I have been hearing a lot of people saying that PRI fatboys will work on anything, M4's included.

I had a brand new 14.5in barrel (Bushy 14.5HBAR W/std gas port), and I installed the fatboy on it, and it would not cycle, once in a while the cases would eject, but the bolt wouldn't go far enough back to strip a new round or lock to the rear on a empty mag.  If I attached a sound suppressor it would cycle 100%, but not at all without it (Bolt most of the time wouldn't even unlock).

(I thought, "well maybe my case was a fluke and I put the same PRI Fatboy on my new Dissipator barrel with a std gas port thinking that at least the extra 1.5inch of barrel should give it enough pressure to cycle.)

So went to the range today and the same thing happened, my $4000 rifle wouldn't cycle, even with a STD colt carrier, and the lightest buffer I had. (I thought the AAC buffer might contribute so I changed it out with a Colt H-buffer and then a std carbine buffer), if the BC came back at all it would only come back far enough for the case to eject.

I also assembled a pre-ban M4/KAC MRE for my brother and it is about a $2000 gun and cycled flawlessly (STD GAS TUBE).

The reason I posted this? Because maybe it could save you some money, fatboy tubes are meant for two things, 10.5in BARRELS WITH HORRIBLY OVERSIZED OR ERRODED GAS PORTS, AND CARBINES WITH SEMI-PERMANENTLY ATTACHED SUPPRESSORS.  If your cyclic rate hasn't topped 950-1000RPM, I reccomend you wait till it does or you will be simply practicing changing out your gas tube twice.  

Hmm... interesting.  I will be testing a combination on 2 uppers of Fatboy, LMT carrier and AAC rate reducer with M16 carrier, std gas tube and standard carbine buffer.  All these will be swapped back and forth with milspec M4 barrels and using .223 and 5.56 pressure loads.

Was your bolt/carrier/barrel/upper all broken in already?  Were you simply swapping the front sight and gas tube on a broken in rifle?

I could not get the fat boy to work on my 16" Bush Master either.

I'll give you $20 for it. [:)]

Odd I have a Colt M4A1 (RO921) upper that works with one.

and I have seen Diemaco C8's work with them.

what am i missing here? someone enlighten me please,,

the bushy 14.5" has a gas port that is .023" larger! doesn't this mean the bolt will see more gas than the colt 14.5"? wouldn't the fat boy tube help slow the gas pulse in that case?

now i have to go pull the front sight on my 11.5" bushy, and see which machinist bits fit best.

[b]Don't quote me on that my memory could be foggy, I made a point of measuring both ports but I didn't make a point of writing down the figures, The hole looked like 1/16inch to me, which equates to .0625 so maybe my numbers got confused, a simple call to BM and asking them what gas port size is standard on an M4 1/7 14.5 and Dissy 16in HBAR will give you the correct answer, they are both std production barrels.[/b]


It is simlply not getting enough gas, plain as that, there is no way a rifle with a .65in port would run with this tube. NO WAY. If the BC is going back halfway it isn't because the rifle is getting TOO much gas, on the contrary it is not getting enough.

Those colts probably had gas ports larger than .95in as they are military guns, dump a couple (5-7) mags in an M4 barrel in rapid succession and you are probably going to see gas port errosion, do it repeatedly and you will get a HUGE gas port. Remember the LMT 10.5s that won't run with a PRI fatboy? (.070in port.) do the math which is harder to work, a .010 difference in gas port size, or a tube with two or more times the interior volume. (that makes a .80in port function like a .30in port.)

[b]Oh by the way, what is a PRI fatboy?, a gas tube that is .090in ID for 7.5inches of it's length and .160ID for 2.5inches of it's length (those are ruler measured my calipers is missing)[/b]

(sorry I would have sold you the tube but (remember I am the guy with the PRI 4-screw post that won't come off) I couldn't remove this tube normally [it is heavier construction and won't slide back into the upper far enough to get it out that way] and had to cut it in about 5 pieces to get it out, and I didn't feel bad doing it either, this tube has caused two of my rifles not to run properly, and when you are sitting at the range next to a guy hammering away with an SKS and your AR is a bolt action it is embarrassing.

