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Old May 23, 2010, 11:18 AM   #1
gunuserfixer
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AR firing pin protrusion

Anybody know what the mil-spec and/or optimum firing pin

protrusion is on a AR-15/M-16 in 5.45x45/.223 Rem. My new Stag Arms mod. 6L checks @ .026. It's obviously adequate as the rifle was test fired by mfg. ; but is it @ optimum protrusion ?

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Old May 25, 2010, 12:23 PM   #2
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Have you checked it with the normal AR firing pin protrusion gauge?
http://www.brownells.com/userdocs/le...on%20Gauge.pdf

Or are you looking for whatever the manufacturer of your particular rifle considers the 'optimal' protrusion instead of the typical Go/No-Go gauge results?

I'm not a gunsmith, but in the 4 armorer classes I've attended for the AR platform over the years (2 of which were Colt classes) it's never been discussed as being necessary to go beyond using the standard gauge to check protrusion.

I'd call the maker of your rifle and ask to speak with a tech or engineer, if you're interested in more specificity.
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Old May 25, 2010, 01:03 PM   #3
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Why are you worried about this?

In my years of AR shooting, I've never had to, or even thought of measuring this.
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Old May 26, 2010, 11:52 AM   #4
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Worried about AR firing pin protrusion?

demigod: I'm not "worried" ; just a little anal-retentive. Thanks for the concern as to my emotional well-being!
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Old May 26, 2010, 01:48 PM   #5
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I'd let that one go. There's a lot of better stuff to get anal over on the AR platform.
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Old May 26, 2010, 02:46 PM   #6
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attention demigod:

"I don't understand/share your concern, therefore your concern is unwarranted" attributed to me 5/26/ 2010
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Old May 26, 2010, 03:53 PM   #7
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I don't understand your reply.... But what I'm getting at is this....

I've been shooting this platform for many years. I've spent an absurd amount of time on the AR related forums over the last 10 years discussing, debating, studying, and even collecting problem data on the AR design.

One thing that is almost never an issue is firing pin protrusion. I may have seen the topic 2 times in the last 10 years, and it wasn't significant enough for the specifics to stick with me. Your Stag is going to be made of parts machined by CMT... A fairly reputable manufacturor who machines parts for many other AR manufacturors. For there to be a firing pin protrusion issue, you'd have to have a dimensionally out of spec bolt or an out of spec firing pin. Both are very rare with good companies like CMT.

If you're not getting pierced primers, or having ignition problems, then I wouldn't be concerned. And even in both of those scenarios, primer protrusion is at the bottom of possible causes.

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Old May 27, 2010, 02:57 AM   #8
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gunuserfixer,

Rule of thumb for any firearm is:
Firing pin protrusion should be no more than half the diameter of the firing pin.
.050 dia. firing pin = .025 protrusion.

Of course very few firearms will have the optimum protrusion, some will have a little more and some a little less.
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Old May 27, 2010, 06:24 AM   #9
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What thumb? Seriously, where did that dimension come from? Anyway, when I was in the military, nothing went out the door with out a gage check. It did not matter what it came in for. I do not ever remember a bad F/pin from anybody in the shop. I am with the other guys, if it is not broke....
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Old May 27, 2010, 01:21 PM   #10
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The standard AR-15/M16 firing pin protrusion gauge recommended in my Colt & S&W M&P 15 armorer manuals uses a GO dimension of .036" and a NO-GO dimension of .028".

With the firing pin held firmly forward in the bolt, the end of the firing pin protrusion gauge (or gage, if you prefer) marked "MAX" (GO - 0.36") should pass over the end of the pin without touching it, and the end marked "MIN" (NO-GO - .028") should hit the end of the firing pin and not pass over it.

Just out of curiosity, how are you measuring the .026" protrusion, and why not simply use the appropriate gauge?

FWIW, I've yet to check an AR firing pin with the standard protrusion gauge and have it fail to exhibit the proper protrusion. If I ever do I'll replace the firing pin with one that passes the normal check.
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Old May 28, 2010, 03:45 AM   #11
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Gunplummer,

I referred to it as “Rule of thumb” cause calling such a simple dimension a “formula” has always seemed silly to me. The dimension was taught to me and everyone else in my class in the first semester of college by the instructor in charge of the gunsmithing program I attended. I have always adjusted pin protrusion in this manner and have never had a firearm fail to discharge. This is how the protrusion was adjusted as well when I worked at McMillan Rifle Co.

You guys got me curious so I mic’ed four AR firing pins, 1 ea. measured .059 dia., 2 ea. measured .062 and 1 ea. measured .063. The .062 were DPMS and the other 2 were from unknown manufacturers. I also used a gauge pin to check the firing pin hole in the bolt face of a Bushmaster carrier assy. and that came in at .070, firing pin protrusion was @ .030 on that unit. I had never checked the diameters of pins for an AR before cause it is never an issue with this type of rifle….if a pin breaks, then just throw in another one. They don’t have to be adjusted during assy. as some bolt guns do.

