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Having followed the rec.sport.paintball newsgroup since it was formed. We've many a time seen the statement made that closed bolt pump paintguns, and closed bolt semi-auto paintguns are inherently more accurate than open bolt semis.
Eh? What does that mean?
Basically these terms refer to the position of the paintgun's bolt at rest position before firing. In a closed bolt system, the bolt remains forward, sealing the breech (back of the barrel) until the paintgun is fired. Then it is opened, the new pellet drops in, and it is ready to fire again. In an open bolt system, the bolt remains open until the time of firing, when it closes, chambering the paintball, then it opens again, for a new ball to fall into place. Many people argue that since the ball is jostled in the fraction of a second before firing, and since the bolt may open again before the ball actually exits the barrel, an open bolt paintgun will not be capable of the same accuracy as a closed bolt paintgun.
Some open bolt paintguns:
Enter WARPIG Ballistic labs. We set out to prove or disprove the following statement:
Since we were testing the accuracy of a paintgun, not our marksmanship, we constructed a steel test stand for the marker. By locking the marker to the stand we were able to take the same shot, over and over. Additionally we outfitted our test marker with an Adco Square Shooter sight. By aiming the sight at our reference target prior to each test grouping, we were able to check it after each shot to make certain that our test stand and target had not moved.
For our testing we used Nelson Challenger, orange shell, orange fill. Any pellets which were visibly out of round were discarded and not used for testing.
The test marker:
Choosing the marker took a bit of deliberation. We wanted a marker that could fire both from an open bolt, and from a closed bolt, and those just aren't to easy to come by, so we took to the shop. Since we needed to do some work, on the receiver, and it would be work not needed for regular paintball use, we decided to use a Stingray. A stingray receiver can be hacked on, and thrown away when the special use is done, because a new one only costs $5.00. This also worked out well, since the Stingray's valve design is very similar to the Nightmare valve from which it is derived, and would therefore work as a bolt action/pump, in addition to a semi-auto. Our conversion of the Stingray was relatively simple. First, we took the bolt, and removed the link arm which connects it to the hammer. Then, we drilled and tapped a hole in the bolt, for an operating handle. We milled a slot in the receiver, to allow the operating handle to be reached. Then, we straightened the link arm, and drilled it to allow it to be connected to the hammer's operating knob, and our new bolt operating knob, on the outside of the receiver. Now, the Stingray would fire as a semi-automatic, or by removing the link arm, it would operate as a bolt action marker, without changing any internal parts. This way everything would be the same, except for the action. The same barrel, the same bolt, the same CO2 supply, etc.
The Stingray we worked on was not entirely stock. It has a shortened barrel shroud, a polished and vented (in the last two inches) barrel, a vertical CO2 supply, and re-worked trigger assembly. Since the Stingray's barrel mount allows a slight amount of barrel movement, we added a barrel brace to our test stand. This test is not about the performance of a modified Stingray, but a difference in performance related only to the way the bolt is operated.
We began with a series of 5 test shots, to check velocity in semi-auto mode. Our average velocity was 267 feet per second. Fine for testing, so we didn't adjust it. Then, after cleaning our target, we fired a group of 20 shots at the target, with an approximate 2 second wait between shots. The wait was to ensure that we didn't cause a CO2 pressure drop off from rapid fire, that would not be present when firing in bolt action mode. We measured our first semi-auto grouping. Eleven inches between the furthest points.
A quick removal of the link arm, and the Stingray became a bolt action paintgun. We fired another 5 shots, this time with the bolt sealed prior to, and during the firing process. We expected to see a higher velocity, due to the fully sealed bolt, but this was not the case, the velocity still averaged 267 fps. We cleaned the target again, and fired a group of 20 paintballs, again with an approximate 2 second delay. This grouping was 11.5 inches. Slightly larger than the first semi-auto grouping, but not by much.
To make sure that we did not see performance differences caused by changes in the CO2 tank, we fired a second closed bolt grouping, followed by a second semi-auto grouping. Our second closed bolt grouping yielded an 11 inch group, and the second semi-auto, an 11 inch group.
By testing under controlled circumstances we were able to isolate the subject of our test (the bolt operation) from the other variables involved. Firing our marker as an open bolt, blowback operated semi-automatic, we found the same level of accuracy, as when firing as a manually operated closed bolt marker. Our conclusion - the great "inherent accuracy" of closed bolt markers over open bolt markers is a myth.
Then why does closed bolt marker XXX outperform open bolt marker ZZZ?
That is an entirely different question, as there are many, many more factors to paintgun accuracy than the way the bolt is operated.
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