AR-10 high-pressure firing pin

I googled and did it again just now. There isn't much useful really. 95% of the content is the "we are the greatest" type of advertisement / propaganda. That is why I posted here to ask.

Out of the 5% forum discussions is hardly anything relevant. Many believe 6.5CM has much higher pressure than .308 win so something high-pressure is needed. But both 6.5cm and .308 win have the same peak pressure of 62kpsi. All discussions on action cycling are related to rifle + 2 gas port, nothing really on the bolt or firing pin.

Digging into JP shows some hints. They have this high-pressure enhancedbolt, and the high-pressure firing pin should be used in this bolt. What makes the bolt high-pressure? 9310 alloy steel, as it will last 6x longer than the regular choice of steel.

There is one post on Reddit that seems to explain it more. The high-pressure enhancedbolt has better steel and smaller firing pin hole, so that primer crater is reduced. And regular firing pin's tip won't fit. Now that make sense to me.

I could well be wrong, as I'm still gathering info. I think that's what has been going on. AR-10 (I mean dpms), mostly LRP, uses the same trigger group as ar-15, mostly SRP. Weak hammer force over thicker firing pin tip means inadequate pressure (not chamber pressure) to indent LRP and causes misfires, especially with the reduced strength hammer spring for better trigger pull. Customers whine. Manufacturers have to do something. Small tips help, so they started selling high-pressure or enhanced firing pins. They should have bolt with reduced firing pin hole. Most of them didn't bother. JP did. The other guys just have undersized tips. Cratered primers? Don't ask don't tell.

Indeed one of the usual jobs for a gunsmith has been to fix sloppy firing pin in bolt. Bushing the hole or retipping the pin is the common approach. AR should work the same.

-TL

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While pressure is a form of energy and what we are talking about here is the energy delivered by the firing pin, I think the use of the word pressure is inappropriate.

Energy is measured in terms of pressure for some things, psi, ft/lbs, etc, but while a hammer striked with X amount of ft/lbs energy it does not "press" a nail into a board, or a firing pin into a primer. It strikes, or it drives, but it does not press. These are the conventions of terms we use and are widely and commonly understood.

I think use of "pressure" should be in line with the conventional useage, which in firearms, is normally a reference to powder gas pressure.

Another place where a different word is the usual convention is springs. Springs do exert pressure, but when talking about the force of a spring, the term usually used is "tension", not pressure.

Heavy impact, or high energy firing pin would be a more accurate and less misunderstood term, but then, I guess it doesn't have the attention grabbing pizzazz of "High Pressure".
 
I don't know. The energy is the same, all come from the same trigger group, so can't call it high-energy.

I actually don't mind what they are called. I just want to understand what's behind the commotions. A side benefit is now I may have answer to why have been seeing so many range picked up brass have crater or even pierced primer.

-TL

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I went measure the bolt and pin that give me cratered primers. The hole diameter is 0.079", typical dpms. The pin is enhanced, aka high pressure, firing pin from the same manufacturer. The tip has diameter of 0.075". It is sloppy. If the Fulton pins I ordered really have 0.078" tip as advertised, I will be happy. I don't worry about misfires, as I don't have a fancy trigger with light hammer spring.

For the future 7mm SAUM build, I will have to order kak's magnum bolt for $130. Can't find anybody else making it. I will see how well the 0.075" pin fits. If necessary I will fit the other 0.078" to it.

-TL

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Tango, you are saying several different things here which is hard to follow. It sounded at first like you bought a 7MM SAUM upper assembled--but now are saying you are doing a future build? So this upper you are having problems with--and are reloading for--is something else? If it came with a high pressure firing pin fitted into a conventional firing pin bolt then what is happening is the pressure upon the case face upon firing is forcing the primer to imprint into firing pin channel. This can cause several issues and will likely lead to malfunctions sooner or later.

"High Energy" has a specific meaning in the type of firing pin used. It is used in a bolt/carrier system designed to help enhance reliability and mitigate the effects of possible spikes in the pressure from the gas system when firing higher power cartridges.

