If the bolt is in battery I think the carrier is holding the cartridges down off of the feed lips. As soon as the bottom of the carrier clears the rear of the case in the recoil stroke the cartridge is ready for feeding in the battery stroke.
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You are correct. The cocking surface of the bolt carrier keeps the cartridge stack depressed and tension off of the feed lips. Even then, there is no way the bolt group can "outrun" the magazine. Now we need to find out what the distance between the on deck cartridge and feed lip is...
A good approximation would be half a cartridge base diameter.
And yes, check the carrier key for leakage, then re-test with full-5.56 ammo such as XM193.
If there is no leak, and the XM-193 works the rifle, then you're back to the "doctor, doctor" joke. The rifle is telling you "don't feed me that."
The bolt pushes the cartridge in the mag down approximately 1/8 inch.
Mk18Pilot, how do you figure it's impossible for the bolt to out run the magazine?
I'm not sure I'm tracking you. The cartridge cannot rise in the magazine until the bolt clears the rim in it's rearward motion
Ok, I'm following you there. A few questions remain-
How long does it take before the spring begin accelerating the stack into proper position? How fast does the spring relax? Will a weaker magazine spring increase the time it takes to accelerate the stack into place?
How long is the time between the bolt clearing the rim in the stack until it returns to begin stripping the cartridge from the magazine? How much does that time change with a change in bolt speed?
How much does the violence of the higher bolt speed affect the positioning of the stack in the magazine? Will a weaker spring allow the stack to be displaced during the bolt cycle?