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What causes an M4 A3 to short stroke?

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Overview

Solutions to what may cause an M4 A3 to short stroke.  (After the last round the bolt will not stay back or will get stuck almost half way.)

Solution

We usually recommend replacing gas rings about every 3,000 rounds or so for reliable functioning. Our new gas rings  have an angled gap now so if you reverse the center ring there is no gap even if they line up. 
 
You could check to see if there is a gas leak at the bolt carrier key;
 
Subject: Bolt Carrier Key, leak
The key is the piece on top of the bolt carrier held in place with two screws.
First check to see if the bolt carrier key screws are loose by trying to tighten them with a 9/64 or 1/8" allen wrench depending on the screws used. If the key screws were loose they need to be removed so the key can be properly reinstalled.
To check to see if you have a leak at the bolt carrier key causing short stroking pull the bolt carrier assembly from the upper receiver. Push the bolt into the carrier and hold it there while performing this test. Spray a light oil into the key. Blow a couple of pounds of compressed air into the key. If you do not have compressed air place a rubber tube onto the key and blow into it. If there is bubbling between the key and the carrier it has a gas leak.

To correct the gas leak remove one of the screws and put it back in. Remove the second screw then go back and remove the first screw. This gets the screws past the staking so that they will not be stuck in the key. Clean the key and the carrier with brake cleaner to remove any carbon build up. Put a drop of oil on the bottom of the key and wipe it off so that there is just a thin film left on it. Install the key by torquing the screws at 45 INCH pounds. The key can be restaked with a dull prick punch or cold chisel so that some of the metal of the key is pushed against the screw heads to keep them from getting loose. A drop of Loctite can be used on the screw threads instead of staking the heads.

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