AR-10 high-pressure firing pin

stagpanther said:
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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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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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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Just to confuse you even further:D--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).
 
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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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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.
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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.
 
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. [emoji3]

-TL

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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??
 
44 AMP said:
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??
Click to expand...
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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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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just a thought, and i'm sure you already know, but a dome point is not best in high pressure situations, the claim is that a rounded flat point is best, something to do with structural strength and how the load is distributed in the primer cup when the charge goes off. you probably know way more about that than i do.
 
georgehwbush said:
just a thought, and i'm sure you already know, but a dome point is not best in high pressure situations, the claim is that a rounded flat point is best, something to do with structural strength and how the load is distributed in the primer cup when the charge goes off. you probably know way more about that than i do.
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I think you are right. Normal FP tip profile is a hemisphere with >0.01" straight shank. For instance a 0.075" diameter tip will have protrusion of 0.075/2+0.02=0.058” with a perfect hemisphere at its top, tangential to the straight shank. With this there is no sharp edges anywhere.

For higher pressure cartridge I would flatten the hemisphere by 0.005” without affecting the tangentiality, and probably would increase the straight shank by a few thous to maintain protrusion. The flattened tip reduces chance of pierced primer.

-TL

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More updates.

Bought Kak magnum bolt for future 7mm SAUM project. It is supposed to pair with their high-pressure FP, which has been out-of-stock for a while. Didn't want to miss out the nice July 4 sales, so I ordered the bolt without the FP.

The bolt arrived today. Guess what the FP hole diameter is? 0.068”. Luckily I found a FP with 0.066" tip in my part drawer. Can't recall where it came from. But hey, one more data point that matches my theory. The so-called high-pressure bolt / FP just has smaller tip.

-TL

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More updates still.

Installed the 0.079” FP donated by a member in the bolt with 0.081” FP hole. Shot the ar-10 this afternoon. It worked perfectly. No primer cratering at all. I'm delighted and thankful.

-TL

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