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Page AR-15 » Build It Yourself
AR Sponsor: bravocompany
Posted: 12/8/2017 3:09:25 PM EDT
Hi Folks,

I normally don't even think of straying from mil-spec with any of the parts in my ARs (except for the LMT enhanced bolt, because, let's face it, it's made by LMT). However, I am the proud owner of a 1959-production Portuguese AR-10, and through my research contacts - I am the author of the recent Collector Grade Publications book on the AR-10 - I learned that the rifle I now own has a round count through it of roughly 40,000 rounds, many in full-auto mode, and never has had a part replaced (it cost me more than I are to admit to buy a spare extractor, extractor spring, gas rings, and charging handle as spares in case I need to replace parts in the future). This has given me a profound respect for the design of the parts on this rifle. This is what pushed me to make my carriers in all my AR-15s chrome-plated, and with the LMT Enhanced Bolt has relegated my previous BCG "A-Team" of Colt Carrier/firing pin/cam pin/retaining pin and ArmaLite Bolt to the backup role.

Aside from the chrome BCG, the other main difference in parts I saw on the Portuguese AR-10 that I actually could emulate in my AR-15s was the firing pin retainer. I am not overly displeased with the standard cotter pin type used today, although I have worn out and replaced 2 in the 25,000 rounds I have put through my modern AR-10. I did notice, however, that quite a few companies are offering this part in the original solid pin with split end, just like on the early AR-15s and my Portuguese AR-10. I know the ones from KNS have mixed reviews due to the split ends breaking from installation fit problems, but I have a source that makes stainless ones, and I am seriously considering giving those a try.

Does anyone have experience or engineering insight into these, and whether they might be more durable than the standard cotter pin? Also, is there any possible downside I am potentially overlooking, like them being too stiff or hard and putting too much pressure and wear on the firing pin or carrier, for example?

Thanks for your advice!
Link Posted: 12/8/2017 3:13:14 PM EDT
[#1]
Cost savings.

Plain and simple.

I replace all of mine with the pin and keep cotters as backup.
Link Posted: 12/8/2017 3:29:40 PM EDT
[#2]
Cotter type pins have been used in place is the solid pins for a long time. I have yet to ever have a problem with either as I have both.  There is no real reason to switch to the solid pin unless it's something you want.

The back and forth regarding solid vs cotter is a lot like the pro vs con of KNS type pins. Although lately with a couple two stage cassette style drop in triggers I can see more going with KNS pins because of regular pins walking/ falling out easily due to the trigger design.

To me,  the solid pins are a ever so slight more of a pain to remove than the cotter pins. Mainly because the clevis tension is higher and there is no eye to hook anff remove.
Link Posted: 12/8/2017 3:58:09 PM EDT
[#3]
Link Posted: 12/8/2017 4:04:24 PM EDT
[#4]
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Quoted:
The DPMS units are crap.
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Totally!  No matter how good a design something is, if DPMS makes it, the part is crap.  The ones on which I have my eye don't come from DPMS, or even KNS, but are from a source I have with a contractor that sells BCGs to the military.
Link Posted: 12/8/2017 4:38:37 PM EDT
[#5]
Modern FP retaining pin is NOT a cotter pin, which is soft metal, designed to be deformed and bent to prevent vibration-induced backing-out of threaded fasteners. Nor is it "modern", per-se, since it's been in use since the mid-1960s - meaning for over 50 years now.

Proper FP retaining pin may start as a regular cotter pin, but is hardened to provide the necessary structural rigidity, which also must occur with the old-style "solid pin" in order for it to withstand thousands of high pressure cycles.
Link Posted: 12/8/2017 7:29:30 PM EDT
[#6]
Discussion ForumsJump to Quoted PostQuote History
Quoted:
Modern FP retaining pin is NOT a cotter pin, which is soft metal, designed to be deformed and bent to prevent vibration-induced backing-out of threaded fasteners. Nor is it "modern", per-se, since it's been in use since the mid-1960s - meaning for over 50 years now.

