Quoted: I have the BCM upgrade kits from Bravo Company in all my AR’s.

I'm installing those in mine shortly as well.

MGI D-Fender "D-Ring".

Quoted: Me too. It's fine to spend more money for a truly superior product ; however the D fender is not a superior product. Ask Denny how many D Fenders he's used that "mushed". The same is not true for the Crane O Ring. It's not only 1/12th the cost(or less), it's better. If the Crane O Ring were to flatten ,the replacement will only cost a buck.

Funny how Crane never was able to "mush" any D-Fenders in about 100k rounds over a years time of testing, prior to the issuance of the Developmental Test Command combat safety certificate, and memorandum to SOPMOD recommending the D-Fender for use all all M16/M4 variations. And I have a copy of that certificate sitting right here next to me now. I've posted the key parts of that certificate on several occasions.

I think I'll give Denny a call, since it's been about a month since I've talked to him, and he's a regular purchaser.
To my knowledge, we've never gotten a "mushed" D-Fender back from Denny, and we do replace them if they fail.



EDITED TO ADD:

The document in its entirety:

" Department of the Army
Headquarters, U.S. Army Developmental Test Command
314 Longs Corner Road
Aberdeen Proving Ground, MD 21005-5055
CSTE-DTC-TT-T
7 May 2001

Memorandum for Department of the Navy, NSWC-Crane Division, Code 4081
B-2521 SOPMOD Project Office, Attn. Mr XXXXXX
300 Highway 361, Crane IN 47552-5001

Subject: DTC Safety Confirmation of the D-Ring

1. Purpose: This memorandum is to provide a DTC Safety Confirmation for the use of the D-Ring elastomeric washers to extractors of M4/M16 series weapons for USSOCOM Forces.

2. References:
a. Memorandum to amend TECOM Limited Safety Confirmation for the D-Fender D-ring, 8 Mar 01
b. Notes concerning Extreme Temperature Test of the SPR and 10" upper receivers equipped with the D-Rings on the extractors.
c. NAVSEA Crane Test. TR/4083/C01/1225, AA40 5.56mm Frangible Ammunition Evaluation in the 10" M4A1 Carbine, 26 Jan 01.
d. NAVSEA Crane Evaluation, 5.56mm M4A1 Carbine Extended Life Barrel and Reliability Evaluation, 27 Nov 00.
e Extract of Draft UPM Shock Test, Involving Sand, Dust, with use of the D-Rings on the extractor spring, undated.
f. Letter, TECOM Limited Safety Confirmation for the D-Fender D-Ring for the Carbine, 5.56mm, M4A1, 23 Aug 99.

3. Background: USSOCOM Forces have experienced incidents of failure to extract/eject with the M4A1 Carbine. Special Operating Forces are constantly seeking tailored variations of standard M4 Carbines that more closely match mission requirements. This has led to the development of the Special Purpose Receiver and Close-Quarters Battle Receiver, which are variants of the standard M16/M4. Issues of reliability have occurred with each of the variants, however, extraction problems across the board have been greatly reduced with the addition of the elastomeric washer D-Ring around the extractor spring, with no detectable negative impact in serviceability or reliability.

4. Test Performed and Results:
a. Documented use of these rings has occurred at NSWC Crane over the last year in an attempt to develop the limits of their performance.
b. Extensive testing of standard M4/M16 gun/barrel/ammunition combinations were performed by NSWC Crane Division as well as modified barrels and ammunition.
c. These were tested at temperature extremes, dust, and gun-related chemical compatibility.

5. Conclusions and Recommendations:
The D-Fender D-Ring elastomeric rings/washers have been used extensively on three configurations of the M16 type weapons and have always been found to aid reliability, without negative effects according to the test data provided by NSWC Crane Division. This has been described to be a simple, easy method to ensure weapon extraction and functioning. Based on the test information provided by NSWC Crane Division, a DTC Safety Confirmation is issued for the use of D-Fender D-rings on M4/M16 series weapons.
The following restrictions or limitations apply:
a. All personnel within 30 meters of the firing site must wear single hearing protection(plugs or muffs).
b. Local range restrictions and M4/M16 seris operating restrictions will be observed.
c. The weapon must be operated and maintained in accordance with the appropriate manuals.
d. It is recommended that further chemical compatibility testing be accomplished IAW TOP 3-2-609, Chemical Compatibility for Non-metallic Manuals, as a guide for future testing and use of the D-Ring.

6. The DTC point of contact for this action is XXXXXXXXX, CSTE-DTC-TT-T

For the Commander:
XXXXXXXXXXXXX
Chief, Tank Automotive Armaments Division
Directorate for Test and Technology"

End of document.

Please note that I have added(as an edit to my previous post) the US Government Military Document in its entirety.

Please see my previous post for the document, a few posts up on this page.


Anybody can buy whatever they wish.
I've posted the documentation, and have the backup for my statements.
Anybody wishing to post the Crane O-ring DTC evaluation, is welcome to put it right up. I'd love to see it, and check out the eval comments.

I appreciate all the feedback and comments.

Here's a bit of insight into the history of this stuff, so you can see what went on in the early days when this stuff was first happening.

