I used the search to find if anybody else has had this problem and it doesn't look like it.

I just finished building my Sep. 14 upper and went to drop the bolt in. NO GO.

It's an LMT 16" M4 upper and barrel. I installed a KAC FF RAS, PRI flip up sight, and a fat boy gas tube.

The fat boy was quite tight going into the upper receiver but it finally went in fine. I went to drop a bolt and carrier in to check weight distribution and found that the carrier is striking at the fat boy and won't go forward.
I checked with both an LMT carrier and RRA as they are the only two I have around. The tube looks straight. It is slightly larger at the last 1/4 inch or so. I assume this is for a tighter gas seal but this is where it's getting hung up. Any help is appreciated but I'm leaving for the weekend around 5 if I don't respond after that.

Thanks,
yakrat101

I decided to just disassemble everything and check it out. After getting it all apart I took the gas tube and checked it with the carrier keys. It wouldn't slide into either one!

I decided to do a little judicious filing and was able to get it to fit inside the carrier key very snugly. I then reassembled everything back again and checked the fit. Everything went together smashingly.

So it looked like the problem was an overly fat Fat Boy at the carrier/tube junction. Has anybody else experienced this at all?

yakrat101

I had the same problem, though not quite as bad.  There was some binding in the carrier key, and a definite 'clink' when the tube entered the key.  Bending and filing the gas tube helped a little, but in the end I chickened out and installed a std. gas tube (which fit better).

Regards,
Randy

I have had some bolt carrier keys that had the diameter of the hole in them just a tad smaller than specs.  Even with standard gas tube I would have a problem with the gas tube not wanting to fit in the carrier key.  I found a key that was big enough to allow the gas tube to fit in, found what drill bit would just fit in, and then used that drill bit to open up the other carrier keys.  No problems with any of them that I have opened up.  I checked with a master gunsmith who specializes in working on AR15's and M16's and he told me that this is a common problem with some of the manufacturers of parts.  His recommendation was to do just as I did.

Charles Tatum
Alamo Professional Arms

Quoted: I have had some bolt carrier keys that had the diameter of the hole in them just a tad smaller than specs.  Even with standard gas tube I would have a problem with the gas tube not wanting to fit in the carrier key.  I found a key that was big enough to allow the gas tube to fit in, found what drill bit would just fit in, and then used that drill bit to open up the other carrier keys.  No problems with any of them that I have opened up.  I checked with a master gunsmith who specializes in working on AR15's and M16's and he told me that this is a common problem with some of the manufacturers of parts.  His recommendation was to do just as I did. Charles Tatum Alamo Professional Arms

Or install only Colt gas tubes  This is why I just bought a slightly used Colt tube off the EE section of this site for about what I would have paid for a new aftermarket piece. I knew it would fit the key of the Colt carrier just right

It did

I did set the PRI front sight a little aft of the shoulder to correct for no handguard cap. I used the bare metal underneath the old front sight for lining it up. After filing so the key would pass over it everything is working fine.

yakrat101

Quoted: Or install only Colt gas tubes  This is why I just bought a slightly used Colt tube off the EE section of this site for about what I would have paid for a new aftermarket piece. I knew it would fit the key of the Colt carrier just right It did

That's great, but in doing so you lost the reduction in cyclic rate and easier extraction (combined with potentially less extractor damage to the case) which is why people buy the PRI.

If the hole in the barrel nut is not dead center with the clover leaf on the receiver, the end of the fat boy gas tube is not going to be perfectly aligned inside the receiver.  This will lead to the carrier key striking the end of the gas tube.  I know this because when I got my M4 upper back from a certain smith, I had this problem.  I had to torque the barrel nut to 80lbs to get it aligned properly and now it runs like a champ (less recoil and faster followup shots).

Whats up with the CAPS Wes?!

Quoted: Whats up with the CAPS Wes?!

He has a great disdain for "Political Correctness".

Ok...I've been "on the fence" with regard to adding one of these "fat boys" to each of my 2 carbines...  Can someone reiterate to me their tangible benefits (other than being another cool part)?  Meaning, does anyone have some experience/care to make an educated guess as to how much they will save on wear & tear of extractors, improve ejection, etc....?  

Also, so you know my set up, I am going to be running a Magpul 93A w/ "H" buffer on one of my lowers and a RRA 6-pos. w/ "H" buffer on the other...both currently have RRA carbine gas tubes & RRA bolt/carrier groups, and both have 14.5 1/7 bushy M4 barrels on them.    

