AR-10 high-pressure firing pin - Page 3

georgehwbush · July 4, 2024, 06:28 PM

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.

tangolima · July 4, 2024, 08:30 PM

Quote:

Originally Posted by georgehwbush

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.

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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georgehwbush · July 5, 2024, 09:43 PM

yep that's what they say.

tangolima · July 11, 2024, 11:11 PM

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

Sent from my SM-N960U using Tapatalk

tangolima · July 12, 2024, 10:29 PM

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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tangolima · January 26, 2025, 03:48 PM

Further update.

Have been shooting the 7mm SAUM upper I recently put together. It has magnum bolt with fitting firing pin with small tip (improved or high-power version). About 60 - 70 rounds, the gun went silent with no dent on primer. The small tip had broken off. Good that I have one back up.

Seems like an issue intrinsic to the smaller tip. Probably not a good idea to do dry firing too often. I have a few extra pins lying around. Will fit one as back up.

-TL

Sent from my SM-N960U using Tapatalk

stagpanther · January 26, 2025, 04:40 PM

My guess is the FP deflected and hit something in the bolt itself. Did any of the fired cases show unusual cratering, movement or piercing of the primers?

tangolima · January 26, 2025, 04:45 PM

Nothing unusual before the click with no boom. At least not that I could find.

-TL

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stagpanther · January 26, 2025, 05:41 PM

You got me stumped--if the pin was properly matched to the bolt I don't see how else it could break other than hitting something harder than it is.

tangolima · January 26, 2025, 07:27 PM

Same here. The tip broke off clean at the root. It is not supposed to happen. I suspect dry firing. When the FP stops abruptly, the tip pulls itself away from the FP body. The smaller tip has less cross sectional area for the stress. Just a speculation.

BTW the 7mm SAUM shoots quite well. It has potential to get close to sub-moa. Still tinkering the details. Ejection is still iffy. May need to open the ejection port a bit more. Fat brass with thin bullet is tricky on AR.

-TL

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stagpanther · January 27, 2025, 02:41 AM

Ah--in other words the thinner tip broke at a shoulder where the pin gets wider? I'm thinking it has to be striking some interference with the bolt; the pin is just along for the ride inside the bolt. It occurs to me another possibility is that there might be a side load on the pin head if it's protruding too far out while the case is being ejected; But I would suspect you'd see something on the the brass/primers as evidence.

tangolima · January 27, 2025, 04:08 AM

The pin must have been fully extracted into the bolt before ejection happens, so there shouldn't be any side loads. It is also possible that the pin tip embeds into the primer deeper. The pin extraction action pulls the tip back out. The thinner tip snapped by the sheer pulling force. You see I'm leaning towards the tip being thinner in hp FP.

Anyway, I will how long the new pin will last, before digging deeper. I'm happy fun tinkering with this AR 10. So far it has been always in silly calibers; .243 win, 7mm SAUM, and 6.5CM later on. I will continue putting anything but .308 win on it. Just to make my range buddies scratch their heads.

-TL

Sent from my SM-N960U using Tapatalk

January 26, 2025, 04:40 PM	#57
stagpanther Senior Member Join Date: March 2, 2014 Posts: 12,897	My guess is the FP deflected and hit something in the bolt itself. Did any of the fired cases show unusual cratering, movement or piercing of the primers? __________________ "Everyone speaks gun."--Robert O'Neill *I am NOT an expert--I do not have any formal experience or certification in firearms use or testing; use any information I post at your own risk!*

January 26, 2025, 05:41 PM	#59
stagpanther Senior Member Join Date: March 2, 2014 Posts: 12,897	You got me stumped--if the pin was properly matched to the bolt I don't see how else it could break other than hitting something harder than it is. __________________ "Everyone speaks gun."--Robert O'Neill *I am NOT an expert--I do not have any formal experience or certification in firearms use or testing; use any information I post at your own risk!*

January 27, 2025, 02:41 AM	#61
stagpanther Senior Member Join Date: March 2, 2014 Posts: 12,897	Ah--in other words the thinner tip broke at a shoulder where the pin gets wider? I'm thinking it has to be striking some interference with the bolt; the pin is just along for the ride inside the bolt. It occurs to me another possibility is that there might be a side load on the pin head if it's protruding too far out while the case is being ejected; But I would suspect you'd see something on the the brass/primers as evidence. __________________ "Everyone speaks gun."--Robert O'Neill *I am NOT an expert--I do not have any formal experience or certification in firearms use or testing; use any information I post at your own risk!*

July 4, 2024, 06:28 PM	#51
georgehwbush Senior Member Join Date: July 14, 2023 Location: down town USA Posts: 537	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.

July 5, 2024, 09:43 PM	#53
georgehwbush Senior Member Join Date: July 14, 2023 Location: down town USA Posts: 537	yep that's what they say.

July 11, 2024, 11:11 PM	#54
tangolima Senior Member Join Date: September 28, 2013 Posts: 5,123	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 Sent from my SM-N960U using Tapatalk

July 12, 2024, 10:29 PM	#55
tangolima Senior Member Join Date: September 28, 2013 Posts: 5,123	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 Sent from my SM-N960U using Tapatalk

January 26, 2025, 03:48 PM	#56
tangolima Senior Member Join Date: September 28, 2013 Posts: 5,123	Further update. Have been shooting the 7mm SAUM upper I recently put together. It has magnum bolt with fitting firing pin with small tip (improved or high-power version). About 60 - 70 rounds, the gun went silent with no dent on primer. The small tip had broken off. Good that I have one back up. Seems like an issue intrinsic to the smaller tip. Probably not a good idea to do dry firing too often. I have a few extra pins lying around. Will fit one as back up. -TL Sent from my SM-N960U using Tapatalk

January 26, 2025, 04:45 PM	#58
tangolima Senior Member Join Date: September 28, 2013 Posts: 5,123	Nothing unusual before the click with no boom. At least not that I could find. -TL Sent from my SM-N960U using Tapatalk

January 26, 2025, 07:27 PM	#60
tangolima Senior Member Join Date: September 28, 2013 Posts: 5,123	Same here. The tip broke off clean at the root. It is not supposed to happen. I suspect dry firing. When the FP stops abruptly, the tip pulls itself away from the FP body. The smaller tip has less cross sectional area for the stress. Just a speculation. BTW the 7mm SAUM shoots quite well. It has potential to get close to sub-moa. Still tinkering the details. Ejection is still iffy. May need to open the ejection port a bit more. Fat brass with thin bullet is tricky on AR. -TL Sent from my SM-N960U using Tapatalk

January 27, 2025, 04:08 AM	#62
tangolima Senior Member Join Date: September 28, 2013 Posts: 5,123	The pin must have been fully extracted into the bolt before ejection happens, so there shouldn't be any side loads. It is also possible that the pin tip embeds into the primer deeper. The pin extraction action pulls the tip back out. The thinner tip snapped by the sheer pulling force. You see I'm leaning towards the tip being thinner in hp FP. Anyway, I will how long the new pin will last, before digging deeper. I'm happy fun tinkering with this AR 10. So far it has been always in silly calibers; .243 win, 7mm SAUM, and 6.5CM later on. I will continue putting anything but .308 win on it. Just to make my range buddies scratch their heads. -TL Sent from my SM-N960U using Tapatalk