My brother was sitting next to me with his brand new BM 1/7 14.5 M4/w/KAC MRE and std tube (his gas port was the same size as mine and he was using a std tube and his rifle was functioning 100% even though just like mine it was brand new. Obviously too much gas doesn't cause the rifle to malfunction as easilly as too little gas.

By the way he fired 80 rds and I fired 25, before I got tired of pulling back the handle and sick of drawing attention to the fact that my rifle wasn't working (I fired 5 rds with my AAC buffer, 10 with and H-Buffer and 10 with a std buffer fired 2 types of ammo, Black Hills 52grain moly and Winchester 55grain) the 55grain cycled more slugishly than the black hills which most of the time would throw the bolt back at least far enough to eject the casing and let the bolt close on an empty chamber, it was only getting worse, as the rifle got a little dirty the thing started to not even open at all, like I was actually using NO gas tube- like the old Allen Zitta bolt action Ar's.

Last time I had this problem was about two months ago and I suspicioned the magazines and changed all the internals to brand new BM green followers and new springs. This magazine spring was on it's first compression.

[b]I have an order in with you (MSTN) hopefully you can get me a standard tube before deer season and I will let you guys know what happens.[/b]

For what it is worth a 1/16 inch drill bit fits perfectly in my Bushmaster 16" gas port.

HMMMMM, strange- I have a fatboy on a 14.5 M4 Bushmaster barrel and I can't get the thing to NOT work. It has worked 100% since the day it was installed.

Wes etc. the 11.5 with FAT BOY is 100% with suppressor and without, semi and full

pic

[img]http://photos.ar15.com/ImageGallery/Attachments/DownloadAttach.asp?iImageUnq=8094[/img]

[img]http://photos.ar15.com/ImageGallery/Attachments/DownloadAttach.asp?iImageUnq=8455[/img]

I looked but I dont think I have any shooting photo's of it with a can

[img]http://photos.ar15.com/ImageGallery/Attachments/DownloadAttach.asp?iImageUnq=19804[/img]

Same here. Since I put one on my 14.5 bushy. I run everything through it. Wolf, Barnul, Olympic, etc.

I am with JBSF3 and ptaylor on this one. Same thing, BM 14.5 and a fatboy installed and it WORKS 100% of the time and with less recoil. So I don't know what might be going on in your rig. But hey if you want to get rid of it let me know.

IPSC_GUY sends

I got 2 of the fatboy gas tubes from Wes, they are on 2 Bushmaster 11.5 uppers on M16 lowers.  Between the 2 we have put through about 4000 rounds. They run 100%. One runs about 750 rpm with the standard H buffer the other is a little slower with a SMG buffer.  We have used Federal XM193, Winchester, PMC, South African and Black Hills. Not a single malfunction.  Th e only down side to the fatboy is, once it's installed...it's forever (really tight fit)

"So I don't know what might be going on in your rig. But hey if you want to get rid of it let me know."

Good because I think I do, and I am the one who needs to fix it.

I have built 14 Ar's and the only 2 that didn't run had one thing in common, they both used this same gas tube.  

Anyway I'll know for sure Wednesday. Then I'll let you know.

If you are getting 750RPM with this gas tube you have a very erroded gas port.

i dont want to sound like a dick, and this isnt a flame...but this is a big "duh" if you understand gas guns and how they work this is a no brainer. you attempted to fix something that wasnt broke with the latest "cool" part. why did you buy a fat boy tube without experiencing the issues it is designed to remedy? the M4 has the same amount of barrel between the gas port and the muzzle insuring the propper "gas pressure over time" meaning the gas impulse lasts long enough to cycle the weapon. shorties dont have this distance thus the need for the fatboy tube.


why buy a fatboy for an M4 or an A2, is it just to say "hey guys i got a fatboy"

DvlDog,

I think Green0 was trying to get unsuppressed ROF with a suppressed gun - at least that is why I did it to my 14.5 guns

I know I will be shooting it with a suppressor some of the time, so it would be nice to have the gas tube that puts less stress on the system with the suppressor attached.

