When to Use Standard, Anti-Walk, and Anti-Rotation Pins
Short Story: Hammer/trigger pin selection is partly influenced by your trigger selection, and partly a matter of preference.
To the average person, it might seem that if something exists, it exists for a reason. They might also believe that if it costs more, it must be better.
Neither is necessarily true.
If you have read through our article on trigger selection, you understand the role of the hammer and trigger pins. These pins secure the hammer and trigger to the lower receiver and in the appropriate position relative to one another. For your standard trigger, the trigger pivots on and around the trigger pin and the hammer pivots on and around the hammer pin.
What we haven’t discussed is the design of the hammer and trigger pins and the forces that act on them during the sequence of fire.
Trigger Pin and Hammer Pin Design
Your standard Mil-Spec trigger and hammer pin looks something like this:
When your two piece trigger is installed properly, the legs of the hammer spring lie on top of the trigger pin. One leg (or both, depending on the design of the pin) will sit in the outer groove(s) under tension, thereby locking the trigger pin in place.
If you look through the eye of the hammer, you will see an apparent obstruction about halfway through. This is the J-spring. It’s sole purpose is to sit in the middle groove on the hammer pin, thereby locking the hammer pin in place.
If your trigger assembly and pins are installed correctly, the trigger and hammer are free to pivot around the pins, the pins are free to rotate in the lower receiver, and the springs keep the pins from walking out of the lower receiver.
Why Use Anti-Walk or Anti-Rotation Pins?
If standard hammer/trigger pins fit the bill, why do we have these alternatives?
Some people will tell you that both anti-walk and anti-rotation pins are completely unnecessary unless the corresponding holes in your lower receiver are oversized/out of spec. While they are correct that oversized/out of spec holes will create a problem for you that these alternatives can compensate for, they are otherwise incorrect.
It is true that you should not NEED anything other than Mil-Spec pins for a standard two piece trigger system.
So then, what is the argument for anti-walk and anti-rotation pins? Let’s explore them individually.
Anti-Walk Hammer/Trigger Pins
Short Story: Generally, there is no harm in using anti-walk pins; they are highly recommended for drop-in triggers.
The idea of anti-walk pins is simple: don’t let the pins walk out of the lower receiver. An anti-walk pin is basically a post running through the lower receiver that is capped on each side of the receiver. You run a pin through the trigger and thread a cap on each side; repeat for the hammer.
You certainly don’t want your pins walking out of the lower receiver. The potential consequences include losing the ability to fire your weapon when you need to most (at least until you find the missing pin), or worse, a catastrophic mess inside your gun. For illustration, imagine the hammer pin backing out of one side of the lower receiver in the middle of a firing sequence. The kinetic energy of the reciprocating BCG applied to an unsupported hammer will make an absolute mess inside your gun, and the twisting force applied to the dangling pin may even cause irreparable damage to the receiver where the pin remains in contact with the receiver.
However, if a two-piece trigger group is installed properly using Mil-Spec pins, you shouldn’t need anti-walk pins. In fact, standard 4130/4140 steel pins are stronger than anti-walk pins. Consider that the shaft of anti-walk pins must be at least partially hollow to accept the cap screw(s), and this weakens the shaft. Anti-walk pins are also usually made from softer metals (like stainless steel), which may not be the best option for the high shear stresses seen in a two-piece trigger system. Note that the only anti-walk pins that we know to be made from 4140 steel are made by Kaw Valley Precision.
There is a very good reason to install anti-walk pins with drop-in, or cassette, triggers, as they don’t rely on springs seated in the grooves on the pins to hold things in place. Many of these drop-in triggers are equipped with tensioning screws that press against the bottom of the trigger pocket in the lower receiver; tightening these screws creates upward pressure against the pins in the bore of the trigger and hammer, which should hold them in place. We aren’t generally thrilled with this idea, so we always install anti-walk pins when we use a drop-in style trigger.
Anti-Rotation Hammer/Trigger Pins
Short Story: Anti-rotation pins are not a good idea for two piece triggers; they are fine for drop-in triggers, but don’t offer any practical advantage over anti-walk pins.
Anti-rotation pins are similar to anti-walk pins in that they have more going on outside of the lower receiver and more confidently secure the trigger system in place. The difference is that the hammer pin and trigger pin each have a flattened portion that seats into a linker bar that connects the hammer pin to the trigger pin. The effect is that the hammer pin and trigger pin CANNOT rotate.
Why would (or wouldn’t) you want such a thing?
Some proponents of anti-rotation pins claim that the pins rotating in the holes of the lower receiver will wear the holes larger and the larger holes will throw things out of alignment or cause the pins to walk out. In theory, yes; steel pins rotating in an aluminum hole will eventually cause wear. Theory aside, we are not sure how rational this concern is, as we have never ever seen such a phenomenon (not that we dismiss it entirely…if you put a few million rounds through a lower, you will probably start to have all sorts of wear-induced problems). Concern aside, the pins holding a two piece trigger system in place are designed to rotate. Preventing them from rotating could create other problems for a standard two piece trigger (we have not experienced this, but it is a possibility). As such, we don’t see a very good reason to use anti-rotation pins over anti-walk pins with a two-piece trigger group (if you are dead set on not using standard pins), especially if everything conforms to specifications.
Anti-rotation pins serve the same purpose as anti-walk pins for a drop-in trigger system: hold the cassette in place inside the lower receiver. The only difference is the strip of metal connecting the trigger pin to the hammer pin that prevents them from rotating; given that the trigger and hammer are not rotating against the pins, this is a completely inconsequential feature. If you like the way it looks, go for it; you’re not hurting anything.