I have two new Bushmaster 11.5" bbls. What is happening is that the spent brass stays inside of the gun causing a double feed. The first thing I did was tried various types of ammo - South African, Argentinian Nato, Norinco "fireball", Winchester, Remmington. All seemed to give the same result. Next I read up on checking the gas system; nope, both were all buttoned up tight. I tried swaping out bolts, carriers, put in new extractors and springs. still same deal! I talked to Bushmaster and they said oil the heck out of it with teflon tri-flow for the break in period. I have both of these barrels up to about 300 rnds each now. Still inconsistent failure to eject. Next, as recommended I put in two new 4oz heavy MG buffers (standard Colt's are 3oz). Still a problem with a more noticable recoil. The theory behind this is to slow the action down because it may be slamming forward to quick. The barrels have the oversized gas port diameter from factory so I am assuming it is not being starved, perhaps too much gas? One other recommend is that I polish out the chambers. These are chromed lined barrels that are in the gray so I'm still a little paranoid of putting a 400 grit abrasive rag on a drill and having at it. I think my next step will be to invest in some form of gas tube modification: Tim LaFrance's twin tubes or the "Pig Tail". My question becomes,which will work best for my uppers? Another is: how do you know if the system is getting enough gas or too much. Is there a prefered test? Once more, I will state that I have checked for gas leaks throughout and including the keys. If anyone has any input for these gas mod sytems or if you think I am beating down the wrong path to solve this problem, I would apreciate your input greatly.

Try a Wolff extra power extractor spring first. [url]http://www.gunsprings.com/RifleShotgun/Colt_RsNF.html#M4Ext[/url]

Modify the cheapest part first and work your way up. [;)] Don't sweat what Wolff says about black inserts - the spring does all the work.

The bolt latch is nearly at the end of the bolt travel.  If the lower is locking open on the last round you have enough energy in the gas system.

11.5" barrels are sensitive.  There is not much time between the bullit passing the gas port and the bullit exiting the muzzle.  A longer barrel would allow lower gas pressure for a longer time to get the same amount of energy into the bolt.  The short 11.5" barrel requires higher pressure for a short time to cycle the bolt.

Too much gas or a sticky chamber will result in the extractor ripping through the rim and leaving the spent case in the chamber.

I would beef up the extractor with a heavy duty extractor spring or with a small rubber o-ring around the extractor spring and polish the chamber.  The idea here is to just polish the high spots and not make the chamber larger.  Be careful not to get into the neck area.  Some people put croqus cloth on a wooden dowel.  Some use poslishing compound like Flitz on a bore mop.  Generally they attach the rod to an electric drill.  Should only take a moment to polish the surface.

The pigtail or fat gas tubes are ment to spread out and reduce the peak pressure reaching the bolt.  They are expensive however.

Or, Bushmaster has great customer service.  If you send it back they will make it right.

Thanks for the tips guys. I had replaced the extractor springs with new DMPS stock replacements. I was not aware wolf had extra strong ones. Even when a case does manage to fly out it lands about two feet out compared to the six feet with my 16" sporter. In fact it is was with the same bolt and carrier. One of four bolt/carriers tried. I will try the Wolf ones - it won't hurt any. I will also try to polish out the chamber very carefully as recommended. Bushmaster even mentions that the type of chroming needs to be polished in but normaly occurs from the cases over the ware in period. As for the bolt travel test - yes it does lock open. In fact the gun almost has a feeling of hard shooting. What I mean is it really seems to slam pretty good. It could be due to the heavier buffer or the fact that they just shoot this way because it is an 11.5". I have been paying attention to the brass for tell tale marks and have noticed that on some the base seems to compress. Not a whole lot but you do notice a slight difference in the letters of the stamping and primer pocket. This only happens on certain brands. I know that the bolts are Colt factory spec for sure but I have not headspaced them to the Bushmaster 11.5" bbls. I know it's hard to judge without being there but it almost seems to be cycling too hard or to fast compared to my H bar or sporter. So I will take both of you up on your advice and apply it. I still would like to here from someone who has had anything to do with the earlier mentioned gas systems. It will be great to be able to get them to run it will be even better to get them to run as smooth as possible and at maximum reliability.[heavy]

Let me start off by saying that of all the barrels for the AR-15, I love the 11.5” barrel the most.  What you give up in FPS (around 400) is well worth the advantage of the short overall length of the rifle.  For clearing a room, it bets the MP-5 hands down due to smother action of the rifle in burst mood (yes, it does tend to over penetrate).   As far as the barrel and gas system, it’s the shortest of barrels that I have found that still allows short weapons to run well, and not be on the border line in regards to perfect auto sear timing and gas pulse (read you don’t need another type of gas tube for your rifle).

