If you have a working 9mm LL in safe-semi-auto, please post here.  

I would really appreciate ALL details on your system.  

I have one and it's not 100%.  I get strings of auto, but not 100%.  I get light strikes every so often.

As a tangent, my 556 and my 7.62x39 set up is working very well.  

1.  3 inch barrel, bought from DDLES.  
2.  JP bolt (obviously milled for the LL)
3.  JP silent spring gen 1, unmodified.
4.  SWR 9mm suppressor.  Always on.
5.  Modified Colt 3 round burst kit.  Lower modified to fit with the LL.  I'm not having any issue with fitment of the LL.  

6.   Now this is where it gets shady.    We could not get this to run at all.  Switched the firing pin to a titanium pin (custom cut down) and a weaker firing pin spring and got it to work.  

I can get it to work well, but it has to be real wet to run.  Not spotless, but pretty clean.  

I'm open to advice.  Even if that means ditching the JP components.   But, but, I really like the JP spring, it's so nice.

Does it shoot fine without the link installed?

When it doesn't go bang in full auto with the link installed, does the trigger reset when you let go of it or did the hammer fall already?

Mine worked fine last time I used that config, though I've been running with a DIAS instead the last ten years or so.

When it stops in full auto, I let go of the trigger and I have pulled it again.  Nothing happens, like the hammer has already fell.  

I put weapon on safe and eject a live round with a light dimple.  

If I lube it generously then it works fine.  It just seems like it's working on the edge of reliability.   The lube is making up for something not being proper.

At least in a DI system (not blowback) that is the classic symptom of bolt bounce.  What weight is the JP buffer?

Another possibility is that the paddle is just a hair too thick, making the hammer drop prematurely.

It already is a lighter weight bolt, since it lacks a core (the channel for the silent spring).   On top of that it's milled for the LL.   So it's lighter.  

Works in semi auto.

I can try a thinner paddle.  

Would a longer barrel be any use?  At all?

When you very slowly return the bolt into battery with the trigger depressed, does it drop the hammer?  If not, then your paddle is probably okay and it's bolt bounce.

Barrel length is likely not going to change anything.

In semi position and auto position the hammer does NOT drop when bolt is returned to battery.  Actually doesn't even happen if I return the bolt normally, only in semi.  I think because it's a modified 3 round burst kit.  Hammer drops in auto setting with bolt returning to battery quickly.  

If it is bolt bounce.  Do I need a stronger spring?

Sorry, I should have explained it better...

Make sure the chamber is empty and the gun is pointed in a safe direction.  With the selector on "AUTO" and the trigger pulled (and held down), pull the charging handle back to allow the bolt to reset the hammer on the disconnector.  Now, very slowly return the bolt back into battery using the charging handle.  The hammer should not drop.  If it does, then you may have too thick of a paddle, causing the hammer release too early and hit the bolt instead of the firing pin. In my experience, the best paddle thickness is one that does not drop the hammer with the bolt/BCG slowly returned into battery, but does when it is allowed to 'slam' shut with the force of the action spring.  In fact, just an inch or two pulling the charging handle back and allowing it to slam home and drop the hammer is ideal.  These were the instructions posted on Quarterbore's website and I have found them to be extremely beneficial.

If you pass the above test, then I think it is bolt bounce.  I'd start by ditching the JP silent spring system and use a dedicated 9mm heavy buffer with known performance in full auto setups.

v/r,
Esox

No Sir.  Your original post was quite clear.  

The paddle thickness is not the issue.  It is the JP silent spring.  That is unfortunate since I do like the spring.  It really is a quiet / silent spring.  However, I need it to function.

I have another 9mm lower (semi) that I can loot the buffer and spring from and then do more testing.  

It is unfortunate about the JP SCS.  On many levels.

Sounds like bolt bounce. I would drop the JP setup and move to a traditional reciprocating weight buffer and spring setup.

Some of the echo users were also experiencing bolt bounce with the JP scs setup.

Tony.  I have the second generation bolt with a first generation spring.  

At this point I've had the JP bolt machined to accept the LL.  So there is that.  I'll keep it, but what use will it be?

I don't think the second gen spring will be much different.   I'll ask, but I don't have high hopes.  

