I read this
“Understanding Extractor Lift in the M16 Family of Weapons”
http://www.dtic.mil/ndia/2003smallarms/din.ppt
It explains that extractor lift occurs, but does not explain what causes it.
It says lift is not caused by “spin out”, but occurs during the “initial rearward translation of the bolt after bolt rotation stops”.
Any ideas what causes this?
Where in the hell is IG when you need him.
“Dual M4 High Fidelity Gymnasticator” 
I love it…
Too much thinking going on there for me.
Cliffs Notes - Use a stronger extractor spring…
I read that same presentation a while back & wasn’t all that impressed with the weak conclusion. It basically just points out when extractor lift happens. Kind of weak. And after an FTE Then this is their remedy:
“The ejector in the lower M4 was removed and the previous experiment was repeated. The unfired, lubed and primed case in the lower M4 extracted.”
Oddly enough they kept the “remove your ejector” part out of their conclusions.
The whole presentation seems like an exercise to explain why they spent the time to build their gymnasticator. I don’t even know where to start poking holes in their scientific method.
My educated guess is that the lifting is caused by rearward momentum as the bolt is pulled rearward and the extractor digs into the spent casing a little harder. It’s also fighting the ejector as the empty casing begins to clear the chamber. The bolt speed and gas pressures within a 10.3-16" CAR gas barrel and empty casing are higher then they are with a 18-20" rifle gas barrel. This is why CAR gas guns should have either a Colt, BCM, Wolff XP M4 extractor spring, a black insert and I highly recommend a Crane-O which imparts even more extractor tension. The Army used to have one part number for the 20" guns extractor spring and another for the M4s extractor spring. The new extractor spring works in both guns.
In severely fast cyclic guns other things could also change the extractor tilt. Things like the beaver tail type extractor from KAC and LMT which both use dual extractor springs. KAC went a bit further and moved the extractor pin closer to the bolt lugs for more mechanical advantage of the extractor springs. The LMT enhanced carrier with its longer race could also be used, this allows the bolt carrier to move a little more rearward (independently) before it pulls the bolt rearward than it does with a standard carrier this gives just a little more time for the gas pressures with the barrel and gas system to drop for easier extraction. Heavier buffers also would work via slowing the bolt speed. The whole gun is one big system. Changing one thing can effect other aspects of the cycle of operations.
What he said.
Failures to extract are extremely rare problems …
Right. I had 2 this weekend. Stupid Wolf.
As for what causes it, I thought it was clear. The extractor pops up and then back down after the shot - the extractor works on pulling out the shell as long as the cartridge maintains contact w/ the bolt face. Everyone always talks about how brass casings “swell” during the shot to fill the chamber - perhaps this is what’s causing the extractor to jiggle a little?
When the bolt carrier and cam pin hit the bolt to start the rearward acceleration of the unlocked bolt it creates a very high acceleration load (perhaps 200,00 g’s) that gets transmitted to the extractor. The extractor bends like a spring under this load, but being a very brief impact load the extractor springs back to it’s original shape and springs away from the case. This dynamic motion causes the extractor to be out of position for the first .15 inches or so of case extraction. The only thing holding the case against the bolt face (while also keeping the ejector compressed) is the residual gas pressure in the bore. There is roughly 1,000 psi of gas pressure during initial case extraction.
The gymnasticator was used to duplicate the bolt and extractor dynamics without the bore pressure, just to provide additional evidence to verify this phenomenon.
Welcome to M4C. Interesting analysis.
Almost 5 years old. Definitely in the Necropost Awards runner up category.