If it had worked without a suppressor I would have been happy.  (I would have had a gun with a smoother recoil impulse, lower ROF and a heavier duty gas tube that was very unlikely to burst in during automatic suppressed fire.) The gas tube is the weakest link in the ar-system.

Besides I am always trying to get my rifles to function smoother and with less recoil. (In a burst the added control is nice.)

[b]KevinB is about right though, simply adding a suppressor increases ROF by about 250RPM, if your ROF is allready 650-700 that means now it will be 950RPM- really fast, increasing heat, wasting ammo, and decreasing control. If this were a dedicated suppressed setup I would have had no problems, but it isn't.[/b]

I have a Fat boy on my bushy 14.5" 1in7 M4 barrel. It has a PRI front sight, and I use a Young engineering national match bolt/carrier. I only have 1000rnds through this setup, but it has worked without a flaw.

Allright, UPDATE.

I got the std gas tube in the mail from Paul at MSTN at 9:45AM yesterday, got home at 9:30PM and installed it, hit the range at 9:30 this morning and rough zeroed some stuff at 50yds.

The rifle was functioning but the bolt wasn't locking after the last round.  So I again removed my AAC buffer, replacing it with a std carbine buffer and it began to cycle fine and lock on the last round.  It runs good, and even though I was rushed and not comfortable (deer sight in with limited range time) I was able to shoot a couple 3/4MOA groups with the ACOG and Black Hills 52grain moly (the only BH's sold at Gander Mountain) - I think the rifle has a lot more potential but I will have to try some other ammo and try shooting better later.

[b]SO my deer rifle is now working thanks to MSTN and the emergency overnight shippment of the gas tube Thanks guys you are the only people in the business I would trust for that kind of service, even on a small non-cataloged item.[/b]

The DR Optic was good for about 1.5MOA and the irons were doing 1MOA for me.  

That Dr. Optic was a complicated zero (it was about 4 ft off target at 50yds when I started). Now it is about 6inches high at 100yds, with its 50yd zero.  

[b]It looks like (as it wasn't locking with the AAC buffer) the gun was obviously still just barely over the limit as far as getting enough gas.  Maybe BM's ports are the bare minimum diameter that will still cycle?

Anyway I'll bet the port will errode with use and then I can put the AAC buffer back in.



Anyway though, my experience for the record is 2 brand new BM barrels, 14.5 and 16in that both did not take well to the Fatboy (actually did not take at all).  I wouldn't try it again unless I was having bolt bounce, burning gas rings, or tubes were bursting on a regular basis (indicating way too much gas was getting to the system).  It looks like these were designed for guns with ROF of 950RPM + or suppressed Ar's.  They work great with suppressed guns though, a suppressed ar with Fatboy will cycle like it has no suppressor attached at all. Again a simple product  description from PRI would have helped clear the confusion that led to my using this in a brand new gun with a std gas port.[/b]

that black hills might be the problem.  I had failure to cycles with black hills ammo before out of one of my ar15s.  I made up some handloads with the same bullet, used mil spec ammoand never had issues again.  it seems some of their loads are a bit underpowered

I am aware of the fact that Black Hills ammo is underpowered by about 10%, but I also tested Wichester 55grain white box (maybe it is underpowered too)? and it didn't cycle either with the PRI tube and the lightest buffer I have- a std RRA buffer as comes with their stocks (seems about 60% the weight of colt's H-buffer.)

