Quoted: Quoted: Yeah the spring was fine as it was a brand new rifle. It just needed a little help. And I will gladly spend the extra whopping 75 cents  to get an O- Ring which is actually intended for this purpose rather than buy a hardware store O- ring which is not desined to withstand the heat of a firearm. I guarantee there is no company out there producing o-rings specifically for weapons use. If they did, they'd cost a lot more than $1 (hence the d-fender's price). The Crane o-ring is simply an o-ring commerically produced that just happened to fit "criteria" for an application. What're the criteria? 1) Does it fit around your extractor spring?(If yes, then move to part 2) 2) Does it help with extraction? (If yes, then move to part 3) 3) Does it withstand heat? (If yes, then move to part 4) 4) Does it withstand solvents? (If yes, you're done. Buy a bag.) From my experience, the overwhelming majority of o-rings found at the Home Depot & its ilk will fit these criteria. I use hardware store O-rings in my hybrid rocket motors. ETA: Another requirement

I dont know who makes them, or what they are, only that I get them from the same place CRANE does.  The .15 cent O-rings may work, but for an additional .85 cents most find its worth the peace of mind.   I get a laugh out of these threads about the hardware stor O-rings. People spend hundreds/thousands of dollars on flash lights and optics(which do NOTHING to enhance reliability), but are concerned about an extra .85 cents on an O-ring that has proven its worth?

If the hardware store O-ring is good enough, so are tac-points and a maglight held in place with a weaver scope ring.


I'll admit I'm biased because I sell them, but I'm not getting rich off these things.

If the lives of the soldiers who use these are trusted to a $1 O-Ring, thats enough for me.

I'm from the company which makes the D-Fender.

I'll be the first to say that some of the O-rings will work to a degree to improve extraction reliability, and I don't dispute that.
In some cases they will work for quite a long time with no problems.
In some other cases they may not.

We at MGI were the first to understand that extraction could be improved with a device such as this.  We tested many things, and primarily the O-rings.
After much testing and observing failure modes, we determined that for the reliability necessary for this application, a different design was required.
This spawned the inventing and introduction of the D-Fender.

The shape and material of the D-Fender is optimized for AR15 extractor applications. It has a specific increase in the extraction tension, is built to resist heat problems, has the shape to eliminate deformation and breakage/binding that plagues the O-ring, and will keep its "springiness" over the long run.

This D-Fender device has been tested at Crane NSWC in both the 35000 round test and 50000 round test, without failure. It has been accepted by SOCOM as a combat reliable item, and is in use by SOCOM, and has been for years.

We are the discoverers and the inventors of this entire concept.
We did a whole lot of testing.
We know a thing or two about it.
We make the D-Fender.

I have not said, nor will I say that the O-ring will not work in this application.
All I'm saying is that our exhaustive testing has shown that O-rings are not reliable enough to use in this application in the long term. The time of failure is not  predictable, and when it fails it is likely to be a catastrophic failure.
If you use the O-ring, changing it regularly will be your best bet.
Our testing has shown that when an O-ring fails, it is prone to migrate to a spot which binds up the extractor and takes the gun out of action.
We could easily have marketed O-rings, since we were the first on the scene with this idea.
It would have been alot easier to sell $1 items that we could buy for pennies in bulk.
We decided after testing that we couldn't go to market with them, due to reliability concerns, and so we went a step further to design the D-Fender.
It's your choice.

The difference between the Crane O-Ring and the D-Fender are the materials and the shape.

The shape is designed in a wedge profile and "D" shape, which is suited to the application and doesn't do the asymetrical "squishing" that the O-rings do, which leads to the breakage.

The material is designed to have a specific(4x) increase in extractor tension in the AR15 extractor system, that has been determined thru testing to be the desired tension increase needed to enhance carbine extraction.

The overall design is made specifically to overcome the inherent weaknesses in the O-Rings that were identified in tests as needing improvement.

Testing showed that this set of design criteria solved the reliability issues that were cropping up with the O-Rings that were first tried. It has resulted in a very reliable, long-term solution to the carbine extraction issue that has been time tested, Crane tested, and combat tested over the last 5 or more years.

We invested a large sum of money in special molding and tooling to make this product, to say nothing of the time and efforts and innovation that were involved.

We are very proud and happy to have been able to be leaders and innovators in this area, but we want to be sure that people are aware that there are differences in these items, and the reasons why we make what we do.

