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Posted: 3/21/2003 2:43:16 PM EDT
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OK down and dirty, The use of a shorter barrel (distance of length of barrel past the gas port) requires that the gas port to be opened up to work with the shorter distance beyond the gas port before the pressure is bleed out the muzzle. This leads to the bolt unlocking with a bit more force/speed, and the extractor can loose the spent case rim as it comes back (the spent case is still barrel gas pressure bound. The H buffer slows the carrier cycle, which slows the bolt unlocking and allows the spend case to become a little less chamber wall bound. An expanded volume gas tube will do the same as the H buffer, which that it slows the carrier down by softening the pressure spike. There are two typed on the market. The first is a pigtail design. By spiraling the tube around the barrel, this allows the tube to a much longer length than if it was straight from the sight to the carrier key. The second type is a fat tube, which does the same as the spiraled tube; it increased the volume of tube to slow the overall pressurization of the tube and delivery to the carrier gas chamber. Another option instead of trying to control the gas pulse is to reinforce the standard strength of the extractor over the spent case rim. This added pressure of extractor engagement force is done by either using a stronger spring, or adding another pressing force around the spring, such as a D-fender. The down fall to the added pressure is that while you are reinforcing the strength of the extractor, you are also making the extractor work harder at climbing past the rim of the live round on loading, and why I consider such a way as a band-aid, verses a true resolve to the problem. Bottom line is that on your upper, it sounds like a few of the corrections were made, then pulled off when it was sold to you. In the rifles current form, the action cycles is fast/high and the extractor is loosing grip of the spent cases rim as the bolt/carrier full strokes. Considering that Bushmaster sells a H-buffer for less than $30, It may be wise to just buy the buffer and order a Extra strength extractor spring from Wolf and be done with the problem. As for changing out the gas tube to an over volume design, it is not really needed. Your more than welcome to try other ideas out (re-invent the wheel), But the last time that was done, BAFT called the Moderator (1) a NFA item and deemed that you have to pay the $200 tax for a device that reduced the report signature by only a few db’s. (1) The moderator on the Colt XM177 was used to keep back pressure on the barrel after the bullet had left the barrel. This allowed for a smaller gas port to be used to charge/unlock the carrier/bolt. Hope this helps. Dano |
| Although I can't comment on the Oly lower, all other parts should be good. The "H" buffer is not always requiered for 14.5 inch barrels. It mostly is needed to solve bolt carrier bounce problems on full auto and to slow the cyclic rate. Ejection/extaction problems can be very complex with many opinions on how to fix them. The most common fix is by using high power extractor springs or the D-fender. I've used both liking the D-fender the best(because it will never wear out). Its rare that the ejector or its spring is the problem for your type of malfunctions. Check for a loose bolt carrier key for the short stroking. Those small hex bolts must be tight and staked in. If your upper reciever is a M4(extended feed ramps), it would help with feeding different types of ammo. That dose'nt mean that the ammo would function well beyond the feeding. I would sugest you get the weapon working reliably with quality ammo, then experment with less quality if you want to. |
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Quoted: [b]Q: Do the full-auto Colts use different gas rings? (perhaps looser as they may get hotter and would expand, just a thought)[/b] No, the difference between your rig and the auto rig is the hooded carrier. On your rifle, there is a chance that you had some hammer/carrier binding and the one-piece gas rings over powered the problem. Instead of the new gas ring, chances are, if you would have kept the upper wet with CLP, and trimmed the hammer tail, the rifle would have broken in (upper/carrier mated) and the rifle would have run fine with the three-ring set. |
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[b]Q: How would binding in the bolt/carrier/hammer contribute to lowering the point of impact?[/b] To answer your question, differences of release chamber gas pressure at time of action cycle changes the barrel harmonics end exit point of the muzzle. Lets say that your upper receiver key slot is very tight, verses the key slot being loose and allowing the carrier to freely open up as designed. On the tight key way, when the rifle is fired, the escaping gasses that are normally lost out the chamber are retained. This means that as the bullet passed the gas port, the bolt/carrier was a little slow opening up. This would account for a little more of bullet speed to group higher. In stationary world, faster means that the bullet gets their sooner, and would make the bullet strike higher, but we don’t live in such a world. We live in a world that has to adhere to the forces of nature, being torque the most importation rule in regards to forcing a projectile down the bore of a flexible steel shaft. As the bullet moves down the barrel, the torque of the moving object creates an opposed reaction to the barrel. This is called barrel harmonics. As the bullet moves down the barrel, the barrel reacts to the toque of the round by moving in an oblong circle-flexing pattern, not simple up and down or side-to-side movement. Once the bullet reaches the end of barrel, is will exit the barrel whereever the muzzle is pointing is regards to the harmonic movement. Items like barrel dampening and change in bullet speed will affect the flex/rhythm of the barrel during harmonics. Due to the bullet leaving the barrel faster than a normal cycle, the muzzle is in a different whip cycle/placement and this different end whip exit point, along with the faster speed will change the POI. Also, burn rate of gunpowder also has an effect on barrel harmonics. You can have two rounds that achieve the same speed, with the same bullet weight, but due to the different characteristics of the powder burn rate/pressure rate, the barrel harmonics will be different. The easy way to prove this is just turn off the gas system on a FAL rifle. The POI will be different that if the gas system is turned left on. Also, the New POI will not be straight up, as some would think. It all depends on the barrel whip and influences of dampening. |
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Hey bt22, First, go easy on Dano, he is probably the most regular and helpful contributor on this thread. Second, his last note only explained how carrier fit changes can change POI, which is what you asked... Changing the rings will cause impact shifts. Look on the competition thread for my note and Tippie's response on how changing gas rings changed my POI. Third, I too am skeptical that the rings really made much of a difference. You were breaking in the carrier to the upper while you were also changing the gas rings. The coincidence of the two COULD have caused you to attribute the fix to the rings when it was indeed break-in phenomena. It could also have been that the changed rings DID make the difference... No matter which way you approached it, the two issues would have been confounded and you would have had difficulty ever resolving it with one rifle because they smooth out with firing. Your rifle works. Terrific. That was the goal of everyone who replied. |
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