Tony, the PAWS receiver I bought from you is still great.

If your 3rd burst modified / RLL select fire fire control group works well with other uppers than the FCG group should be good.

My guess is that you are experiencing one of the issues below and/or a combination of both.

1.Bolt Carrier Bounce

The 9MM system is much more prone to bolt bounce in my experience that a DI upper.   There are two primary reasons for this:

A. The DI rotating and locking bolt assembly has gas rings which provide a friction fit between the carrier and the bolt.  When the carrier slams home against the barrel extension the gas rings provide friction against the carrier opening.   This is one of the reasons why folks report uppers that ran well at first start to experience bolt bounce as both the gas rings wear and the gas port opens up over time.

B. The DI system also slows the carrier speed right before lockup as the bolt carrier pin hits the angled ramp within the carrier.   As the carrier pin rides the angled ramp and twists the bolt, energy is robbed from the system slowing the carrier closing speed.   Slower carrier speed = less energy for the carrier to bounce open.  This is also incidentally why 9mm systems are harder on lighting links as the carrier speed right before lockup is higher than a DI/rotating bolt system. (resulting in a harder jerk to the link)

I don’t own a JP silent capture spring buffer but I don’t see the silent capture springs having the same reciprocating mass that a normal heavy buffer would.  The weights on the rod appears to always spring loaded forward, also appear to be steel vs. tungsten...so I suspect their ability to impart a delayed “dead blow” right as the action is closing is much reduced compared to say an Colt X or H3 buffer with three completely free reciprocating tungsten weights.


2.To impart another winkle factor with the 9MM system is that the 9MM bolt carrier can be subject to a phenomenon as “firing spring bounce”

The 9MM bolt has a spring loaded firing pin which is unable to provide direct ignition to the primer.    

With a standard 5.56 bolt carrier assembly the firing pin is able to make direct contact to the primer when it its full forward position with the hammer down.   As example when the bolt is in lockup and the hammer is down the firing pin will be protruding from the bolt face.    

In contrast on the 9MM system,  it utilizes a shorter firing pin that when in full bolt forward/lockup and the hammer down, the firing pin does not protrude through the bolt face.    What this effectively means is that when the hammer drops, it strikes the firing pin sending it forward down the firing pin channel (compressing the firing pin spring) and there is a small period of time when the firing pin is no longer in contact with the hammer but has not yet protruded from the bolt face.   The firing pin continues forward (via inertia) and eventually strikes the primer igniting it.  This is a similar arrangement as many handguns and provides a level of safety with softer handgun primers.

So what does this inertia firing pin arrangement have to do with the symptoms you are experiencing.

A.  Due to this inertia firing arrangement the firing pin requires a much more solid strike vs. a DI system.  So any level of bolt bounce that results in an off-center firing pin strike can result in not enough energy imparted to the firing pin to ignite the primer.

B.  Given that the firing pin is not a direct ignition arrangement like a DI/5.56 upper this means that upon the bolt closing, what is hold the firing pin in the rear of the bolt carrier (so it gets a good hammer strike) is the firing pin spring.  

When the 9MM bolt carrier slams home the firing pin also wants to drive forward as well (compressing the spring).  If the firing pin spring is not strong enough to hold the firing pin rearward, the bolt carrier slams home, the firing pin moves forward compressing the spring, the hammer drops and strikes the bolt carrier with a firing pin that is now “floating” forward in the middle of the bolt carrier firing pin channel.  This results in a hammer down with no primer strike or light primer strike.  This isn’t an issue in semi auto as the spring returns the pin to the rears position before the trigger is pulled.

I have found that a lot of 9MM firing pin springs are not strong enough for full auto fire out of the gate and/or don’t last very long to support full auto fire as the spring fatigues.   The only spring I have found that are guaranteed to support proper full auto fire are the Colt made springs.  Second place would be the Rock River Arms firing pin springs.

To resolve your issue/s I would suggest purchasing  “Colt X – 9MM Submachinegun” buffer and a “Colt 9MM / AR6951 9MM”  firing pin spring.    I suspect that replacing these two parts will resolve your issues.