Take a cartridge, lock the bolt back (and with the safety on and the gun pointed in a safe direction at something you don't mind destroying) drop the bolt on it and without firing it eject it. There should be a notable dent in the primer as a result of the free-floating firing pin striking it. So the firing pin has to successfully ignite the primer with that dent while at the same time not inadvertently cause a slam fire--or even a possible full-auto like mag dump.

Something like the SAUM I would not recommend as a first-time xx10 build. I would try a creedmoor first, because it will definitely entail system balancing issues that go beyond your typical 308 xx10 rifle without dealing with the additional complications of a magnum cartridge.

Try a 25 creedmoor--one of the most awesome high-performance new cartridges I've ever fired which will not beat you up in long shooting sessions--nor will it cook itself.

BTW--"high pressure" does not mean necessarily a cartridge that generates at or near absolute max pressure as rated by SAAMI/CIP--it can also mean a pressure higher than what the current rifle and gas system is balanced for.
 
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tango...i have been reading along here and i get it that you need or think you need a super duper firing pin in your big rifle with a tight fit for the pin and hole it travels through...and i agree with that... and the last post you did, you were talking about light hammers and light springs

if you want to change out the trigger to a fancier one you should look for big rifle specific triggers...they are different than ar15 triggers....the big platform triggers come with a heavier hammer and stronger sprngs to move the heavier hammer so lock time does not suffer...while i dont have primer flow in my big platform rifle i did change the trigger out and i used the trigger tech... they call out one they make for the big rifles and i have never had a misfire with it...i think there are a few others out there that make specific triggers designed for the big platform...they are different and i could see IF you use a ar15 trigger set up in a big rifle you could get light strikes on the harder primers

this is just a thought i had and so i inserted my .02.....i had no intension of disrupting the thread or discussion......just wanted to let you know there are ways to have a great trigger and get the job done at the same time

carry on fellas
 
Sorry I didn't put in a lot of details in my monologue. Here is a recap.

I bought a complete ar-10 (dpms) upper in .243 win to build my first ar-10. I had premature cratered primers. The bolt has 0.079' FP hole and 0.075' FP tip. I started looking for FP with bigger tip but couldn't find any. Everybody seems to sell this sort of enhanced FP with smaller tips. Buddy had an extra JP bolt. It had much tighter fit to the 0.075' tip, so I bought it from him. Cratered primers went away and I have used this bolt in the gun since, while the 0.079" old bolt has been set aside.

I got interested in trying 7mm SAUM so I have been eyeing parts on and off. Haven't bought anything yet. There I caught sight of the term high-pressure FP. I thought it was something specially built to handle higher chamber pressure. So I started this thread this thread to solicit info. Based on what I got so far, it just has smaller tip to promote reliable ignition, similar to the enhanced FP that everybody is selling.

During searching for info, I bumped into 0.078" tip Fulton FP. I ordered two immediately. They will make my 0.079" old bolt good again. When I move ahead with 7mm SAUM build, I won't spend money buying their high-pressure FP. Instead I just put my old 0.075" to use, or I just fit the other 0.078" to it.

-TL

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ocharry said:
tango...i have been reading along here and i get it that you need or think you need a super duper firing pin in your big rifle with a tight fit for the pin and hole it travels through...and i agree with that... and the last post you did, you were talking about light hammers and light springs



if you want to change out the trigger to a fancier one you should look for big rifle specific triggers...they are different than ar15 triggers....the big platform triggers come with a heavier hammer and stronger sprngs to move the heavier hammer so lock time does not suffer...while i dont have primer flow in my big platform rifle i did change the trigger out and i used the trigger tech... they call out one they make for the big rifles and i have never had a misfire with it...i think there are a few others out there that make specific triggers designed for the big platform...they are different and i could see IF you use a ar15 trigger set up in a big rifle you could get light strikes on the harder primers



this is just a thought i had and so i inserted my .02.....i had no intension of disrupting the thread or discussion......just wanted to let you know there are ways to have a great trigger and get the job done at the same time



carry on fellas
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Thanks for your inputs. I'm glad finally someone gets what I am trying to do!

