A police friend of mine just purchased a 10.5  A2 upper for his issue M16. The 20 in upper runs fine and the 10.5 fires fine and will eject like it should but it will not pick up another round from the magazine. It is as if it is either cycling so fast that the mag doesn't have time to push another round up, or the carrier isn't coming back far enough to pick up another round but, far enough to eject. He is wanting to clip a couple of coils off of the action spring. My solution is to open up the gas port going to the gas tube. What do you guys think? How exactly do you open up the port? Who is right? To clip or not to clip is the question.  I don't think that clipping is the answer since it runs fine with the 20 in. Oh and by the way his lower is using the collapsible stock and we have tried several mags. Thanks...321

I would say to purchase a pig tail gas tube and slow the action down a little: http://www.brownells.com/aspx/NS/store/ProductDetail.aspx?p=1591&title=AR-15%2fM16+PIGTAIL+GAS+TUBE

I think a "Pigtail" gas tube may help your problem... hopefully someone who has had a similar problem will jump in.

Check out: [url]http://www,m16pigtail.com[/url]

Sorry, its: [url]http://www.m16pigtail.com[/url]

Thank you Rag, I have thought of this myself, but he isn't into spending too much cash on this thing. At the time he thought he got a heck of a deal on it. Might have been a deal if it worked. ;-) Also I didn't recommend him clipping coils off of a department issued lower. I don't think that would go over very well with the higher brass, not to mention possibly alter the reliability of his current 20 in set up.. Anybody else?

Don't clip coils, The only shorty M16s I could not ever get working had Pigtails, never again I say, plus to use the pigtail you gotta open the gas port anyway.

If you put one rnd in the magazine and shoot it if the bolt does not lock open on empty mag it's short stroking and you have a gas problem.

Now this could be bad rings, loose carrier key. Once you decide it's not the bolt group then the front site must be removed by driving out the taper pins and opening the gas hole one number bit at a time, reassemble and try till it locks open on empty magazine.

David, now that sounds like a great idea. I had never thought of trying the single round and checking to see if the bolt catch engages. I haven't bee around when he fired it, I have just been listening to him cry about it over the phone. I also didn't ask him if the bolt was staying back after he shot it or not. I will relay this info to him tonight. What would you reccomend to use to open the port? Thanks guys you know your stuff....321

Do the single roud in the mag test, as a short stroker will come back far enough to toss the empty, but not enough to pick-up the next round.

Typical 10" gas port will run .093". Bits of varying size available at tool supply or tool truck. Ace Hardware used to have number and letter bits in small flip top case at store. Just work you way up a few thousandths at a time, and test fire each time. May even pick-up a #60 o-ring in the plumbing dept, put it around the extractor spring.

Use the std gas tube, and he really ought to go to a 11.5" barrel, less hassle.

If the ROF is too fast you might want to go with a hydraulic buffer to slow it down. If its short stroking drill the gas port and go with a PRI Fatboy from MSTN on the Equip. Exchange board. You wont have to pull the barrel apart like using the pigtail.

Ok, my friend is requesting that I drill the gas port. He seems confident enough in my abilities but I am not as confident as he is. So my question now is. If I was to accidentally drill it too much, what effect will that have on functioning? Should one error on the not enough side as too much? Mistakes happen and God knows that I am entirely capable of performing many more. More info would be greatly appreciated.. Thanks again..321

You really don't want to over drill. Like I said take it apart and go 1 number drill size at a time and re-assemble and test each time till the bolt locks open.


If the gas hole is too much then the recoil will be increased, the rate of fire will climb, and the it will most likly have extraction issues as too much gas will try and extract early before the chamber pressure drops enough to release the case.

David, Now just to clarify and to get me on the exact track. I am to enlarge the actual hole in the barrel under the front sight base and not the hole in the sight base itself. Is this right? My friend says that just by looking, the barrel hole and the sight base hole seem to be about equal. So my hesitation is that by having a larger hole in the barrel than the sight base then I would think that I would get gas bypass under the base and leaking out under the sight base into thin air. Does this make sense or do I just need to get some assorted size bits and get drilling. :-)

what do you mean by pull the barrel apart? I am confused.,

The o-ring is a new one on me also, is this to help the extraxtor grip the case better with the expatation of a bolt carrier moveing faster with the 10.5 than it would on a 20 in? An o-ring made of rubber in a hot upper just doesn't seem like a good idea to me.