I'm leaning towards sticking with the "H" buffer, standard tube, and maybe adding an extra-power extractor spring because it's cheaper and it's what I currently have (e.g., "it aint broke...so...")...  That said, is there any "real" reason I should go with a standard weight buffer, extra-power extractor spring & PRI Fat Boy tube instead?  The second option is more expensive, but it seems to me that it solves the problem in a more "gun friendly" way.  Specifically, my read of the posts I've seen here suggests that the PRI tube + standard buffer & ex-power extractor reduce recoil and wear on the extractor by actually lowering the gas pressure in the tube....where as the "H" buffer + standard tube + ex-power extractor slows everything down by not reducing the gas pressure and instead simply adding more mass to the works (and relying on inertia).  So, it seems that the PRI "fat boy" + standard buffer + ex-power spring should last longer, but be more expensive initially....  

Anyone have some "real" G2 to add in here?  Do I have this straight or am I understanding this incorrectly...?

Thanks.

The Fat Boy is not the only solution available.
The MGI Adjustable Gas Tube is available, and provides the ability to "tune-in" your action speed.

To answer your question about these kinds of devices, on carbines and shorter barrelled AR15/M16 weapons, the gas port is drilled into the barrel at a point closer to the chamber than on a rifle-length gun. This causes the gas to be sent into the gas tube earlier in the cycle.  The gas then causes the bolt to open when the pressure is still high in the case. This makes extraction more difficult and may cause Failure to Extract with the rim ripped off, or a broken extractor, and can cause rapid bolt lug wear. Since the gas port is closer to the chamber, the pressure and heat is higher at this point, than in rifle length systems, it causes the gas port to erode after as little as 500 rounds fired. After the gas port has eroded open to a certain diameter, the amount of gas sent into the action is much more than originally designed, and causes very fast bolt carrier speeds and violent cycling. This can cause noticeable increase in rate of fire, violent cycling causing feeding malfunctions, excessive wear, and/or other problems. The shorter the barrel is, the worse these problems get. That's why you read alot of questions about reliability of short barrelled AR15's.

The Fat Boy uses a larger diameter gas tube to reduce the pressure in the tube, to help with the extraction problem, and also has somewhat of a rate reducing function. It has been popular with short barrelled guns because of this.

The MGI Adjustable Gas Tube does not use a larger gas tube, but instead has a valve installed in the gas tube that is adjustable by the user with a small allen wrench. This allows the gas pressure to be tuned exactly to the needs of the individual weapon and loads being used. It gives the features that the Fat Boy does(in a different way), and also allows the user to adjust for the changing gas pressure that occurs when the gas port continues to erode over time. Once the gas port erodes to a certain point, it pretty much stays at that diameter, with only a small amount of additional erosion occurring after that. Tuning the gun with the MGI Adjustable Gas Tube makes the gun cycle reliably, and allows cycling speeds to be only the minimum necessary for good operation, thus reducing battering, and noticeably reducing recoil for better control of the weapon during firing. So, you get improved reliability and less wear, rate reduction, better controllability, and you can adjust it to take up additional gas pressures as the gas port erodes during its lifetime. It installs just like changing any other gas tube, and has no problems fitting into the carrier key.

The Fat Boy and the MGI Adjustable Gas Tube cost about the same.

I am the National Sales Manager for MGI products.
I can answer any questions regarding the performance, installation,  purchase, and use of our products.

I hope my comments were informative and useful for you.

Thanks TWL, but (respectfully) I stand firmly in the K.I.S.S. crowd and I don't like adjustable anything....hell, I even have a problem with my QR levers for my BUIS and scope mounts (hate 'em, removed most of them & replaced with grade 8 nuts).  Anyway....

Has anyone here switched to a "fat boy" and regretted it???

To me it is worth it if I can say reduce wear to the point that I will go through 1-2 extracters/other bolt parts(?) to the 3 or so I may need over time (read many 1000's of rounds +).  It's an insurance policy that I am not sure I need...but reading stuff like "midlength is better than carbine...bla bla..." and such make me want to reduce my gas pressure instead of simply relying on my H buffer so soften the pulse  That said, I am not in a hurry to spend more money as I have bought parts for 2 full rifles this week instead of the mere 1 I thought I would get.  


SO, basically, if one already has an extra power ejection spring & a 14.5" carbine barrel w/ CAR-length gas system, should they get

a) an H buffer?     (I already have these, but nevermind that for now)
...or ...
b) a PRI Fat Boy?

Why?

Thanks.

Actually I would vote for BOTH reducing the gas pressure AND slowing the cycle by increased mass.

My personal weapon is a carbine, and I use reduced gas pressure(adj. gastube) and I use the MGI RRB buffer(which is about the same static weight as a 7 ounce 9mm buffer), which is alot heavier than the H buffer. I also use an M16 carrier which is heavier than the AR15 carrier. I have also milled a flat on the leading edge of my cam pin, which retards the bolt opening timing even a little more. I use the D-Fender on the extractor.

