Can the AR firing pin retaining pin be replaced with a regular stainless steel pin?

[Maelstrom]

I was looking around at Home Depot today and I noticed they had stainless steel cotter pins. The 3/32" pins appear to be the correct size to replace the firing pin retaining pin.

How much stress is the firing pin retaining pins subject to? I figure it can't be much since it's such a small piece. It seems the only thing the retaining pin does it make sure the firing pin doesn't fall out of the bolt carrier during the chambering phase, so it's really only experiencing the "impact" of the firing pin as inertia moves it back to the rear.

Can anyone tell why or why not this should be done?

Seems like a stainless steel replacement might be nice.

 

[rcmodel]

No reason it won't work if you can find the exact same size.

The early FP retainer was a solid steel split-pin that was costly to make.

It was replaced with a cotter-key because they are cheaper.

They are not load bearing, and don't wear out.
So I think there is nothing special about them except they have to fit the hole, and not rub the head on the upper receiver wall.

rc

 

[FN5.7shooter]

I put these in all my ARs. Couldn't tell you why. I have been fooling with EBRs since the early 70s and never saw a problem with cotter pins but I just like having a purpose built piece in there. The cotter pin seems cheap I quess.
http://www.dsarms.com/AR15_M16-Bolt-Carrier-Firing-Pin-Retain-Pin/productinfo/KNSPERMA/

 

[dfariswheel]

DO NOT USE A COTTER PIN.

As some manuals warn, cotter pins have a more rounded "head" that will stick up out of the hole and this will gouge your upper badly.
Compare a cotter pin to the standard AR-15 retainer and you'll see the higher, more rounded head on the cotter pin.

Here's an excerpt from one AR manual:
"CAUTION: Use a firing pin retainer, not a cotter pin. The retainer is a specially heat-treated piece. Do not spread or close the ends"

Also, cotter pins are NOT hardened like the AR pins are. If there's a problem with the hammer following the bolt, the hammer is designed to catch on the firing pin. to prevent a slam fire.
This will bend even the AR hardened pin and will badly deform a softer cotter pin. which can be very difficult to extract from the carrier.

Very few guns use ordinary hardware store parts. Even though something LOOKS like a gun part, it seldom is.

 

[Maelstrom]

I saw the same warning you referred to, and I flattened the cotter pin head to mimic the shape of the retaining pin.

As for the "special heat treat", it seems to me that the heat treating would be lost when you consider that the retaining pin sits directly below the gas key and is directly subject to the hot gas.

 

[ants]

Sounds to me like Maelstrom is trying to talk himself into saving 75 cents.

Isn't the FP retainer the cheapest part of an AR anyway? It's only a dollar.
https://www.model1sales.com/item-de...e=bcgparts.gif&CFID=46816547&CFTOKEN=81924423.
The chrome plated one is six dollars.

 

[nicholst55]

Firing pin retaining pins are hardened; cotter pins are not. You might get by using a cotter pin but I'm not going to try it in my rifle. I'll also wager that the cotter pin will be horribly bent each and every time you fire it.

 

[BBstacker]

DO NOT USE A COTTER PIN. They are not heat treated. Stainless steel is softer than mild steel. The cost of a new one is about $2.50.

 

[Max C.]

Actually stainless steel is generally much harder than mild steel. Chromium itself adds hardness to the metal. This is why stainless steel tends to make better pocket knives (of the same level of production) than mild steel. Also mild steel doesn't have enough carbon to be useful in applications calling for a harder metal. This is the same reason why 410 and 420 stainless steels tend to make poor sharp objects. Stainless steel parts also have to be painted since parkerizing is a no-no with high chrome steels. In addition, high end knives, drill bits, and saw blades, made from high carbon steel go through a more rigorous heat treatment process than stainless steels (differentially tempered, case hardened, etc) do. Most stainless steel tools/cutting tools go through a simple heating, annealing, and tempering process. Tool steels (alloy steels) tend to get their hardness (and in the case of cutting tools, carbides) from the same elements as more "exotic" stainless steels, just in smaller amounts so that their toughness isn't hindered.

But I digress to my point, any damage done to your rifle will be due to design and not material choice. Mild steel is easy to work with, cheap, and well suited for general purpose parts that won't experience a ton of stress. They anneal easily and are extremely rugged (when's the last time you heard of an I Beam just failing for the heck of it?). Hardening the pieces is really only so that the part is properly annealed and won't bend/deform/crack over time/with use. So long as proper design considerations are in place for a stainless steel part (modified cotter pin or original part) there's no real reason why it should fail either in a non-load experiencing area of the gun. The real problem is probably that there is a clearance issue or there is simply increased wear due to vibrations because the stainless steel cotter pin is 5-10 points harder (rockwell C scale). A hard steel knife will shave pieces off of a piece of aluminum very easily, just like a hard cotter pin. A soft, crappy, cotter pin will simply deform with use. A cotter pin that is far too hard (Stainless steel or carbon steel) will snap off with use.

I don't own an AR so I can't really muse beyond that, but I felt I should throw in a bit of my metallurgical knowledge into this thread.

 

[dfariswheel]

Cotter pins are designed to be bent to lock them in place.
The AR firing pin retainer is hardened steel and is designed NOT to bend.

Many manuals specifically warn NOT to use an ordinary cotter pin.

Again, just because a firearms part looks like a hardware store item is no sign it IS. In the case of the AR pin retainer, it's NOT replaceable by a common cotter pin and you're courting serious damage to a rifle by attempting to use one.
Looks are NOT function. Some firing pins LOOK very much like a common nail.

THINK about it. Is it worth risking many hundreds of dollars of rifle just to save a few cents on a small part?

 

[Maelstrom]

I wasn't considering it as a cost-saving measure. I was looking more at accessibility.

If you need the part, do you buy it for 40 cents from Brownells, then pay $5 shipping to get it in a week? Or, as an alternative, can you drive five minutes, spend, fifty cents, and have it now.

Of course someone will pipe up with how I should be prepared and have spare parts.

 

[rbernie]

You should be prepared and have spare parts.

Actually, I've never needed to replace a firing pin retaining pin due to breakage or wear. I'd be much more concerned about other parts.

 

[rskent]

No, it’s not OK.
The stainless that is used in cotter pins is defiantly not harder than the steel used in the standard retaining pins.