I would add an issue of safety in using the SINGLE HOOK trigger group.
In home builds, the outer dimensions of the receiver can vary, especially if it's built from a flat. Some manufactured receivers can vary also (I have seen large variances in Ohio rec's for example).
If the receiver is wider side to side than spec, the gun will still run fine, HOWEVER, the issue comes up that the trigger will have side to side play on the trigger pin. If there is enough play, with the single hook trigger it is possible to cock the hammer, then push the trigger sideways releasing the hammer while keeping the trigger forward (NOT pressed).
This happened on one of my first builds from a flat. I assembled it and noticed the play allowing the trigger to move side to side enough to let the hammer fall. This is dangerous for obvious reasons, the least of which is dropping the gun on its side, causing the trigger to move, and firing the gun without pressing the trigger. NOTE this can happen with the safety ON or OFF.
With the double hook trigger, even if there is sideways play of the trigger, one hook always is still engaged on the hammer.
The important thing here is when building or assessing a gun and its safety, to look at this aspect of the trigger installation. Again, the above condition will still allow the gun to function perfectly, but in some conditions, it may fire with the trigger forward and safety on.
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