I have never had any misfires. The trigger is ar-15 rated budget 2-stage that I just worked on to make it much better. It has pretty strong spring and there are several ways to enhance it if need be. No problem there.

Reality is I don't have a lot of money to spend. In a way it is my game to get close to high performance of expensive stuff with my limited resources and my own skills.

-TL

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Thanks for your inputs. I'm glad finally someone gets what I am trying to do!
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You can end up with cratered or pierced primers for reasons that have nothing to do with the hammer or hammer spring force. People tend to gravitate to heavy carriers, buffers and buffer springs to help mitigate increased gas impulses to the carrier; but that is going to affect how the whole system functions; not to mention an impressive carrier "bang and twang" when a hefty charge sends that piece of steel hurtling straight back at you. I would ask that you pull your firing pin and post a picture of it; I'm really curious as to what it actually is. Was your 243 win a new purchase--or a used one that might have been Bubbatized?
 
The hole diameter is 0.079", typical dpms. The pin is enhanced, aka high pressure, firing pin from the same manufacturer. The tip has diameter of 0.075". It is sloppy. If the Fulton pins I ordered really have 0.078" tip as advertised, I will be happy.
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I understand you are concerned about the "loose, sloppy fit" but consider this, a difference of 0.004" means clearance all around the pin is 0.002", and a difference of 0.001 with be a clearance of 0.0005" which isn't very much at all.

The very tight clearance increases the chance of foreign matter such as brass shavings, carbon build up, or something else interfering with pin movement.

This could result in misfired due to light strikes, OR in a worst case situation, it might actually freeze the firing pin in place. IF the pin gets stuck "out" that can produce slamfires, or even runaway full auto fire (continuous slamfire until the mag is empty) when you chamber the first round.

There's no free lunch, everything has its pros and cons. Up to you to consider the benefits vs. the risks and make your own choices.

The original AR 10 rifles were made for military use. Didn't sell well, though a couple of countries did buy and use them, for a time. Today's "AR 10s" are not identical to the originals, but the base design is still roughly the same, and I suspect some of the tolerances might have carried over from the original, and possibly changed later, (or not) in the never ending quest for both performance and profit.

Good Luck!
 
At school we were taught to have FP fit of 0.002" or less. It is indeed a loose to moderate slip fit. I did the same for customers when I was a paid smith. Nothing really out of ordinary here. I can see the point of opening the limit up slightly for service arms or for higher production yield. But 0.004" / 0.005" is definitely on the loose side. Wouldn't have much concerns though if not for the cratered primers.

In a pinch I used number drill bits as pin gauge to measure the hole in the bolt. 0.079" was the thickest that could go through. The next size up was 0.081". The actual diameter is somewhere in between.

I don't think a stuck forward FP is possible on AR unless the tip is broken off. So not going to lose sleep over that.

Will take more precise measurements and some pics tonight.

-TL

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Consider the primer material thickness. Wild example,if its .005 it can bridge .003 clearance without issue.
However, if the firing pin tip does not emerge far enough so the hole in the bolt is dealing with the full diameter of the firing pin, the full diameter of the fp is not supporting the primer.

A very high speed camera,if it could view the primer strike through the pressure curve might show an interesting dynamic.

I doubt its a static event. Probably more like watching ballistic gel dance.

I think I'll try to dig up Varmint Al's web page. He has some interesting finite element analysis there (maybe? still.)
I'll try to bring a link.

You can go here and poke around.

http://www.varmintal.com/aengr.htm
 
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Did more precise measurements.

Left. Original bolt and pin. Cratered primers. Tip 0.075". Hole 0.081". 0.006" slop.

Right. JP bolt from buddy. No craters.
Tip 0.076". Hole 0.078". 0.002" slop.