Now for the secret to the barrel,
1.It’s 11.5”, so forget about using it for long-range match shooting. The short barrel is meant for close up work. This means that if the chamber is slightly looser in the sidewalls (not head space), the barrel-pressurized cases are easier extracted from the chamber during firing.  On the factory short barrel (read colt), the headspace is the same, but the sidewalls are slightly wider/looser.  This means that the Colt barrel was reamed with a fatter reamer before being chromed, and not a standard Mil-SPEC Nato reamer used to chamber the 20" barrels.  Chances are that Bushmaster just used the same standard reamer that they use on their other barrels. The fact that it’s chrome plated means that you will no be able to open the chamber cross section up, but you will be able to polish the chamber to be smooth as a babies ass.  If you just using the weapon in semi only, polished is all that is needed to get a clean extracting, your not going to get the barrel hot enough to justify a looser chamber.  In regards of polishing, think 22cal nylon brush, a few rod sections, and spin a wad of brass wool on the brush.  If you keep it short, the brass wool will not eat away on the chrome plating, only shine it up and knock down any micro burs. Using sandpaper just causes grooves in the chamber.

2.With the shorter barrel, the gas pulse is higher than a Standard car barrel (less barrel after the gas tube for pressurization). This causes the action to open up with a greater force (read larger barrel gas port).  The solution to this is to use a Colt black insert extractor spring (black color inserts just means that the spring is stronger, the  both colored insert just keeps the spring from collapsing sideways/ side blowout), or a Wolf extra power extractor spring.  This allows the extractor to retain the case during extraction (read greater friction of the case to the chamber wall due to shorter gas dwell time/higher spike to unlock the system, and the fact that the action is opening up while the gun powder is still burning/pressure peaking, and throwing that lovely fire ball out the front of the rifle).


3. A heaver buffer is only needed to slow the cycle rate down in regards to bolt bounce on lock up in a full auto rifle. If you are using the rifle for semi auto shooting only, all you are going to feel is the extra weight of the buffer slamming against the back of the receiver extension.  Bottom line is that on a Semi auto rifle (223, not 9mm), the dead blow effect of the buffer only controls the direction of the spent casing of ejection due to the rearward stall of the buffer when it makes contact to the back of the receiver extension. In regards to slowing the unlocking action of the bolt, the weights of the buffer would need to be all the way forward (shooting downward at the ground) to have any effect of the bolt opening/unlocking.  Trust me, the first M-16’s had no weight in the buffer and they ran just fine.  The weights in the buffer were added just to slow the rifle rate down.  The heaver buffer (read 9 smg) is used to slow the action of a blow back system (mass/spring weight/hammer angle to the non-ramped carrier), since the bolt is never locked.

Bottom line is to spend time on polishing the tight/rough chamber, and then change the extractor spring out to solve your problems.  If the weapon is a full auto, then post back and I can walk you threw changing the sear timing to allow the weapon to perform without bolt bounce jamming (read light primmer strikes).


[url] http://www.ar15.com/forums/topic.html?b=3&f=14&t=153425[/url]

Thanks Dano523. Sounds as though you have a good degree of experience with the 11.5".

ref to point #1 - I agree, these systems are not going to be used for long-range-match. If all goes well they will play more of a PDW role (on steroids)once in service.

ref to point #2 - I also agree on getting a stronger spring. Till now I thought that replacing them with the DMPS (stock) ones would be good enough. I was not aware that a stronger one was available. I heard something about a "D-Ring." Are they any better than the Colt (Wolff) springs?

ref to point #3 - The heavier buffers were recommended by the Armorer at Bushmaster after the "oil the heck out of it" advice didn't produce results. In theory, I thought that in using the HMG buffer it may have some effect on the resulting net force of a gas overpressure in the system.[banghead]

In point one you mention that I should not have to use an alternate style of gas tube. What kind of system would use the above discussed (La France, PigTail)? I still have yet to polish the chambers - I will as recommended and will keep posted the results of these modifications. Your also saying that more attention is required for dialing in the gas system to the sear in a full auto? Please explain your method.

P.S. I couldn't get your site link to work that you supplied.

Thanks again.