I never have any problems with reliability, even with handloads or any factory ammo. It feeds, fires, and ejects everything I throw into it.

When using my upper on a friend's  M16 full-auto lower, it remained perfectly reliable, and had a ROF of about 725rpm.

In our testing at the factory, we have found that reducing the gas pressures and increasing the moving mass is a good combination. Unfortunately, when the higher moving mass hits the back of the buffer tube, it makes a recoil "smack" which adds to felt recoil of the weapon. This is why the RRB has sliding weights and a plunger, to set some opposing mass into counter-motion against this higher moving mass of the heavier carrier and buffer.

I realize that this may sound like a commercial, but we have done extensive testing during our product developments and aim our products at the military market, and to perform under rigorous military testing and use. We designed our products to improve the function of the overall M16 weapons system in both full-auto and semi-auto. You don't have to use our products, but perhaps some of this information may be useful to you for your own purposes. We have expended millions of rounds testing various behaviors of the M16 platform, and tried many different ways to improve it. Our commercially available products are the results of this testing.

Quoted: Actually I would vote for BOTH reducing the gas pressure AND slowing the cycle by increased mass.

I have read several threads that suggest that both at once may lead to problems in cold weather (where I find myself a fair amount of time as I often spend winters in MN/WI).  Are you familiar with the posts I am referring to/can you refure this in the cold?  I don't mind it if your posts are self-interested since I just want the answer (regardless of who gets my $$$ in the long run).

Thanks for the continued input.
- Chris

In very cold weather, the pressures present in the cartridge case during firing may be somewhat lower than in hot weather. This will tend to reduce the muzzle velocity, and also reduce some of the gas pressure in the cycling system. On very weak loads, there may be some tendency to short stroke the system, but this is unlikely because the typical operating problem on a carbine or SBR is too much gas pressure, not too little, as I discussed previously.

What I would do with my gun, if this ever became a problem, is to slightly open up the adjustable valve to compensate for the lower pressure in cold conditions.
For those with standard non-adjustable gas systems, it would be relatively easy to just substitute a lighter buffer weight when going to a cold climate.
Additionally, many lubricants act differently in cold climates, and can cause drag on the cycling system when they become stiffer, due to cold temperature. Changing to lubricants that are more suitable to cold climates can help with that situation. I'm talking about below zero temperatures here, not  just a moderately cold day. I've shot my AR on 20 degree days with no difficulties at all.

If you don't regulate your gas system, it will generate more and more gas pressure,with the more rounds that have been shot down the barrel, due to port erosion. Thus, the more that barrel has been fired, the faster and harder the gun will cycle. This will make it very unlikely to short stroke in cold weather, but will make it very likely to batter in normal weather.
For your case, it would probably be advisable to have a couple of buffers to use in different weather conditions. I'd suggest a heavy 9mm buffer for summer weather, and maybe a normal H buffer or even a standard buffer for very cold weather. This is actually making an "adjustment"(to your cycling mass), and I know you don't said you don't like to do adjustments, but that is what it comes down to.

Or you can do what most people do, which is to do nothing, and take the battering of the gun as the port erodes, and ignore it until something breaks. If you only shoot semi-auto, it may go along for quite a while without breaking something, which is why some people think that there is nothing wrong, yet. Or you may start to get FTEx, or double feeds, and wonder why. But, at least now, you will know why, and can do something about it.

One of the causes of this erosion problem is that the barrels are chrome plated before the gas port is drilled, and there is no plating in the gas port hole. We are going to be making some barrels in the future which will have some improvements like a chrome plated gas port to resist this erosion, as well as some better barrel cooling features and lighter weight. We've done extensive torture tests on various barrel types, and have a lot of data on this subject. But our current focus in the near-term is to concentrate on the newly available Quick-Change-Barrel system, and the very-soon-to-come modular lower receiver system with interchangeable mag-wells.

Hmmm...  9mm buffer = tungsten buffer...correct?

Yes, 7 oz. is the heaviest tungsten 9mm buffer.

For reference, the MGI RRB buffer with plunger and tungsten sliding weights is also 7 oz.

In a carbine, with a normal AR15 carrier, and a fully open gas system, you should have no trouble at all running that 7 ounce buffer with XM193 or M855.

The gun will cycle slightly slower, but you won't really notice it on semi-auto.
It will still shoot as fast as you can pull the trigger.
Bolt-opening will be slightly retarded for better extraction reliablity.
You won't (shouldn't) have any bolt bounce.
A good extractor mod like the D-Fender or Wolf X-tra power spring wouldn't hurt either.

One last related question:

If the midlength gas tube/port is "ideal" for a 16" barrel as many have suggested, would the CAR-length gas tube/port be similarly ideal for a 14.5" barrel?