The O ring thing using the right rubber improves extraction for those who are having problems.

Olympic sells their O ring for $1.49 each.
They also sell their one piece Ti alloy gas ring for $7.00.  This can be helpful for M16's with gas problems.

321Bang,

Before you do anything, step back and take an overall look at the problem

To start off, there is very little barrel after the gas port on the 10.5-barrel.  This means that that once the bullet passes the port, there is very little time for the barrel to pressurize the gas tube before the bullet leaves the barrel and is lost out the muzzle.  Because of this, the barrel gas port is enlarged to allow more pressure to flow into the gas tube and unlock the bolt.  The problem is that the enlarged port produces very high action working pressure that has a high/quick spike and when the bolt unlocks, it does so very violently.  Since now the bolt is unlocking while the barrel is still under high pressure, the spent case is still under pressure and the case is still pressure bonded to the chamber wall. This is the reason that the extractor slips off the case rim on the unlock cycle, and the use of an extra strong extractor spring or D-fender is used.

Now that you have the bolt unlocking, getting the case out of the chamber, and the bolt/carrier full stroking on a A-2 butt stock rifle you are set.
But if the rifle is a CAR, you have just run into the next problem, which is the bolt carriers speed is way faster than normal and the standard Car buffer is having a hard time controlling the stroke. Instead of the buffer stalling at the rear of the receiver extension tube, it is deflected and the return stroke is faster than normal. This  leads to the bolt bouncing open on lock, due to the buffer not working as a dead blow device, and the rifle will not run in full auto mode due to the hammer striking the firing pin with the bolt partially open.

The way that a larger volume gas tube works is that is softens the gas spike and since the pressure wave is away from the gas port, the tube will stay pressurized after the bullet leaves the barrel, and not fully drain back out the port. The stronger extractor force, applied by the stronger spring or D-fender, allows the extractor to pull the pressure bounded case out out/off the chamber walls.  The heaver buffer allows the faster carrier speed to be controlled and lessens bolt bounce at lock.

But still keep in mind that due to the lack of barrel past the gas port, there is very little operating/pressure margin error.  What is needed is the added barrel length after the gas port, or a device that will create the backpressure needed, hence a suppressor or moderator.

Bottom line is that unless you want the brain damage that comes with tuning the rifle to the 10.5” barrel and the cost of parts, just have him get a longer barrel and be done with it.

P.S. If the 10.5 barrel is a commercial type, the chamber will need to be enlarged. The standard Nato chamber will not work, due to being too tight in the cross section of the chamber walls.

Dano, that is a GREAT explanation of what occurs with shorty uppers. Mind if I save it for a future FAQ?

321, to answer your other question: Yes, it is easy to melt an "O" ring. Shorty uppers get much hotter than longer-barreled ones because of the increased gas required for them to run properly. So you need to be much more conservative when shooting them, particularly if you like mag dumps.
I have an 11.5" upper and when shooting a mix of two-or-three-round bursts and partial-mag dumps, I switch to another upper after two to three 30-round mags, four mags at most. More than that and you're risking barrel damage (as well as looking at a melted "O" ring).
The key to shooting shorties is to have several uppers and rotate them so they will have time to cool. Or just have a bunch of MGs. Hey, nobody said NFA was a cheap sport. [:O]

Danno, I have to @gree with Tony. You are gifted in the explanation department. You guys have been a big help. Well if worse comes to worse we can still have fun with his normal upper.
I, as well as my friend think the 10.5 is, well, for lack of a better term "sexy" but I am getting the feeling about like a sexy woman. Fun but a lot of times not worth the hassle. ;-) I'll keep you guys posted.. Thank you for your replies....321

321Bang
If you want a couple o-rings (made of Viton) which resist heat, oil etc... send me your address and I'll pop a few in the mail for free. I bought a bag of them last year and they worked great for me.

Tony,
It's all yours, use it as you see fit.

Dano

I have done a 200 round dump with suppressor and the Olympic eXrings did not melt...

FWIW

I have been told that a brake as opposed to a flash hider can help to increase dwell time and improve function.  Any thoughts dano?

