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Posted: 9/14/2011 10:21:20 AM EDT
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Went to the range the other day and after the 46th round fired, the bolt carrier failed to go into battery by about .5 inch. Separated the receivers and found that the operating rod was stuck tightly about .5" rear of its normal position. Went home removed the scope and top half of the handguard. Found that the piston had been blown completely out of the gas cylinder. On its way back forward the piston could not realign with the bore of the cylinder and hung up. It was easy to reinsert the piston back into the cylinder and the return spring easily returned everything back where it belonged.
During the previous 2500 rounds through the sytem, this never happened. I was firing handloads that were loaded at less than book max. Just looks like the 3 cylinder vent holes could not relieve the cylinder pressure fast enough once they were unported. Basically blew the piston out of its cylinder. Anyone else with the Ares / Bushmaster retrofit system ever have this happen? |
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Quoted:
Happened to me also but only when I had a sound suppressor attached. So the common theme seems to be something that creates a pressure (or backpressure in the case of the suppressor) that overloads the system and basically blows the piston too far rearward. I still place my bet on the handload being a tad on the "warm" side. |
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This can also happen with a weak load. The slow bullet velocity increases the dwell time, and if this is longer than the piston system was designed to cope with then those vent holes aren't just venting the gas from the cylinder, they're trying (and failing) to vent the gas from the barrel too.
The way the short-stroke system is supposed to work is the bullet passes the barrel port, gas vents into the gas cylinder and starts accelerating the rod, and the bullet leaves the barrel while the residual gas in the cylinder finishes moving the rod. The higher the pressure in the barrel the faster the bullet leaves and the flow of gas to the gas cylinder stops. The higher the pressure in the gas system the faster the rod accelerates and the faster the vent holes open up to relieve the pressure and slow the acceleration of the rod. This helps the short-stroke system operate correctly over a fairly wide range of pressure curves, and is one of the advantages of the short-stroke system over the long-stroke system. However this only works correctly as long as no more gas is entering the gas cylinder, if you don't have a quick pulse of gas but instead a stream of gas then the system can be overwhelmed by the sheer volume of gas it's asked to deal with. The M14 had a shutoff valve in the gas cylinder to help ensure that it only got a quick pulse of an appropriate duration, but most recent short-stroke designs do away with this on the basis of cost and simplicity. Rather they simply take care that the gas cylinder is well-supplied from the barrel, and to shut off the gas flow they depend on the bullet exiting and quickly dropping the barrel pressure, and then depend on the ports to handle any excess gas from a high-pressure situation. An underpressure load causes a variety of problems for a short-stroke gas system. One is because the combustion rate of smokeless powders tends to be pressure-sensitive, a lower-pressure round lets the powder burn longer, so the powder may well have still been burning furiously as it passed the gas port, and may have entered the gas cylinder in this state. Second is that the slow bullet meant that the pressure in the bore was high for a relatively long time, possibly long enough for the piston to pass the first vent hole so that it was trying to vent barrel-pressure gas (and a barrel-volume of gas as well) through a hole designed to handle a small fraction of that pressure and volume. This vent hole would not have been able to drop the pressure as much as designed, so the cylinder pressure would stay high and the rod would keep accelerating. Long-stroke sytems have their own flavor of problems in this situation; google "Garand bent op-rod" for some examples. |
| Not with the Osprey and thats a short stroke system but the gas system is metered and only takes enough gas to cycle the system so high or low pressure ammo or using a can has no effect on the cycling thats why for the drop ins the Osprey is the best in my opinion. |
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