My test was limited to what I could find, Gander mountain and Walmart sell only light bullet loads for .223 (55grain was the heaviest load at either and I'ld rather shoot 52grain black hills than 55 grain UMC or Wichester soft points [the points will last about 1/4 inch untill they deform as they hit the ramps and my groups might just reflect the fact that the bullets have eccentric tips.]

I am now using my upper with that Carbine buffer and a std tube and it is running flawlessly.  I guess I've said enough though. If you have a brand new barrel and buy this tube for it you are looking for trouble, and you will probably have problems, I tried it on 2 brand new barrels and it would not cycle with either of them, std tubes however allowed both guns to run properly.

I am not hunting with Winchester white box so I didn't even bother trying to fire it with my aac buffer this time, I just was trying to get my gun to run with 52grain BH's and walla, std tube and std buffer did the trick. I suppose 62grain ammo loaded to mil-spec will probably allow me to re-install my AAC buffer (higher pressure maybe enough to give the extra gas it needs) but I don't have any components or the time to load the 62 grain ammo and won't pay the $190+ a 1000 that people want right now (which is totally obscene) so I guess that is something to try later.

I think with military components I can load this ammo for about $80-$90 a thousand and after I load it it will shoot way better than standard S&B or name your company 62grain ammo.

[b]I really wanted to believe this tube would work out, but three embarrassing, and depressing trips to the range with 2 guns is enough. If I ever use one again it will be with my CQBR 10,000rds from now.[/b]

Well I cant agree on that one

I have them on NIB guns both 11.5 and 14.5" ones never had troubles.  As well I know of people who have them on a plethora of 16 - 10" guns and they dont have troubles...

Interesting to be sure. It is my opinion that the fatboy is a step forward for shorter guns e.g. anything less then 20 inch. When The AR was designed the position of the gas port on the barrel was determined and placed where it is so that the dwell time of the gas coming back hits the bolt after the casing has finished it's expansion to the chamber walls and slightly rebounded. With carbines and such the extraction process is beginning while this expansion of the fired case is still going on. Hence all of the odd size gas ports with the first CAR 15s and later the M-4. In fact the M-4s higher failure rate of 168 in 66,000 rounds (most of which were failures to extract) was three times as high as the A-4s 62 in the same 66,000 rounds. This test was completed by the Marine Corps in it's evaluation on which rifle to replace the A-2. So again I say IMHO the fat boy is not a cool gadget but rather an evolutionary step forward. Look at all the fixes Colt has had to apply to the M-4 Heavy buffer, stronger extractor spring to improve reliability. These are symptom fixes, lengthen the dwell time in the gas tube and fix the problem. So In closing GreenO I am bettin you got tight gas ports and if ya wanna sell that sucker let me know ASAP cause i want it if you don't.

IPSC_GUY

IPSC_GUY- Appreciate the offer but I spent the better part of a year buying and saving for parts for this rifle, I don't want to sell it.  I have $3600 invested in it (the other $550 was the E-stock that I have removed and am hoping to sell.)

I like the gun, and it represents what I think is possibly the best compromise between a M4A1 and an SPR.  (if I had it to do over again though, I might have done it with a rifle length gas system and a KAC RAS mounted front sight (for easier access to the barrel and gas tube without removing the front sight).

I realize that if you get your gun to run with a Fatboy it should be more reliable, but if you can't, 168 failures in 66,000rds is better than 66,000 failures in 66,000rds.

[b]Maybe it was simply ammo, but all I currently have access to is factory ammo, if they are all loading it weak, I would have no way of knowing wether it was that or the tube.    

Or I could have gotten a defective tube. I guess that is possible.

At least I'm not the only one or this would be a total flame fest.[/b]

"I could not get the fat boy to work on my 16" Bush Master either."

Maybe they don't like BM barrels?

Hello
I meant buy the fat boy from you not the rifle. I have one more upper I want to build and it is going to be a short one so that is my reason for wanting another Fat boy. What exactly is the specs on your rifle top to bottom?