-TL
bc9c3b4a9ec74e8d8130db1350b657fb.jpg


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Press the firing pins through the back of the bolt and measure the distance that the point of the FP protudes from the bolt face ( I think it should be around .035). Also measure the distance from the face of the bolt's lugs to the bolt face. Are the measurements equal between the bolts?

The link HIBC referred to is pretty relevant to your situation--if you scroll down you'll see a pressure map exerted on a primer upon firing--and it's taken from a 243 win. Notice the max pressure around the circumference of the primer--and that it expands out and rebounds upon firing. Looks a lot like the ring on the bolt face, doesn't it? That area analogous to the primer can dish-out/scallop the bolt face over time on both bolt guns and AR bolts. In a sense the primer is being impressed onto the bolt face. When your primers progress to getting pierced and showing gas escape--that's a good sign you need the high pressure bolt and pin. Cratering in itself is not generally something to get alarmed about in regards to the functionality of your MSR--but it is if it's a sign of possible over pressure in your cartridge. Your mission--should you choose to accept it--is to find out which is which.

This post will self-destruct in 5 seconds.

PS--AR bolts these days often come with "overpowered" ejectors and extractors--that can cause problems as well.
 
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That's absolute FP protrusion. They are about the same in acceptable range. 0.050" ish.

The load is not over pressure for sure. The original bolt / pin shallow craters even near the min, and rather bad past mid point to max. The JP bolt has none of that. I don't think it is normal. The high-pressure bolt/pin may be able to correct that but how? By having a smaller pin hole to have better fit over the smaller tip? That's what the JP bolt seems to have.

There is no such thing as high-pressure bolt / pin in conventional rifles, Rem 700 for instance. However good FP fit is always required. Slop of 0.005" is considered substandard. I simply can't see reason why AR is different.

-TL

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I simply can't see reason why AR is different.
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Other than the obvious "because different people made them" consider that the bolt action 700 is striker fired, the spring is much larger (and more powerful) the firing pin is larger and heavier than the AR, and its entire function is in a straight line.

There is no hammer used to transfer the spring force to the firing pin, it is directly applied. AND, again, the Rem 700 was not directly adapted from a design with semi auto clearances, particularly one made originally with military use in mind.

Also, Rem 700 parts aren't made by everybody and their Uncle Max, each to their own ideas of what is correct.
 
The same requirement for any auto loaders. They all have hammer, pin, and hole in the bolt. It is just one of the items for servicing center fired firearms, rifles and pistols. When replacement parts aren't available, smiths would bush or retip. Why is AR gets to be sloppy and still be acceptable?

-TL

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Other than the obvious "because different people made them" consider that the bolt action 700 is striker fired, the spring is much larger (and more powerful) the firing pin is larger and heavier than the AR, and its entire function is in a straight line.

There is no hammer used to transfer the spring force to the firing pin, it is directly applied. AND, again, the Rem 700 was not directly adapted from a design with semi auto clearances, particularly one made originally with military use in mind.
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^^^^^^Yes, this!^^^^^^

To the above I would add that a bolt gun does not need to be timed to the pressures in the gas system--a modern MSR does.

PS--ocharry is correct that an xx15 trigger is not necessarily going to be adequate for an xx10 unless the manufacturer ceritifies that it will operate correctly in the bigger platform.

Why is AR gets to be sloppy and still be acceptable?
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I don't know--that's up there with "why does the government keep raising my taxes and I keep getting less in services?"
 
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M1 garand, just an example of many, follows similar FP fit procedure. Not DI action? Egyptian Hakim. It is similar, if not the same, for all center fired calibers, bolt, semi, or even full auto.

-TL

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It is similar, if not the same, for all center fired calibers, bolt, semi, or even full auto.
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No, it is not. the AR FP is loose and free-floating in the bolt; it is not cocked with a mainspring that remains under tension and delivers the same strike force upon release by the trigger, regardless of the rest of the components or pressure in the system.
 
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