AR-10 high-pressure firing pin

[ocharry]

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

[tangolima]

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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[tangolima]

Originally Posted by ocharry

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

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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[stagpanther]

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

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?

[44 AMP]

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 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!

[tangolima]

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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[HiBC]

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

[tangolima]

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

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[stagpanther]

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.

[tangolima]

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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[44 AMP]

I simply can't see reason why AR is different.

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.

[tangolima]

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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[stagpanther]

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.

^^^^^^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?

I don't know--that's up there with "why does the government keep raising my taxes and I keep getting less in services?"

[tangolima]

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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[stagpanther]

It is similar, if not the same, for all center fired calibers, bolt, semi, or even full auto.

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.

[tangolima]

Originally Posted by stagpanther

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.

I meant the requirements on FP fit in the bolt. < 0.002" slop. AR is not the only one that has free floating pin.

Well I guess we have digressed too much. My fault. I think I get what high-pressure bolt / pin means. Thanks guys.

-TL

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[stagpanther]

This might help: high pressure bolt (click on high pressure bolt tab)

[tangolima]

Ah ha. That's exactly what I have thought. They reduced the hole diameter to fit their smaller tip pin. The small tip develop high impact pressure to promote more reliable ignition. Thanks!

As far as I know, default ar-10 FP tip diameter is 0.078” to fit 0.080” hole. The so called enhanced FP everybody is selling has slightly smaller tip diameter of 0.075”, to improve ignition reliability. They are betting on most people just ignore the cratered primers due to the increased slop.

JP high-pressure FP has even smaller tip 0.062". Unlike the other guys they have dedicated bolt to match. No wonder they don't sell the individual pin. The slop is simply too much if it is put in a 0.080” hole. Interestingly they mentioned the poor ignition reliability is related to reduced strength hammer spring.

Right on. I'm doubly set. Thanks again Panther.

-TL



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[stagpanther]

Just to confuse you even further--an enhanced bolt is not necessarily the same thing as a high pressure bolt--generally that means the bolt itself is stronger or has enhanced ejector/extractor features--it can still have a firing pin hole for the wider firing pin (which is what you have). you definitely do not want a high pressure pin in a conventional firing pin hole bolt. In "the old days" before high pressure pins showed up some MSR shooters (including me) simply took the conventional bolt and ground the pin head thinner--but that didn't deal with the inertia of the pin strike High pressure refers to the propensity for some higher-pressure cartridges to "push back" on the primer and cause the primer material to flow--as well as exert force on the primer cup of the case. That's why I think the creedmoor is an excellent "starter" for a first time xx10 build because it's almost guaranteed you will deal with all these things as you explore the realm of what it can do with a wide range of hand-loads. BUT increasingly manufacturers are offering things like small primer cases to help mitigate the flow/stretch of both the case and primer. Key thing--what works just fine in your bolt gun might not in your xx10.

Something like the SAUM in an xx10 I would consider a "black belt Jedi master" level MSR 10 build--and I've built lots of xx10 rifles. They are not nearly as lego block easy as the xx15s are (the further away from your basic 308 you get).

[tangolima]

I didn't care about AR/AK that everybody seemed playing, so I was quite late to the AR game. It was the pandemic and cancer treatment that got me started. I needed something to put my mind on. I have a few now. The ar-10 in .243 is not my latest. It is the 6mm ARC that I put together. 7mm SAUM will be the next when the sales are back on.

Thank you for your help along the way.

BTW, I have been contemplating which barrel profile to get for 7mm SAUM. HBAR heavy with 0.75" GB journal or the HEAVY with 0.835" GB journal are the final candidates. I lean towards the latter as I don't have anything with bigger GB yet. Rifle + 2” gas port with adjustable GB of course. 24” barrel length as I will need to pay more for anything longer. What do you think? Thanks in advance for your input.

-TL

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[stagpanther]

BTW, I have been contemplating which barrel profile to get for 7mm SAUM. HBAR heavy with 0.75" GB journal or the HEAVY with 0.835" GB journal are the final candidates. I lean towards the latter as I don't have anything with bigger GB yet. Rifle + 2” gas port with adjustable GB of course. 24” barrel length as I will need to pay more for anything longer. What do you think? Thanks in advance for your input.

Just my opinion--since I don't have a 7mm SAUM--though there is a guy here on this forum who just built one--I'd ask him what he thinks.

I'd look at it from the point of view of what you want to do with it and the constraints of the set-up you're looking at. Do you want to shoot the latest heavy very high SD .284 bullets? If so, they have their own requirements in velocity and twist which should factor into the barrel you choose. You'll also need to consider the limitations in COL imposed by the short action and magazine and how that impacts the case capacity used when seating the bullets. Call the barrel manufacturers and ask their advice--after all you'll be paying them a lot of money and probably waiting a long time for them to get around to making your barrel. You can go down the rabbit hole pretty fast if you decide you want a match-grade barrel.

[tangolima]

Yeah I did ask the man, but he hasn't been back.

Can't do anything fancier as I'm cheap. It is my game to spend little and tinker to get close to the high-end stuff. I literally have nothing match grade in my possession. So far I can still see the tail lights.

-TL

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[44 AMP]

Just curious, you're looking to build a 7 SAUM, but haven't built it, yet??

IF so, what are you getting cratered primers in?? What load are you shooting??
What primers??

have you considered changing to a different primer, to see if that changes the results???

Might it be that the GI primers won't be bothered by the "slop" between you firing pin and the bolt hole??

[tangolima]

Originally Posted by 44 AMP

Just curious, you're looking to build a 7 SAUM, but haven't built it, yet??



IF so, what are you getting cratered primers in?? What load are you shooting??

What primers??



have you considered changing to a different primer, to see if that changes the results???



Might it be that the GI primers won't be bothered by the "slop" between you firing pin and the bolt hole??

It was .243 win. 75gr bullet over 45.2gr of hunter, going at 3350fps. Pretty light load. Shallow crater started to show. It became pretty bad at 46.8gr 3480fps. Primer is WLR. Not quite milspec but I have used it on M1 with no issue. Same loads with JP bolt (0.002” slop), no problem at all.

I have run out of hunter and 75gr bullets. My current go-to load is 100gr bullet over A4064 going at 2800fps. Not showing powder charge as it is not a published load. No problem with the JP bolt either. A4060 is a bit too fast but that's what I have now.

-TL

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[tangolima]

Some updates.

Fulton armory has FP with advertised 0.078" tip diameter. When received them, they all measured 0.075" tips. Asked for exchange, the man replied they didn't have 0.078" any more. They gave me refund and updated their product description immediately. Grrr.

Bumped into member on other forum who shared the same concern. He collects and compiled list of tip diameters. So kind and generous that he is sending me 2 FPs out of his collection, 0.077" and 0.079".

Will see how it goes.

-TL

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