When Mack and Jim were developing the Counterpoise System(which was a carbine reliability package) back in the 1990s, there was development work on extractor reliability enhancement for this project.

They initially tried o-rings in their work, particularly because they were easily obtainable and showed promise.

They did alot of testing with o-rings.

The testing showed that the o-rings worked.
However, the testing also showed that the o-rings had unpredictable number of rounds to failure. There were some that failed real early, and some that lasted a real long time, and others that lasted somewhere in the middle.
They shot alot of rounds with alot of o-rings in an attempt to be able to figure out some kind of predictable data about replacement schedules.
Ultimately, they were unable to make any reliable determination about when to make replacements, because the failures were unpredictable.

However, the failures did have a particularly nasty feature of having the failed o-ring migrate to a spot under the extractor, that could cause the gun to tie-up, and that concerned them alot.
They felt that they didn't want to accept potential problems like that, so they proceeded to evaluate the causes for failure.

The failures were predominantly caused by the symmetrical donut shape of the o-ring, when subjected to the asymmetrical compressions seen in the extractor application.

Analysis yielded a new shape which was the evolution of the o-ring, and it was the "D-Fender" D-ring.
The differences are that the D-shape fits the shape under the end of the extractor where it is installed, it is wedge-shaped in profile to perform in the asymmetrical compressions without "squishing" in a mode that subjects it to failure, and it is made of a different polymer which is designed to provide the ideal(determined thru testing) 4x increase in extractor tension.

These are the things that set it apart from the o-ring.

Now, of course, we always see and hear the anecdotes of people having good long-term results from an o-ring.
I don't dispute that, and I've never disputed that.
It's possible, and we've seen it many times in testing, and we know about that.
Our point is that it is unknown what result you will get when you put any given o-ring in there. It might last a helluva long time, and it might not.
We don't think that is what we want to provide.

We wanted something that could be counted on to last a long time, with every one of them. And that's why we evolved the design into a more application specific product.
Let's face it, the o-ring is not designed, nor was ever designed, to be an extractor enhancer. The fact that it works as one is purely coincidental. It will never be as good as a properly engineered product made expressly for the application.

So, we certainly concede that the o-ring works, and can be a very cost-effective extractor enhancement device, as long as you understand that you don't know how long it is going to last.

We were more adamant that our product had to be predictably reliable for the long term, and that's why we made it.

Of course I know it costs more, and I fight that battle all the time.
All I can say is that it is made for the application specifically, is correctly designed, and will last a very long time, and it can be relied upon to do so.

I can't tell you how to spend your money.
I think it's worth it, and alot of other people who shoot for a living do too.
If you don't think it's worth it, then don't buy it. But at least we are making it available to those who do want something that was developed exactly for the application it is being used for, which has not been done by anyone else. And it has been proven to be effective, and does exactly what we say it does.
So, there's some choice beyond the basics, for users who wish to avail themselves of it.

Just because this product is on the market, is not any kind of indictment about anybody's choice in extractor products. It is simply a product that is on the market.
But, we didn't just pull this thing out of our ass one morning and decide to make all kinds of claims about it, and charge a ton of money for it.
I engage in these threads because it is common for people to make certain non-flattering comments about the D-Fender, just because they don't like the price. I try to show why the product exists, and why it might be something they'd want.

Quoted: Can't argue with anything you said.  Although i do have some questions? You say it's the asymetrical force being applied to the Crane O ring that causes it to sometimes prematurely fail; correct? So in other words one side of the O ring is getting squished more than the other. I'm wondering if perhaps periodically rotating the position of the Crane O Ring will potentially help extend its life; by distributing the compressions of the extractor to different parts of the O ring? I'm not saying removing the extractor just for that puropse but rather rotating it during those inspection periods. Now the D fender is supposed to last longer than the Crane O Ring. Does that mean you just install it and forget it; or is it a good idea to check it every now and then?

To be honest, I don't know the answer to the question about rotating the o-ring.
I can ask Mack about it, and see what he says.

Regarding the D-Fender replacement schedule, we generally recommend replacing it when you replace the bolt.
However, they have lasted for many many thousands of rounds where people just kept using it over and over.
In fact, if you notice in the DTC letter I posted, Crane's statement was that they tested it " over the last year in an attempt to determine the limits of their performance".  In reality, what that really means is, they reached about 100k rounds and gave up. They didn't fail. They never really determined the "limits" of performance.

We are conservative, and recommend changing with the bolt. We always recommend that it be inspected during normal routine maintenance disassembly.
It should be installed correctly, with the D-ring installed around the spring, and not under the spring. Most D-Ring failures have occurred from improper installation.
In the unlikely event that it did fail, we do have a replacement policy in place, and you'll get a new one.
I've been working with Mack for almost 7 years, and I haven't seen any come back from the customers that I've sold to. I'm sure it has happened, but it is rare.
In fact, in all those 7 years, I've had no D-Rings come back, only 5 RRB buffers(weak roll pins), one adjustable gas tube that a customer burnt-up in full-auto, one QCB upper(thin wall in the forging), and one Modular Lower(tolerance stack). All were immediately replaced at no charge. We have a very good reliability record with all our products, and we are very attentive to reliability in our items.