This was taken off the Perfect Union website. It was written by a gunsmith who works on Rugers. It's kind of long but an interesting read:
There have been many posts lately about firing pins, and while I usually stay out of such things I wanted to share some thoughts if you have time for this lengthy post.
First, Ruger firing pins don’t break very often. I’m sorry for those who have experienced a broken pin and had to go through the time and expense of replacing a firing pin, but the percent that break is actually quite small. I want to discuss why they break, but this post is by no means an end-all of reasons.
Most of you have heard about firing pin breaking and needing to send the rifle back to Ruger for repair. This is true. Ruger does not offer “restricted parts” for sale to anyone, not even experienced gunsmiths. Why are they restricted? – Because Ruger said so. For the purpose of this post restricted parts are those parts that each manufacturer has decided to restrict the sale of and insist on factory installation. Most every gun manufacturer has a couple or more restricted parts that they will only sell to qualified people or simply not at all. That’s their prerogative to do so. I’m sure Ruger does it for liability since one – they’re self insured, and two – one in maybe fifty gun owners could fit one properly. A trip back to the factory is usually not a big deal, but if you’ve had your rifle modified – i.e. changed the factory specifications, Ruger will charge you for replacement factory parts and the labor to do so. Again it’s a liability issue and to a lesser extent reputation management. Few gun accidents occur where the operator readily admits to causing the problem, and Ruger doesn’t need bad press. So whether or not your Mini has been modified you’re looking to avoid a trip to the factory and/or have a few spares on hand for the popular SHTF scenario. I’ll add some more paranoia that in just a bit. But first…..
Maybe you understand how things work, and if you do then skip this part. For those that don’t let’s examine the firing pin’s role in the firing of the rifle. The hammer spring stores the energy needed to move the hammer once the primary trigger sear releases it. It not only has to overcome the inertia of a rather large body at rest, but most provide enough energy to the hammer to transfer it to the firing pin. Let’s consider:
1. Reduced power, shortened, or old and weakened springs may not have enough storage capacity to get the job done. To step to the side just a bit, never lighten the hammer spring or allow anyone doing a trigger job to do so. In general reduced power springs do not cause broken firing pins.
2. Extra Power hammer springs can cause breakage. The reason many folks install extra power springs is to reliably fire imported and/or mil-surp ammo, even though your owner’s manual specifically recommends against using it. While it may allow you to shoot cheaper ammo, or in the case of the mini30 the only available ammo in your area, extra power is detrimental. The gun was designed to a certain spec and tolerance and the factory spring was chosen for a reason.
So the hammer has contacted the firing pin and transferred its energy to the pin. The pin then transfers the energy to the primer for ignition. That’s the simplified way of explaining it. The way the firing pin absorbs and transfers the energy is crucial. It must do both in a linear, straight line fashion. No flexing, no bending. The reason is simple; the center of the pin is only half diameter. The center is machined (or cast) as a half circle of the firing pin diameter so that the extractor can retain it in the bolt. This makes the center portion of the pin quite weak in comparison and must transfer the energy in a straight line as a result. Just as important is the amount of energy it must transfer and dissipate. The pin, and the proper fitting thereof, allow for a specific amount if travel – or more accurately a range of travel. Too little and you get spotty ignition. Too much and you get pierced primers.
A word here on pierced primers. The primer not only has to ignite the powder in the case, but it has to contain it as well. The expanding gases in the case are expanding in all directions, including towards your face. If you look at modern bolt rifles you’ll see a hole in the receiver ring and/or holes in your bolt. These are to vent the gases out away from your face in the event of a pierced primer or case head separation by allowing them to escape to the side or down into the magazine. That hole in the side of your Remington 700 receiver isn’t for oil; it’s to save your face. Do you see that on your Mini? No. Proper firing pin fitting is paramount with high pressure cases like the .223, 6.8SPC and the lesser 7.62x39 cartridge. These high pressures and the ability to contain them were one reason the Ruger XGI never went to market.
Back to the firing pin. Travel is limited by two features – 1) The firing pin tail and 2) the tapered head of the firing pin. So the firing pin goes from a lazy afternoon rest to a sudden sprint towards the primer. Then it has to abruptly stop. Contact and energy exchange with the primer slow it only minutely and helps cushion the stop, much like a buffer. Without the tapered section of the pin head getting too big to fit thought the firing pin hole in the bolt the pin would stretch a little as the tail stopped. Stretching metal weakens it. The tail being improperly fitted will allow the head of the pin to enlarge the hole and travel further, again stretching it. This brings us back to the extra power hammer spring. The pin was designed and metals chosen to accept the forces of moving and stopping and provide sufficient power for domestic ammunition. You wouldn’t use a sledge hammer to drive a finish nail would you? Maybe, but the nail loses….and so does your firing pin. Hard primers in imported or mil-surp ammo limit the pin’s travel by resisting the pin’s energy. That’s why the extra power spring was installed, to overcome the resistance and get ignition. But since it stops prematurely i.e., the tip travel has been limited while the tail is still quite under power creates a bend in the weakest part of the pin – the middle. Yes, you got ignition with the cheap ammo, but that pin isn’t cheap if it breaks.
This brings me to the replacement pin whether for repair or a spare. Ruger won’t sell it to you or me without doing the installation. As has been mentioned elsewhere you could shove a broken aftermarket pin in the bolt and have Ruger fit a factory one. Ethical or not is a matter for your own morals, it’s going to be expensive to buy and break a pin and have a new one fitted. So let’s look at that aftermarket pin, specifically the GlendArms part. I’ve seen lots of them and they’re so very close to factory that minor if any fitting is needed. The issue I have with them and others have discovered is that they are quite brittle and break much more frequently than the factory part. Bad metallurgy, bad heat treat – I don’t know. Just my experience… they break. I’ve tried annealing (softening them by heat) and case hardening. Still unacceptable failure rates to those that need a functioning rifle when the zombies come. I explored sand casting my own pins, but haven’t gotten around to it yet. The part could probably be CNC machined, but I don’t have that kind of equipment or the funds to let someone else experiment. Maybe someone will make an acceptable pin replacement soon. Maybe one of you.
To add to the paranoia as I teased with earlier, how prepared are you to change a pin? I never hear anyone wanting to stock up on emergency pins mention the extractor plunger and spring. Remember, the extractor retains the firing pin. The extractor is retained by a tiny little plunger on a stout spring. Lose the plunger and you’re out the game. Luckily these parts are available from Brownells in the factory parts section. If you’ve watched my video on Mini bolts I showed you how small they were and the difficulty in disassembling and reassembling the bolt. I’ve lost more than one and I do it for a living. Try doing it under stress or in the field. Get the parts and keep them with your spare parts cache.
So to sum up the most common causes of firing pin breakage:
1. Non-domestic, non-commercial ammunition.
2. Extra power hammer springs.
3. Excessive dry firing, which admittedly is hard to quantify, but allows the pin to protrude without a primer to cushion the stop.
4. Quality control. Simply put, bad parts get out. They may be stressed or otherwise susceptible to breakage soon out of the box. There will always be bad parts in mass production due to tolerances and random sampling.
5. Obstructed bolts. I didn’t touch on this, but you’d be amazed at the amount of crap inside the bolt. Old oils, brass shavings, dirt, rust – all contribute to impeding the firing pin’s travel resulting in flexing which we’ve already decided is bad. It also acts like a grinding stone every time the retention slot in the pin rubs the extractor tail thinning out an already weak spot even more. Clean your bolt every year or so depending on use and definitely on out of the box guns.
http://www.perfectunion.com/vb/ruger-mini-14-mini-30/79881-firing-pins-long-post.html