Again, we take no issue with those who want to use O-Rings instead of D-Fenders.
We simply want to point out that we did not stop at the simple answer of the O-Ring for a reason, and went on to address the issues that came up with the O-Ring, so that we could have an even better product that went beyond that in performance and reliability.

We know it costs more, and people sometimes take issue with that.
We're very aware of that.
But, we're also aware that many people want to have the best performance possible, and are willing to pay more to get it.
That's where our product is targeted.
We went beyond the norm in performance, and it cost more to do it.
We know the difference, and we are providing the best solution, but not the cheapest one.

It's up to the individual to decide for himself what he wants to use.
We simply make the product available to those who want it.

It is mostly an issue with carbines, due to the short gas system and its effect on early bolt opening.

BTW, I am the factory sales rep for MGI, and I have all of their products available by contacting me at the info listed below, in my sig line.

Allow me to repost this:

" Welp,

I dropped in on a semi-private machinegun shoot held today at a farm in Wythe County, VA.

A older guy there who had just bought an M-16 (He did not know how to break the bolt down so he had no experience with his toy) had a ultra short pistol length upper (7 inches?) he was trying to get to run.

**NOTE** this was a micro M-16, the hardest of the hard to get running right.

He was getting around two dropped extractions per magazine.

I had a spare D-fender in my range bag.

His ultra shorty now runs like a 4 legged sunuvabitch .

The D-fender fixed his new expensive toy, I am now one D-fender short and 13 bucks richer.

He is happy as he can be and I got to play with a couple of M-16's as a reward for my foresight.

The D-fender works with no BS. It is worth 13 bucks easily IMO especially considering he spent at least 30 bucks worth in ammo trying to figure out his problem before I butted in."

Quoted: The D ring solved the dropped extraction issue for my problem-child 11.5.  Now runs perfect with anything. I can understand that an O ring would work, too.  But I like the idea of using something that's been tested to over 30k rounds.   My understanding of the Crane rejection was that it was based on cost.  In other words, it wasn't that the O ring tested out as a superior product.  When I think about my annual firearms budget it's clear that $13 isn't going to break the bank.   Sam

That is pretty much the jist of it as I see it too.

Brett

I would rather just fix it once for the life of the Bolt and never have to worry if it was going to break and render the weapon inoperable at an inoppurtune moment.  13 bucks to never fool with it again.  Seems like a bargain to me.

Quoted: I would rather just fix it once for the life of the Bolt and never have to worry if it was going to break and render the weapon inoperable at an inoppurtune moment.  13 bucks to never fool with it again.  Seems like a bargain to me.

I personally know of a couple CRANE O-Rings with over 10K rounds on them and they look as good as new.   I figure 95% of ar owners will never shoot that many rounds in their lifetime.  Personally, I would replace the bolt every 10K, and drop another $1 for a new O-Ring.

I can understand the design idea of the DFender, and if I spent as much time and money as they did to develop something that is no better than a $1 O-Ring, I'd be hyping and marketing it all over the place to get sales.  
Either way, both have proven themselves. There are too many CRANE O-Rings out there in combat dispute their reliability. If they were failing, you can be sure we'd be hearing about it.  I've never heard of a DFender fail either.
Stick with the CRANE O-Ring or the DFender and you'll be fine, but I wouldnt risk the hardware store O-rings.

Quoted: Quoted: Personally, I would replace the bolt every 10K, and drop another $1 for a new O-Ring. Steve, this seems very conservative to me.  I thought expected lifespan was around 30k or so, even with the extra wear and tear of the carbine gas tube lengths?  I realize this is going a bit OT, but what fails around 10k?  If the extractor why not just replace it? Sam

Bolts should last 20K rounds.  It takes me 3-4 years to wear out a barrel on my service rifle(4-5K rounds), and I replace the bolt when I replace the barrel.  $40 bolt  every 4 years, big deal. As I said 95% of AR owners will never run 10K rounds through a rifle in their life time.

Things like PRI fatboy gas tubes and heavier buffers can restore the lifespan of bolts used in carbines as they delay the opening a bit till the chamber pressures have zeroed.

Steve,

Are you going to be getting any more Fatboys from PRI soon?

I notice you are out and think I may have got your last one a couple of months ago.  I am getting desperate for one to go under a FF rail for a carbine I am building up for a Pat Rogers class in early July and I really don't want to cannabilize one off of one of my other rifles (Esp since they are tight in the gas block).