I am building my first AR15. I have this problem.....

When I installed my Timney 667S trigger, I used the KNS .154 standard Stainless Pins. I noticed that when I turned the gun sideways, the pins freely fall out when the hammer is locked back.

Should I have purchased the .155 pins desinged for a match trigger?

Does the smith M&P 15 lower receiver accomodate the .155 pins to stop the pins from walking?

Your pins are fine, When you put your hammer in, lay the springs on top of the trigger set. That's what keeps them from sliding out.

The tail of the trigger spring lays forward.
The tail of the hammer spring lays on top of the trigger pin.

Normal trigger pins have a knotch where the hammer spring rides into.

Then I believe you have the anti walk pins, you attach the end caps to the outside of the receiver.

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