I imagine (in regards to military gauges) that the max (Go) and the minimum (No Go) protrusion on an AR (M4,M-16,etc.) would be to insure that the military never had any misfires cause by too little firing pin protrusion. On a firearm chambered in a non-magnum cartridge….a little too much protrusion wouldn’t cause any undue problems as long as the tip has the correct radius, and I’ve only seen maybe three firearms that had to have the pin reshaped in order to keep them from piercing primers. Being that military ammo can also utilize slightly harder primers, the added length (Go-gauge .036) to the protrusion would also help ensure ignition if the headspace of a particular weapon were at max to begin with.

I have never owned a military pin protrusion gauge cause all it will tell you is if the protrusion passes or fails and that is determined visually, I use an adjustable gauge that allows me to check the dimension with a depth mic…….no guessing involved as to how much of the pin is sticking out of the bolt face. I have had to use the military gauges once before though, the Navy supplied them to us when we were building the .50’s for them and wanted the pins set using their gauges, this method did make setting the protrusion of the pins much faster.

I agree with you that “if it ain’t broke, don’t fix it”, all you end up doing is spending more time in the shop working on crap when you could be spending that time on the range. The OP was simply asking what was the optimal protrusion for an AR, I gave him the “Rule of Thumb” that I was taught. This particular question is kind of subjective though when dealing with military style weapons. He never said he was going to alter the pin and some of the replies to this thread kind of beat him over the head.
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Old May 28, 2010, 01:34 PM   #12
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Yep, not a lot of reason to agonize over this in the AR design. It was made simple for a reason. I'd be more concerned to check the tip during cleaning to make sure it wasn't damaged.

The M16 (military full auto) rifles use a firing pin that has a slightly thicker collar (rim) around the rear of the firing pin than some commercial models. Supposedly better able to withstand the impacts involved when it comes to a lot of FA fire.

The generous protrusion dimensions probably also allow for sufficient protrusion in the event of accumulated fouling under (in front of) the collar during battle field conditions, too. Not something a lot of sport shooters or LE users would probably have to worry about in semiauto rifles not being used under adverse conditions.

I'd be surprised to find an AR firing pin made by a reputable manufacturer out of spec in regard to protrusion. Easy enough to check and not worry about it with the little gauge, though.

Considering the popularity of the AR rifle, the OP might find that a friend already owns the standard gauge (like sold by Brownells), or maybe a local gunsmith might have one and he could take his BCG by for it to be checked.

Quote:
... and some of the replies to this thread kind of beat him over the head.
If you mean one of my posts, that was not my intention.

Quote:
I agree with you that “if it ain’t broke, don’t fix it”, all you end up doing is spending more time in the shop working on crap when you could be spending that time on the range.
Agreed. Range time is always more enjoyable than bench time.

I also like the old adage that it's generally not a good idea to keep trying to fix it until it is finally broken.
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Old May 28, 2010, 10:15 PM   #13
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ogree

Actually, that formula does seem to hold out some of the time. I really do not do much gun work anymore, but now that you have that stuck in my head, everything I work on I will have to check. Gee thanks, I guess.
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Old May 31, 2010, 03:25 PM   #14
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Quote:
The M16 (military full auto) rifles use a firing pin that has a slightly thicker collar (rim) around the rear of the firing pin than some commercial models. Supposedly better able to withstand the impacts involved when it comes to a lot of FA fire.
Not true. The smaller firing pin collar on SOME of the civilian ARs is designed to hook the hammer notch (on the Type II style hammer) and stop the gun from runnaway firing should the disconnector fail.
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Old May 31, 2010, 05:18 PM   #15
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Quote:
Not true. The smaller firing pin collar on SOME of the civilian ARs is designed to hook the hammer notch (on the Type II style hammer) and stop the gun from runaway firing should the disconnector fail.
While the Colt engineers did come up with that unique hammer which was designed to catch and destroy the firing pin of a runaway commercial AR-15 (runaway due to mechanical problem or an intentional attempt to 'modify' things), and the lighter collar is easier to destroy than the heavier one, the instructor in one of my Colt classes said it was also able to made lighter because it wasn't going to be subjected to the same stresses as a FA gun. Different sides of the same coin, so to speak.

Colt did a number of things to try and appease concerns over whether their rifles could be easily converted from semi to FA. Studies in engineering, if nothing else.
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Old June 8, 2010, 01:14 PM   #16
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AR firing pin protrusion {[guf]gunuserfixer}

haven't checked this post in a while. good info. thanx I'm not "trying to fix something that's not broken" or worried or agonized. This my first AR and was just curious. I have since come across a dmnn. of .029. Don't remember where. I check firing pin protrusion with an adjustable gage and 0-1 OD micr. Brownells sells them but I made mine. I'm a retired tool maker. [which might explain my anal-retentiveness] guf
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Old June 14, 2010, 09:36 AM   #17
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Yesterday I went shooting with my Armory buddy, just back from Afghanistan, and he gave me a headspace guage and a military min/max firing pin protrusion guage.

I immediately thought of this thread. For what it's worth... my M4 passed the min/max check.

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Old June 14, 2010, 11:50 AM   #18
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Quote:
... and he gave me a headspace guage and a military min/max firing pin protrusion guage.
Handy tools to have at hand.
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Old June 14, 2010, 11:52 AM   #19
demigod
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That military field guage is awesome! You don't need to take the bolt apart to check headspace like you do with commercial headspace guages.
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