The other option is to make a custom 9MM firing pin that allows for either direct primer ignition or doesn’t have as much free travel within the firing pin channel in the bolt carrier assembly.

I personally run a LRM integrally suppressed 9MM upper with a RLL and modded 3rd burst FCG in combination with a Colt X buffer, RRA bolt carrier with a Colt 9MM spring, and a custom longer firing pin that almost allows direct primer ignition (reducing it free float time in the carrier and has less reliance on the firing pin spring).

This setup works 100% for me for years although I had similar issues as yourself that had to be debugged over time.  

Hope this helps.

The custom titanium firing pin may be helping as it doesnt have the same mass/inertia that the steel units do, so you get less firing pin bounce due to a  weak firing pin spring.  

However, you may now be fighting an inertia battle as you may have solved the Firing Pin bounce issue with the lighter titanium firing pin....but now your firing pin doesn't have the same mass/inertia when shooting forward down the firing pin channel in the bolt carrier after being struck by the hammer.  In order to overcome the mass/inertia issue of your lighter firing pin you now have to use a weaken firing pin spring so that it doesn't rob energy from the lighter pin.   The lighter firing pin spring then leads you in  a vicious cycle back to firing pin spring bounce and not getting good hammer contact.    

Depending upon the lube you are adding to get it to run the lube may be adding friction/"sticksion" inside the firing pin channel helping to keep the light titanium firing pin from moving as easily and minimizing  bouce.... or.... maybe conversely helping it move forward easier overcoming its inertia deficit.

I would go back to a factory steel firing pin, Colt spec firing pin spring, and a normal spring and heavy style buffer as you have a lot of custom parts potentially fighting each other.

With this in mind, swap in a Spikes ST-T2 or heavier and give it a go.  

Jbntex.  Thank you for taking the time to type all of it out and explain it.  I really appreciate the time and effort.  

I agree with too many custom parts.  I need to get rid of that stuff.  I originally had a semi only 9mm SBR and the JP system (spring and bolt) worked well.  As you can assume, that system wasn't cheap.  When the LL was finally transferred to me, I wished to use the best (most expensive) bolt and spring I had.  That's why the JP system was used with the LL.   I went that route after I contacted JP and confirmed reliability in full auto (not necessarily with a link).  

I have noticed that many of the 9mm bolts have firing pins that are dissimilar.   Does that translate to the firing pin springs?   Does the Colt spring work with most (if not all) 9mm bolts?   Where do you suggest I look for one?  

As far as buffers go.  In my possession I have a Spikes 9mm buffer (still in package) and a Kynshot 9mm buffer (still in the package).  

I have a multitude of Sprinco springs including a 308 spring.  

What buffer spring and buffer would you try out first (of the ones that I already have).   If I'm not mistaken, the Colt buffer is 5.5 ounces and has a dead blow like feel.  The Kynshot has a dead blow like feel, being hydraulic, but I forget the weight.   The Spikes is 8 ounces (realistically less) without a dead blow.  

Sometimes I think I should ditch all of it and just be happy with my other 9mm automatics.

I would skip the Spikes, you need the reciprocating mass.  I would probably drop a standard carbine spring in along with the Colt 9mm buffer.

I would agree with jagufrost in that you need a buffer with reciprocating mass to prevent bolt bounce.

I have a kynshot and it doesnt do much in terms of bolt bounce as they are really light overall and other than the fluid sloshing around a bit or maybe the body slightly compressing on the piston.

Many of the semi-auto 9MM buffers are solid to just put as much weight behind the carrier to slow down the recoil stroke and minimize trigger slap or busted FCG pins.  In semi auto bolt bounce is a non-issue.

Do you have a H3 buffer in any of your other guns?   The Colt X is basically an H3(3 tungsten weights)  with a heavy steel body and weighs in at  ~8 ozs.  You can definately feel the heft and weight sliding into the buffer body.  If you slam an X buffer into the palm of your hand you will feel a noticeable double "thwack" as the buffer hits your palm followed by the tungsten weights slamming forward.

As for the Colt firing pin springs they should work in any of the colt pattern carriers out there.  I personally have only used Colt and RRA carriers and the design and parts are pretty much interchangeable.

