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A look at Rebound
By Bill Mills - March 2005

Back in 1997, Smart Parts introduced Turbo Mode for the Shocker paintgun.  This immediately led to controversy while teams, tournament promoters and referees debated whether or not Turbo Mode was legal in tournaments, or even under paintball field insurance policies.  

In 1998 my wife and I stopped in at Smart Parts factory in Pennsylvania on the way home for the International Amateur Open.  While there we had the opportunity for an in-depth demonstration of Turbo Mode, not just how it felt on the paintgun, but how it worked, recording solenoid outputs and trigger switch data with a multi-track oscilloscope.

That trip, interview and testing resulted in the article “A look at turbo mode” (the original article can be found here.)

At the time the article was written the NPPL, and other leagues still hasn’t decided exactly how to classify Turbo Mode.  Those who wanted to use Turbo Mode argued that the signal spikes coming from the switch all counted as trigger pulls, and thus Turbo Mode never fired more than one shot per trigger pull, and was a legal semi-automatic.  Those who didn’t want Turbo Mode allowed argued that “trigger pull” meant a pull and release on the trigger by a finger, and sub-movements, or switch noise did not count as trigger pulls, thus Turbo Mode fired more shots than trigger pulls, and was not tournament or field legal.

What was needed was a clarification in rules.  Trigger pull needed to be more clearly defined so that everyone following a rule set could clearly tell if Turbo Mode would be allowed or not.  In short, the rules needed to catch up with technology.

For a short time Turbo Mode was allowed, subject to a rate of fire cap while teams and promoters heatedly argued about how it should be classified.  The NPPL re-wrote their rules and Ultimate Judge, Bill Cookston decreed that Turbo Mode did not fit within the league’s definition of semi-automatic.  This rendered the debates over legality moot.  

Similarly most companies insuring paintball fields stated that Turbo Mode did not fit within their definition of semi-automatic, and thus would not be allowed for use.

After a bit of an absence, various enhanced trigger modes have begun appearing on the market.  In some cases, paintguns have had internal software that works on the same principles as the original Turbo Mode’s shot buffer and have a user adjustable software switch bounce filter.  If the debounce setting is turned low enough, the effect is similar to that of the original Turbo Mode.

To complicate things even more, some aftermarket companies have even produced circuit boards that fire several times for each trigger pull, but only when a secret mode is activated.  Because this type of software based cheat is undetectable without knowing the code to activate it, some leagues such as the NXL and PSP have modified their rules once again.  At the time of this writing, both the PSP and NXL allow enhanced modes of fire with certain safety-oriented restrictions and subject to a fifteen ball per second maximum rate of fire cap.

Seeing the writing on the wall for changes in allowed firing modes, Smart Parts revived the concept of Turbo Mode by putting what they call Rebound as a feature in the Nerve,  later model Shocker SFT paintguns, and the Ion.

Nerve and Shocker Rebound is adjustable on a scale of one to five.  In both cases, the mode is changed by pressing the power switch for approximately one second, and then pressing the dwell setting buttons to increase or decrease the Rebound setting value.  

Each time an adjustment is made, the circuit board responds with a chirp.  When either the upper or lower limit of the setting is reached, a series of short beeps is heard.

Because the dwell setting buttons are on the paintgun’s lower circuit board inside the grip frame, changing fire modes on the Nerve and Shocker SFT can only be achieved with the use of tools to open the grip panels, and cannot be done readily by a player on the field.  This allows the ‘gun to basically be locked into field legal semi-auto operation, and still usable on the many fields that won’t allow the ehanced modes.

The lowest rebound setting, 1 is rebound off, or traditional semi-automatic. At the highest setting of five, the Rebound will increase the rate of fire as much as possible.

Like the original Turbo Mode, Rebound doesn’t start working its magic until after a few shots have been fired at a reasonable rate of fire.  This is a safety measure, so that a paintgun won’t fire multiple shots if the trigger is accidentally pressed.  Rebound will only unleash its hail of paint when the trigger is being repeatedly and consistently pulled.

To see just what kind of difference Rebound makes to overall rate of fire, a Nerve was placed on the WARPIG Ballistic Labs test stand, and rigged for data.  To be more specific, most of a Nerve was placed on the test stand.  The receiver was removed from the grip frame, and test leads were attached to the Nerve’s solenoid valve so that a computer could record the signals sent from the circuit board to the valve.  The WARPIG electro-pneumatic finger was installed on the trigger guard to activate the trigger at consistent intervals.  

The pneumatic finger is made from silicone rubber over a steel armature.  A pressure transducer, linked to a recording computer was fitted between the steel and silicone to record how hard, and how often the finger presses up against a trigger.  A pneumatic ram powered by low pressure air from a Palmer’s Micro Rock Regulator and Crossfire compressed air system, controlled by a microprocessor driven solenoid valve allows the finger to pull triggers at predetermined rates of fire.

To get a picture of what effect the Rebound setting has on the Nerve, it was set to each of its possible Rebound settings, and then the trigger was pulled for 10 shots at one shot per second, 10 shots at 6 shots per second, 10 shots at 10 shots per second, and 10 shots at 12 shots per second.

First, with Rebound set to 1, regardless of the rate of fire, there was a direct correlation between trigger pulls and shots fired (or, more precisely activations of the solenoid valve.)  Using the present definitions of semi-auto for the NPPL, the Nerve was field legal with a Rebound setting of 1 (off) for all the rates of fire tested.

With the Rebound moved up to a setting of 2, or the lowest active rebound setting, performance was the same from 1 to 10 balls per second.  However, at 12 balls per second, the correlation between trigger pulls and shots fired changed.  After the fourth trigger pull, the Nerve began firing additional shots, at a rate of roughly two shots per trigger pull.  Effectively, the rate of fire was double the rate of the trigger pulls once Rebound went to work.  This is not legal under current NPPL rules or most paintball field insurance programs, but is allowed in PSP events, as long as the timing settings restrict the rate of fire to not more than 15 balls per second (for this test, the Nerve’s recharge setting had been turned down to its minimum to show to prevent a low rate of fire cap.)

At a Rebound setting of 3, the rate of increase was similar (two shots fired to one trigger pull) but the Rebound effect kicked in at a lower rate of fire.  At 10 shots per second, it was clearly noticeable as well as at 12.

At a Rebound setting of 4, the traces were comparable to 3.  Further testing at intermediate rates of fire between 6 and 10 balls per second could determine if Rebound 4 activates at lower rates of fire than Rebound 3.

Like levels three and four, Rebound 5, the strongest setting, kicked in the Rebound effect on the fifth shot, effectively doubling the rate of fire.

On the field, Rebound mode often can be spotted simply by listening to a player shooting.  After the fourth shot fired the rate of fire will increase and become consistent in its timing.  In the hands of a player with a fast trigger finger, it becomes harder to detect, because they can get those first four shots closer to the rate Rebound can provide, making the difference in rate of fire less distinct.  Because the Shocker and Nerve to not show if they are in a Rebound mode without opening the grip and adjusting them, it is important that those players who are using them know their equipment and make certain it conforms with the rules where they are playing.

Author's note:  The astute observer may notice that some of the trigger recordings have what appears to be a fast double-tap of the trigger.  This is an artifact caused by a shift of the pressure sensor leads when the finger retracts.  The size of the attitional spike, when it exists, depends on how the wires were hanging and flexing during that trigger pull.  

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