Sounds like a good excuse to buy a suppressor. [:D]

Instead of trying the expensive hydraulic buffer, you could try a 9mm "heavy" buffer first...
The 9mm buffer is made from steel instead of aluminum...
(The 9mm AR's function via blowback and have no gas system at all)



FWIW...
The little O-ring needed under the extractor can be bought at most any hobby store...
Look for O-rings that go inside oil-filled shocks used on RC cars & trucks...
The "dime-a-dozen" shock O-rings are made of the same blue silicone as the $13 D-fender...

Using a brake, like that of a Branson type, is not the way you want to go on a 5.56 shorty.  First off, 5.56 shortys throw a hell of a fireball. When you redirect the flash up, you will vapor mirage your sight plane so badly that your going to lose sight picture around the second round (heat vapor white out). Another problem is that the muzzle will be pushed down, which will require you to fight the rifle/muzzle back up.

If you go with big port chambered brake, then most of the backpressure is lost out the ports, with very little back pressuring the barrel for any real length of time.

On a FBI MP-5, they use a brake to control the muzzle climb. The design is 45* off center to each side, with very little backpressure to the barrel.  Also, since the weapon is a sub caliber, there is much less muzzle flash to vapor mirage the target than on a 5.56 weapon that has a fireball for 2' long, and 1'wide coming out the muzzle.

It is the gas port hole. Start at .08 but you have to use a stronger extractor spring. Wolf has it. The next problem is that the cyclic rate will go up to about 1,000 rpm. Reduce the cyclic rate by a heavy buffer from Bushmaster or other rrs.
There are two that I know that are reducing the rate through gas tube, La France and the Pig tail. The reason why they do this is to slow the movement of the bolt by putting a longer gas tube effect so that it will not extract quickly leaving an empty case in the chamber causing a failure to extract and slow the cycle a little bit. This procedure of reducing the rate by gas is sensitive to ammo.

have you tried using a different brand of ammo?  Try using simple sollutions before making permanent ajustment to your baby.

I changed my Colt M16A1 RR from its stock configuration to a 10.5" Colt M4 upper.  The upper was unfired and the barrel is stamped M4.  It has the feed ramp cuts, but it has no step down, I was told it was a "machine gun upper" by the dealer who sold me it with the A1. At the same time I put on the old style two position CAR stock also Colt.  

No change in rate of fire was noticed, or any timing problems in semi, or full auto with 55 gr. FMJ military surplus ammunition any mag works just fine runs like a clock.

Why is all of this necessary on some rifles while others require no attention at all?  

It would seem that perhaps there is another reason that isn't so complicated.  It is unlikely that these 10.5" uppers were not tested before leaving the factory.  

Big_B,
When a short MG barrel is built, the chamber sidewalls are reamed a bit oversize to allow the chamber bound case to be ejected.  The problem on some after market barrels is that a standard size 5.56 reamer is used, and the case sticks. If the barrel is not chromed lined, you can polish the chamber to open it up a bit, but if it’s chrome lined, you have to attack the problem a different way.

I'd love to see a comparaison pic with the "Fat Boy".
What is special with the M4 Colt bolt carrier groups.
Thanks

Thanks.
I'm not sure I understand the MPC part (I thought this metal check applied to barrels). The std Colt bolt was upgraded to perform better (longer life) in an M4 due to higher pressure. Perhaps this is what it's about.
In you 11'5" uppers, do you use the Fat Boy as standard? And, are your barrel's chrome lined?
Would your set up be OK with the AAC rate reducer (for M4 tele stock)? I'd like to have "as slow a rate" as possible.
Thanks

I am getting kind of sick of people complaining about how  someone (Wes in particular) types.I see alot of information and well informed opinons being disseminated by Wes .Who cares if he types in all caps or lower case or swahili for that matter?As long as his continues to try and share what he has learned in a convival manner and you don't have any qualms with the actual meaning of what he is saying who cares?

Wes,

How much would one of your 10.5" barrels cost me?
Do I have a choice of getting non-chrome lined?