Proper FP retaining pin may start as a regular cotter pin, but is hardened to provide the necessary structural rigidity, which also must occur with the old-style "solid pin" in order for it to withstand thousands of high pressure cycles.
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It does even start life as a cotter pin.  It starts life a a piece of half-round wire and is made into a retaining spring pin.

And, they are not loaded at all.  The pin bouncing off it at the end of the recoil stroke?
Link Posted: 12/9/2017 8:53:43 AM EDT
[#7]
First of all pics of your porty ar10 please??. Also I use pins and I will never buy dpms  parts for anything.
Link Posted: 12/9/2017 2:36:40 PM EDT
[#8]
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Quoted:
And, they are not loaded at all.  The pin bouncing off it at the end of the recoil stroke?
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The firing pin slams back against the retaining pin under gas pressure as part of the cycle. Not much mass, but a crapload of gas pressure acting on it.

An unhardened pin would fail quickly.
Link Posted: 12/9/2017 5:49:16 PM EDT
[#9]
Discussion ForumsJump to Quoted PostQuote History
Quoted:

The firing pin slams back against the retaining pin under gas pressure as part of the cycle. Not much mass, but a crapload of gas pressure acting on it.

An unhardened pin would fail quickly.
View Quote
Got any proof of this ? I have been shooting AR"s now since 1980 and have never seen a failure of the pin. Zero. I am convinced it could be plastic if not for the heat.
Link Posted: 12/9/2017 7:00:39 PM EDT
[#10]
Discussion ForumsJump to Quoted PostQuote History
Quoted:
Got any proof of this ? I have been shooting AR"s now since 1980 and have never seen a failure of the pin. Zero. I am convinced it could be plastic if not for the heat.
View Quote
Hardened pins don't usually fail, so I'm not surprised you've never seen one.

Go ahead and try it with a plastic coffee stir-stick. It will fail before it can melt, I assure you. An unhardened regular cotter pin will last longer, but get pretty deformed in short order.

Simply consider that the firing pin channel is one of two ways in which hot, high-pressure bore gas can exit the piston formed by the bolt inside the carrier, the other being blowby past the gas tube out the carrier key, until the gas rings clear the vent holes in the side of the carrier. FP retaining pin is hardened for that reason.
Link Posted: 12/9/2017 9:30:38 PM EDT
[#11]
Discussion ForumsJump to Quoted PostQuote History
Quoted:

The firing pin slams back against the retaining pin under gas pressure as part of the cycle. Not much mass, but a crapload of gas pressure acting on it.

An unhardened pin would fail quickly.
View Quote
Sorry, no, it does not.

1) If the firing pin were set back enough to contact the retaining pin the hammer would have to be moved backwards about 5/64th inch. High-speed film made during testing shows the hammer does not move appreciably until the carrier starts to move.

2) In order for the pressure inside the case to act on the firing pin outside the case, it must move the case wall in some manner.  The only part of the case touching the firing pin is the primer. If the firing pin moves due to gas pressure the dent in the primer would be pressed out.

3) The pressure in the carrier cavity is about 2000 psi, and in order to get to the firing pin needs to bleed through a .0010 inch gap between the tail of the bolt and the hole in the carrier.  That will drop the pressure considerably.  Rough estimate is about 150 to 200 psi.  That pressure acting on the area of front flange of the firing pin is less force than the hammer tension.

There is the set back of the case against the bolt face, but that does not have very much force and acts only on the firing pin, and the distance the firing pin moves is the difference between the cartridge headspace and the chamber headspace, around 0.010 inch... and the hammer keeps the firing pin away from the retaining pin.

The major loading on the retaining pin is at the end of the recoil stroke when the bolt carrier stops as the buffer bottoms out and inertia carries the firing pin back into the retaining pin.  And, that is not inconsiderable, after all, that inertia at the front end carries enough energy to dent a primer.  But, generally there isn't much load on the retaining pin.
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