I bought a used M3K that had been kitted out for 3 gun (Expanded loading port, +8 extension) and I bought a +4 tube kit from MOA with a low drag follower. I followed the instructions about cutting the spring between 14"-12" past the extension.

Now when I load the tube with 8 rounds the cartridge drop lever does not work. It only releases the shell when I pull the bolt back hard after hitting the drop lever. This only happens with the first 2-3 shells.

I put the +8 extension and spring and follower back on to see if that problem happens with it, and it happens with that too. Even though I do not remember it doing that when I first got it.

I googled the issue and some people have the problem and the threads die without a true fix being revealed.

I have already cut the spring down to 10" and I don't want to go further than that.

I noticed I began to have this issue after installing a +6 moa extension on my M3K.  It seems that it sticks every so often and when I can hit the receiver hard with a fist and it will drop a shell onto the lifter.  I haven’t messed with it yet but I’m thinking I will disassemble and lube everything up.  Please let me know what you’ve tried

When the loading port is opened (opened too much on the sides?) the shell resting against the the shell stop can sometimes have so much side to side movement that the shell latch cannot move far enough to  clear the shell's rim, in other words the the shell moves with the shell latch. Load a round or 2 into the mag tube and wiggle the the loaded round side to side and  you'll see what I am talking about.

The fix? In some cases the shell latch can be tuned to have a little less engagement on the shell rim, but in doing so it is easy to go too far and have round exiting the mag tube too soon and falling on the ground.  Other more involved repairs include  installing a side to side limiting fix into the side of the receiver/mag tube.

If the issue only occurs when using the cartridge drop lever (and not when live firing) the easy work around is to port load a round at “load and make ready” or smack the receiver as you'll see folks do.

Why do shell feed normally when firing, but not when using the cartridge drop lever? When firing the gun's recoil causes the shells in the tube to move forward in the tube and reduce the amount of friction between the shell's rim and the shell latch.