the same rate as the shell itself. This gyroscope action will produce an erratic flight path. How many times have you seen a paintball corkscrew through the air or do otherwise bizarre things in flight? I've seen them do things that were nearly baffling.
By preventing all spin and giving a paintball the most even 'push' possible out of the barrel the greatest level of accuracy can be achieved. But why doesn't a Venturi bolt do this? Back to gas flow. An Autococker does not have an inline valve. The valve is positioned below the bolt and during firing the gas comes up from below, makes a 90 degree turn, then encounters the paintball. This 90 degree turn takes place only one and a quarter inches from the point at which the gas encounters the ball. The sudden turn the gas must take when it enters the bolt creates a high pressure area where it encounters the top of the bolt and a low pressure area at the bottom where it rounds the corner. The high pressure gas at the top exits via the uppermost ports in a venturi bolt while lower pressure gas exits at the bottom ports. This pressure differential imparts a spin to the ball. During the first instants of the paintballs flight while velocity is still high the spin is insignificant. But as air resistance bleeds off this velocity, the spin begins to alter the balls flight path. The result is a tendency to 'hook' at a distance down range. Hook is bad. Hook is evil. Long range accuracy is far more critical than short range.
All Sheridan Based guns suffer from this problem. My old Mac-1 Avenger hooks badly. My friends and I were aware of this hook years ago, but we never stopped to think about what caused it. We just accepted it as 'the way things were.' So figure that your Autocockers, PMI pumps, Typhoons, and Sterlings are gonna have some hook. The ideal bolt would be one designed to correct gas flow before it reaches the ball. A shallow curve inside the bolt with a diffuser to redirect some of the gas back toward the bottom would be an improvement. But such a bolt would be difficult to design and even more difficult to build.
With gas flow in mind, examine your markers bolt. Many of the currently popular designs have appalling flow characteristics. The graceful tapered styles that have stright drilled ports leading back to an open chamber actually channel an even greater percentage of the gas out the top ports. To get a more graphic demonstration go hold the thing under the faucet. Even when you add an artificial 'chamber' to redirect the flow of water the flow is still markedly uneven.
Two options remain for owners of Sheridan Valve guns. First is to wait for someone to actually design a bolt that corrects flow. I'm working with a few ideas but have yet to achieve satisfactory results. The other is to modify the guns gas flow path. If the distance between the bolt face and the point at which the gas is redirected were greater, flow would equalize. This is done in Air Power's Vector. In the Vector, gas is directed to the bolt face via a long power tube. Even though the gas must make a 180 degree turn in the Vector, the power tube gives time for the flow to equalize. If you have ever encountered a Vector you know just how accurate they are. In other Sheridan-Based guns it is theoretically possible to plug the original valve hole, build a new bolt with a long gas path. And redirect the gas from the valve to a point furter back away from the bolt face. This could be done with a side tap from the valve chamber and a hard line running back toward the cocking block on an autococker. A new bolt with a long power tube and rear seals would have to be constructed. This bolt could be Venturi ported to improve gas flow and reduce deformation of the paintball. I will be attempting this modification on my 'cocker in the near future. I'll keep you posted.
The venturi concept is not bad. It works in markers that have inline valves. All nelson-based pumps, Tippmans, mags, and a few others I am sure. But for markers that don't have a straight gas path I would steer clear of them. I believe this list also includes the Spyders and their clones though the spyder may have a longer gas path. I would be very certain I knew the exact workings of any marker I owned before shelling out any cash for a hop up part that might not do me any good. And don't just take peoples word for it. Far too many players will tell you that your gun sucks if it doesn't have such and such part. Mostly this is just elitism talking. They have some part and that makes them cooler than you. I've seen too many tricked out guns that wouldn't shoot for squat because their owner judged performance by price tag alone. The attitude is that every modification improves performance and so the more modifications you have the greater your performance must be. This is not true. Some mods actually interfere with one another. You recieve diminishing returns on your modification investments. When it comes to making these decisions, believe what your eyes tell you. Take the gun out and test it frequently and after every modification. If you suddenly can't hit the target you were hitting before you got that 'Great' new barrel maybe that barrel isn't so 'Great.' Or maybe it's just the paint you're shooting. Try a different brand. The thing is to conduct logical and scientific evaluation of performance at each improvement. Don't just assume that because you spent $$$ that you are going to see improvement. I've wasted a ton of cash on improvements that didn't improve anything. Venturi bolts were one of them.
scarecrow
Here's a side note. The term Venturi, when applied to a paintball gun bolts is a misnomer. The Venturi Effect is just the opposite of what a Venturi bolt does. A Venturi Bolt is rightly a Diffuser Bolt. A true 'venturi' bolt would taper down to a single small port. Greg Koteski of Air Power was the one who first applied the term to a paintgun bolt when he introduced the Apex pump-gun. The Venturi bolt was devised in the days before velocity restrictions were in use because the thin shelled paint of the era could not withstand the accelleration of high velocity use. The venturi bolt reduced ball breakage at high velocities.