Gunsmithing Pin fall and accuracy

[MangoFett]

I've seen in many places where company's or gunsmiths want X amount of pinfall or when tuning the trigger hanger that say don't go below .240 thou pin fall or your accuracy may go to hell.

So my question here is how exactly does pin fall effect accuracy/consistency?

Also, besides accuracy are you giving up anything else as you loose pin fall or gaining anything by increasing it? From an engineering standpoint what would be ideal? (If such a thing exists)

 

[Praeger]

The distance a firing pin travels, and its speed, directly affect reliable ignition of the primer. In that consistent ignition is necessary for repeatable accuracy - there's the relationship. As for any advantage in minimum or maximum firing pin travel, too little and no iginition or inconsistant ignition - too much can result in inconsistant ignition and/or punctured primers.

If you were to break down all the dominoes that must fall from the trigger pull, to the bullet exiting the barrel, I'd place pinfall towards the bottom in terms of the cause of inaccuracy.

 

[carlsbad]

I assume you're talking about Remington or clones. I've seen some new actions such as the Tempest that are designed for much less pinfall.

The mechanism that pinfall affects accuracy is consistent ignition of the primer. If you have a primer that doesn't ignite right, it will often cause a flier. short pinfall can lead to less impact energy and occasional poor primer ignition.

For Rem 700s and clones, the recommended number is .240. Some smiths refuse to go below that. Others say keep it at least .210. When I go below .240, I usually go with a stiffer spring, but that increases cocking force. See below for my current favorite configuration.

I bush all the bolts I work on to .060 firing pin diameter (.062" hole). From a physics standpoint you need less energy to put a .060 dent in a primer than a .075" dent. so you should be able to get by with less pinfall with the .060" firing pin. The difference is about 35% less by area or if you want to consider the circumference, then it is only 20%. Reality will be somewhere in between. Many customs use a .070 pin, which is better. They can be bushed to .060. Generally not required to prevent cratering and pierced primers but should be considered if you have short pinfall.

There are people who say a small firing pin makes no difference. I've got a physics degree and a Mechanical PE license and I will vouch for the reduction in required energy for a smaller pin. but I haven't done field tests to prove it. So draw your own conclusions. Try it and see.

For Remingtons, I've reduced pinfall to remove cock on close to .210 without problems but added a stronger spring. Generally now I replace Remington bolts with PTG bolts and when I remove all the cock on close, PTG bolts will generally give you .240 pinfall. This is a big advantage of the bolt change out. I don't like cock on close.

--Jerry

 

[Robert Gradous]

https://m.facebook.com/story.php?story_fbid=1026838270859362&id=100005995968694

Has to be crisp and repeatable

 

[Jackomason]

My big horn origins firing pin measures .228" of total travel and the firing pin itself is .060", I still have a small bit of cock on close but I'm not sure I want to take much more off.... this is a good question and I'm glad you asked it! I'll be staying tuned!

 

[Mr.BR]

Smaller firing pin footprint with bushed FP hole helps , but in general rem 700 clones ,for maximum accuracy , heavy firing pin with maximum FP fall you can get is what is being done in F class and BR , long stroke and heavy pin are the more consistent option and induce less vibration .
Piring pin induced vibration is a mayor factor in accuracy.
On the other hand the Euro positional and High pover actions all generaly run light FP with short strokes for faster locktime but that comes at the price of much stiffer cocking ,already stiffer as they are all 60° lockers and also have stronger springs

Best proven in this Voere , group with same load shot trough same barrel , one with conventional FP and one with Laser ignition (no vibration induced).

 

[ShtrRdy]

I just started reading a book written by a successful bench test shooter, ( Tony Boyer ). He says to use 0.220" of pin-fall and have a firing pin protrusion f 0.050" to 060".