The 9MM system doesnt require any special recoil springs so a standard Sprinco "white" spring should be just fine.

If you want to test/debug via a less expensive route you can get a fresh RRA firing pin spring (to use with the standard steel firing pin) for $2 and a generic H3 buffer for $30.  Kak industries also has some extended steel bodied 9MM tungsten weight buffers for half the price of the Colt X.

Another good 9mm buffer is a VLTOR A5 buffer.  They're longer than standard buffers which helps with broken bolt catches.  They also have four reciprocating weights, so they can have more reciprocating mass than standard buffers.  An A5H4 has four tungsten weights and weighs 6.8oz.

http://www.rogtac.com/vltor-a5-a5h4-6-83oz-buffer/

Can the weight of the bolt and buffer be viewed as a total?  Meaning if I have a lighter bolt, I should try to add weight to the buffer?

Why that question is important to me.  I have the JP bolt.  It doesn't have a central metal core (it never did), to allow it to work with the JP silent spring.  The JP bolt has already been milled for a LL, and fine tuned for my lower, my link and my upper (as far as a perfect fit goes).  The JP bolt weighs less because it doesn't have the central core and it has been milled for the LL.

I would like to continue to use the bolt just for that cost and time saving.

If that is reasonably true then I am leaning toward the KAK tunable 9mm buffer kit.

1.  Get new RRA / Colt firing pin springs
2.  Get a buffer
3.  Use a standard Sprinco spring.

Yes the bolt and buffer total weight should be taken in total.   I am not familiar with the JP bolt but if it doesn’t have the steel core pinned in like a colt pattern bolt it sounds like its going to be a bit lightweight.

In 9MM AR15/M16s higher carrier/buffer weights are your friend as the system is pretty much pure blowback with no advantage of advanced primer ignition like an open bolt  gun has.  That’s why colt pins a chunk of steel into the bolt/carrier, has a series of steel bodied buffers to up the total mass, and developed stainless steel FCG pins.

The total weight of the lightweight bolt and lighter weight JP buffer means a faster bolt recoil speed.   (less mass acting against what is essentially a direct blowback action).   Faster carrier recoil speed means a faster return stroke and the carrier and buffer “bounce” off the back of the buffer tube/receiver extension.    

The faster the bolt slams into the barrel extension on the return stroke the more bolt bounce you will have to counter-act.  You will also be more susceptible to busting hammer pins (and trigger pins to a lesser extent)

One of the reasons I use a Colt X in my setup is because of the lost weight due to the carrier being milled for a link.  Normally a Colt X is for suppressed and/or hot 9MM ammo.  The Colt B buffer is the next step down in weight for more standard 9MM full auto usage.   However, the Colt X makes up for the lost weight in the overall system due to the milling required on the bolt for use with a RLL.

The KAK tunable kits are nice, I have bought many parts from them to make custom buffers and also to take standard carbine buffers (that are pretty much worthless for full auto) and convert them to H, H2s, or H3s by swapping in the KAK tungsten weights.   Given it sounds like you have lost a lot of weight on the JP bolt between it not having a steel insert and being milled back it may make sense to go with the Kak steel bodied 9MM buffer that can be configured up to 10+ oz of weight.

I ran a Rock River bolt and pin spring for a couple years until I got firing pin bounce and then went through a maddening set of troubleshooting to resolve the symptoms you are experiencing.   Its intuitive to clip or shorten the firing pin spring because you think its stealing energy from the firing pin and causing light strikes.   The more you weaken the spring the worse the light primer symptoms get.  I have stuck with Colt springs since then and don’t think I have worn one out yet if memory serves correctly.  However, the Colt springs are definitely hit/miss regarding availability.

Thanks.  That's what I will do.  Great help.  Thanks again.  

One more favor.  If you can please weight your bolt and buffer that you use with the LL.  

That will be a great place for me to start.

I will try and get you the weight/s this weekend.

*Edited to add weights.

Complete bolt assembly (cut for RLL)
14.4 oz

Attached File


Colt X Buffer
8 oz


Attached File


Bolt and Buffer
1.65 lbs / 22.5 oz


Attached File


LRM M16/9

Attached File