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Posted: 5/23/2005 5:21:10 AM EDT
Just thought I would drop a note here for others to profit from. Worked with a rifle (16" midlength)Sunday that was showing problems with the bolt running over the magazine without picking up a round. This happened with magazines known to be good that worked in other rifles. Same rifle was also showing a stuck case - would not extract and extraction could not be accomplished manually without slamming the gun on the deck while yanking on the charging handle. At that point, it would extract but the rim showed obvious bite marks where the extractor had nearly pulled it off.

We played with this for a while and the only thing that had been changed since the rifle last ran fine was the buffer. The buffer had been replaced by one of the super heavy buffers in an attempt to reduce recoil even further. We swapped the heavy buffer with the regular buffer and the rifle began to run fine again.

Since it would have never occured to me in a million years that these types of failures could be caused by the type of buffer, I have posted this here so that others can learn as well.
Link Posted: 5/23/2005 6:18:56 AM EDT
There is a dwell (may be spelled wrong for this usage) or timing in gun actions. As the weapon recoils the rounds in the magazine will dip and compress the magazine spring. If all is timed correctly, the rounds are pushed up to the feed lips before the bolt returns. If the timing is off, the bolt passes over the mag before the rounds have been pushed up.

H&K had problem with the P7 quite a bit back. The slide would close on a empty chamber with a partial magazine. The rounds were compressing the magazine spring, and the spring was not strong enough to push them back up in time. They began using a stronger spring, and the problem was solved. There used to be a video of a functional cut away P7 being fired showing the rounds dipping in the mag. The video shows the slide closing without stripping a round from the magazine because the recoil dipped the round. in the mag. The spring had not pushed them up to maintain proper dwell.

Another cool thing about the video which was from a high speed camera, it showed that the slide actually stopped in the rearward position, before moving forward. Actually hesitating fully open for a measurable period of time. I expected it immediately bounced closed after full recoil spring compression!

As far as not stripping rounds from the magazine, it sound like the heavy bolt messed with the dwell of the action.
Link Posted: 5/23/2005 6:47:01 AM EDT

Originally Posted By DasRonin:
As far as not stripping rounds from the magazine, it sound like the heavy bolt messed with the dwell of the action.



Except that the heavy buffer is supposed to increase dwell (giving the magazine more time to push rounds into place), not decrease it. Also we used a SAW red heavy-duty spring equipped magazine and still had issues with the bolt riding over the mag.
Link Posted: 5/23/2005 7:47:56 AM EDT
Would the heavy buffer possibly require a heavier recoil spring (maybe in this case) or the current recoil spring be weak? It sounds like the dwell is the problem.

Though I don't understand the extraction problem... unless the heavy bolt is hitting the cartridge so hard that as it chambers, the thicker part of the case is being bent, dented, or damaged in some way... then wedging in the chanber. Or... the bullet is being pushed into the case by the heavy impact (by inerta or as it hits the feed ramps)... and chamber pressures are the extraction issue? Any signs of pressure or case damage on the cases stuck?

Very interesting problem.
Link Posted: 5/23/2005 10:43:57 AM EDT
[Last Edit: 5/23/2005 10:44:20 AM EDT by Bartholomew_Roberts]

Originally Posted By DasRonin:
Would the heavy buffer possibly require a heavier recoil spring (maybe in this case) or the current recoil spring be weak? It sounds like the dwell is the problem.



It does, yet even when the rifle was cycled manually, it still would often not feed a round. This puzzles me to no end since I cannot think of a single logical reason the type of buffer would effect how the magazine feeds during a manual cycle of the action.


Though I don't understand the extraction problem... unless the heavy bolt is hitting the cartridge so hard that as it chambers, the thicker part of the case is being bent, dented, or damaged in some way... then wedging in the chanber. Or... the bullet is being pushed into the case by the heavy impact (by inerta or as it hits the feed ramps)... and chamber pressures are the extraction issue? Any signs of pressure or case damage on the cases stuck?

Very interesting problem.



You may have something with the bullet being pushed into the case. The ammo was Win Q3131A and was cannelured; but the cases were clearly sticking to the chamber in some type of heat/overpressure issue. I examined the fired cases for bulging or signs of overpressure in the primer and didn't notice anything. Both cases had a sizable chunk bit out of the rim where the extractor had almost pulled the rim off during extraction; but other than that appeared mostly normal.

The whole thing puzzles me. I can't figure out how a buffer would affect either of these problems; but once it was replaced, the problems disappeared.
Link Posted: 5/23/2005 4:11:19 PM EDT
On less that stellar mags, the solid impact of the buffer to the back of the receiver extension adds the mag on bumping up the top round to the feed lips. If a heavery buffer is added, and the gas system is funtioning on the light side, then the buffer never gets fully slamed off the back of the receiver extension.

This means that yes the bolt is opening up slower, but without the dead blow effect of the buffer off the back of the receiver extension, the carrier is coming back too soon/faster than normal (ammo not fully raised to the feed lips). On the rifle in question, it could be that that is has some gas leaks, the chamber is on the rougher side, or even that the carrier is binding up on the hammer (disconnector wedging on top of the trigger, not allowing the hammer to be lowered enough without binding).
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