 

[Robert Gradous]

Smaller firing pin footprint with bushed FP hole helps , but in general rem 700 clones ,for maximum accuracy heavy firing pin with maximum FP fall you can get is what is being done in F class and BR , long stroke and heavy pin as the more consistent and induce less vibrations .
Piring pin induced vibration is a mayor factor in accuracy
On the other hand the Euro positional and High pover actions all generaly runs light FP with short strokes for faster locktime but that comes at the price of much stiffer cocking ,already stiffer as they are all 60° locker and also have stronger springs

Best proven in this Voere , group with same load shot trough same barrel , once with conventional FP and one with Laser ignition (no vibration induced).

Very well done.

 

[Alex Wheeler]

Ignition energy is critical to raw accuracy. .240" pin fall, with a full weight pin and spring is my base line. I have fixed enough custom actions in the .220-.230" range to know what works. Keep in mind thats a reference and also assumes a .070" diameter or smaller pin and a bind or drag free striker assembly. Any other issues will still cause accuracy loss. You will still shoot some small groups with a sub par ignition, but you will also get some unexplained fliers and general lack of consistency. These issues can be tough to diagnose. If your not shooting group matches you may not see it. Some actions were designed to have a pin fall of .220" but many BR smiths do not leave them stock. Not to mention those actions have less drag on their strikers than a Remington style ignition system so they can get away with a little less. Pin fall is usually overlooked because to increase it without making the action cock on close is much more difficult.

 

[Robert Gradous]

Ignition energy is critical to raw accuracy. .240" pin fall, with a full weight pin and spring is my base line. I have fixed enough custom actions in the .220-.230" range to know what works. Keep in mind thats a reference and also assumes a .070" diameter or smaller pin and a bind or drag free striker assembly. Any other issues will still cause accuracy loss. You will still shoot some small groups with a sub par ignition, but you will also get some unexplained fliers and general lack of consistency. These issues can be tough to diagnose. If your not shooting group matches you may not see it. Some actions were designed to have a pin fall of .220" but many BR smiths do not leave them stock. Not to mention those actions have less drag on their strikers than a Remington style ignition system so they can get away with a little less. Pin fall is usually overlooked because to increase it without making the action cock on close is much more difficult.

Poor ignition results might not show up at 100yds but 300yds or more definitely.

 

[Terry Cross]

Poor ignition results might not show up at 100yds but 300yds or more definitely.

I totally agree.
Any issues that result in exacerbating your ES/SDs will cause vertical troubles at distance.

Example is that even with healthy firing pin energy, ammunition with large SD/ES can be totally capable of shooting small groups at 100yds and then the wheels fall off at distance because your vertical is a shit show.

Same thing can be experienced with rifles that shoot pretty cold bores after cleaning. Those first shots from a really clean barrel can be 50 to 75 fps slower than your normal but still shoot into the tiny group at 100. Shoot the same clean bore shot at distance and you are going to lose it out of the bottom at 6 o'clock and be scratching your head why.

./

 

[MangoFett]

This was essentially the thoughts I was thinking but with solid explanations, thank you for that.

So if a guy wanted to tune this, fiddle with it, how would he go about this? Adjustable trigger hanger?

 

[Terry Cross]

Ignition is going to be dictated by firing pin speed and mass (not counting protrusion from the bolt face which should be within spec).

If your mass is already set, the only variable is speed/acceleration to get the energy you need. That is going to be changed by the stored energy of your spring and/or the distance of fall (which allows the mass to obtain speed).

So you can change the hangar which will increase spring compression and distance of fall or you can go to a heavier spring. The 0.240" number you quote is only in play if the spring weight is fixed and the firing pin is std. weight.

These days there are a myriad of firing pins with different masses and after market springs. The 0.240" is not valid but within a small slice of that pie.

./

 

[Alex Wheeler]

This is an area that energy formulas do not tell the whole story. Real world results and testing is the answer to learn what works. Do some research into the subject and see what you can dig up. Much work has been done in this area and continues to be done. In my experience it takes relatively large changes in spring force or pin weight to equal small pin fall changes. This is why its pretty hard to "fix" short pin fall by adding spring.