IPSC_GUY sends

Well here we go. :

BM 16in HBAR Fluted 1/9 chrome lined barrel
KAC M4 QD FH
PRI 4 screw front post
KAC low-profile gasblock
Colt Std gas tube
KAC FF RAS (rifle length)
KAC QD RAS mount swivel
KAC Vert grip (soon to be replaced with a tangodown)
ARMS 35A(I think) Bi-pod mount
Harris Std L series S leg notch bi-pod
Garrison A3 Upper (w/cut M4 ramps)
Les Baer Chrome Bolt w/Mcfarland gas ring
DPMS mil-spec chromed extractor
Smith enterprise Chrome carrier
DPMS-early armalite style takedown pin
PRI M84 Charging handle
KAC 600M rear sight
ACOG NSN
SPECWARGEAR DR. Optic mount (replaces rear back up on the ACOG)
Dr. Optic sight (7MOA dot)
OlyArms preban POS lower
Paulsons military supplylower completion kit (PMSWEBSTORE.COM)
JP Ent. single stage trigger.
Falcon Ind ERGO grip w/gapper
Vltor 5-pos tube with Colt mounting hardware
Colt buffer spring (Std M4)
RRA cheapo buffer (since my AAC isn't working right yet)
Crane Reproduction stock (I'm still hoping to sell the real one it replaces)
Tactical Tailor 3-point sling, modified to 2-point while still pretty much retaining the full function of the 3-point setup.

In a week or two I'll have a Surefire 951 SU04 (6v w/2 screws and pressure switch on it and then it will be done, untill I find a decent IR laser for cheap [US made not Russian])

WOW

[wow]

And I thought I had a lot sunk into my SPR which is thankfully 96% complete. Although I am now doing the "I need a Vltor or a Crane stock" bit. By the way that is telling that you are selling a real crane stock to replace it with one of the copies. Which extension tubes do the crane copies fit on Colt or everbody else? Also How did that whole off size tube bit occur anyway? Do you have any pictures of your set up? I would be curios to see some.

IPSC_GUY sends

P.S. if ya get a "hankerin" to let go of the fat boy gas tube let me know.

GreenO,

16" barrel, Rifle length FF RAS

So this was a Bushmaster factory Disipator?  And you swapped the front iron sight?  So the fatboy is in the chopped gas block and not in the front PRI?  Have you pulled and reinstalled the first bas block?  Have you measured the gas port size?

Greeno

Your rifle sounds almost exactly like mine!  Very versatile set isn't it?

Anyway, might I suggest you try getting rid of the Mcfarland gas ring.  I found it to be the source of may short stroking problems I was having.  I know it is supposed to give a tighter gas seal and therefor stop short stroking.  However, It is such a tight fit that it binds and slows the bolt carrier's rearward travel down (at least that's my guess as to what happens).  Anyway after I went back to mil spec rings my rifle functioned much more reliably.

Kris

No kiidin...
I would bet that could be a possible culprit. Greeno try it and see...

IPSC_GUY sends

That damned Mcfarland gas ring will cause malfs with the fatboy no matter what setup you use. My upper would barely run suppressed with that thing and a standard buffer installed, but it runs great with a Colt "X" buffer and standard rings with or without the can.

-Cap'n

My McFarland is worked in, and smooth operating. If the gas seal is not as good that could have been an issue, but I always thought they said it should be better with no gaps.

Oh well it runs now.  I might try a Mcfarland upgrade on my LMT 10.5 that is the gas ring that Crane used on CQBR's. If that stops running I'll know why.

I have a McFarland ring on a M4A1 - I noticed several FTE's using a certain sort of ammo - but using M855 or M193 type ammo or the Mk262 Mod0 and 1 it has never hicupped.

The bolt came that way - I would not bother adding it to guns that run fine without.