Danno does a great job of explaining EXACTLY what happened to my rifle, the bolt carrier thrust was so violent the plastic tip of the CAR buffer was getting broken up, resulting  in chips of plastic
getting  floating in the buffer tube. There was so many chips of plastic in my buffer tube that it hardly function on semi. I have a 11.5" now and a heavy bolt carrier and it functions fine.....Chad

The "MPC" mark on the barel refers to some sort of Magnetic Particle Treatment or testing. I read about it in one of the books from Paladin Press, but it does not bother to explain what the process is exactly. Anyway, in regards to the 10.5 inch barrel, try using a "D-fender" to raise the spring tension on the extractor first.  I've come across a number of shorties that had the same problem, and all have so far been made to cycle this way. Good luck!

The barrel is electronically magnetized, and then iron powder/flake is added to the barrel.  If there is a crack, you will be able to see it in the added iron due to the crack breaking the current flow.


Here is an engine head that has been magnufluxed to find a crack.
[img]http://www.merkurxr4ti.com/ref/magnaflux/mag1.jpg[/img]

hello people i am new here and to the AR world i baught my first AR-15 A2 about 3 weeks ago i went with an armalite for my first rifle so i just wanted to say hey.

I've found a simple inexpensive solution to cycling problems in SBR's...

1) Tungsten filled buffer assy from DPMS @ $59.95... [url]www.dpmsinc.com[/url]

2) HD Extractor spring from Wolff Gunsprings @ $20 per 10... [url]www.gunsprings.com[/url]

3) D-fender O-ring at $13 from [url]www.fulton-armory.com[/url] or a whole bag of silicone O-rings for $3 from a local hobby shop.

These simple mods were done right on the shooting bench, and made my 7.5" upper run 100%...

No other mods were done to the gun...
And this was with an el-cheapo barrel from Model-1 Sales...

Simple, simple, simple...

Onething I have seen is that on some of the cheep uppers the gas tube is a bit small and gas escapes from around it, Look for carbon on the outside of the tude were it goes into the sight base. How to fix is another thing. I have tried to spread the tube just a little with a flaring tool (not much luck). You can always change front sight base with a quality one. Find out were he puchased it from, and try to send it back!

As we all know the problem lies with the short barrel.  The shorter the barrel the faster the cycling rate.  This makes the bolt run faster than the magazine can feed.  I have a Bushmaster XM15 with an 11.5 inch barrel.  I have yet,,Knock on wood,, to have any problems with it on full auto.  It has the heavy weighted buffer.  I suggest that you try the heavy buffer before you go to the expense and time to change the front sight assembly, gas block and gas tube.  You will need this any way if you wish to keep the barrel at 10 inches.

F Y I
I cut a 16" 7.62 x 39 barrel down to 11.5. A few failure to feeds per mag. The original gas port in the barrel was .082" I opened it up to .089 and it runs awesome. What a flame thrower....

[img]http://users.zoominternet.net/~picplace/M16A1762x39.JPG[/img]

Have you tested your Crane/LMT 10.5uppers with the new LMT SOPMOD II B/BC?  Did they run reliably?

Was there any percievable benefit?

Do you know of any SOCOM use of alternatives to M4 RAS (like the FF RAS you have on your 10.5?)

I have been in the proccess of building a CQBR copy (but I want to keep it authentic and really don't see any benefit to using a M4 RAS if I can use a superior system and retain authenticity (I know it might sound retarded but this gun is 60% desire/need to have and 40% "cool factor".)

Thanks                   -Austin

I had the privilege of working with one small group of "high speed boyz" in the sandbox area recently.  I can say first hand that they loved their LMT 10.5's with RIS and Aimpoint despite the abuse that goes along with being an operator and not a collector.  

They did not have ANY problem with accuracy or reliability despite the "handicap" of not having a free floated bbl or fancy "improved" bolt.  For any Hooyah groupies curious what they were running:  It was LMT10.5's and regular old Colt 14.5 M4s with RIS, KAC sound suppressors (sometimes), Colt bolts, old style m4 stocks and Aimpoints.  Even the "snipers" favored RIS m4's with a1 stocks and ACOGs.  Again PLENTY accurate, even out of the helo.

Buying the best is fun, but don't feel shortchanged with last months gear.  Are you really too good for it?

Pardon the slightly off topic rant folks.  I'm writing to myself here.  I'm as obsessed as anyone even after seeing that the best of the best don't fret over these silly details like we do!  They're too busy getting the job done.

Have you tried a heavier buffer (Colt H) if it's a tele stock or Oly arms makes one for the A2 stocks. I had the same problem with my short upper and the buffer solved it...