Post by TheEnd on Dec 13, 2010 22:52:26 GMT -5
I've noticed a somewhat common problem with the Magpul PTS ACR. The problem is the trigger fires in semi auto while in both semi and full auto. When the trigger is pulled half way the full auto works like normal. I had a chance to look at one in person, and found the problem.
The PTS ACR uses a micro switch instead of standard contacts. The micro switch is a small black box with a small grey button sticking out of it. The switch defaults to the ON position, and there is a small lever with a spring that holds the button down. When the trigger is pulled it rocks the lever back which lets the micro switch turn on.
The problem with this design lies in the semi auto arm mechanism. The lever that hits the micro switch has to not only pivot back to allow the switch to turn on, but it also has to spring vertically to be cut off by the semi auto arm. What was happening with this ACR was the lever was getting stuck halfway along the vertical travel which made it slide off the trigger sear. When pulling the trigger half way the sear was held at the highest vertical point of the arc it travels in. When pulled all the way it was slightly lower and it allowed the lever to pass over the trigger sear.
The fix is to carefully reassemble the mechanism. During the assembly process the micro switch and the lever mechanism will want to spring out of place. It is important to make sure they stay in the correct spot as any mis alignment will cause the lever to stick or jam up.
Note: Make sure you mark the location of the selector lever gears before pulling the gear box out. Lining them up again can be difficult.
Here is a rough sketch of the trigger mechanism parts. I don't have access to an ACR so this is the best I could do. Hopefully it illustrates the point.
The PTS ACR uses a micro switch instead of standard contacts. The micro switch is a small black box with a small grey button sticking out of it. The switch defaults to the ON position, and there is a small lever with a spring that holds the button down. When the trigger is pulled it rocks the lever back which lets the micro switch turn on.
The problem with this design lies in the semi auto arm mechanism. The lever that hits the micro switch has to not only pivot back to allow the switch to turn on, but it also has to spring vertically to be cut off by the semi auto arm. What was happening with this ACR was the lever was getting stuck halfway along the vertical travel which made it slide off the trigger sear. When pulling the trigger half way the sear was held at the highest vertical point of the arc it travels in. When pulled all the way it was slightly lower and it allowed the lever to pass over the trigger sear.
The fix is to carefully reassemble the mechanism. During the assembly process the micro switch and the lever mechanism will want to spring out of place. It is important to make sure they stay in the correct spot as any mis alignment will cause the lever to stick or jam up.
Note: Make sure you mark the location of the selector lever gears before pulling the gear box out. Lining them up again can be difficult.
Here is a rough sketch of the trigger mechanism parts. I don't have access to an ACR so this is the best I could do. Hopefully it illustrates the point.