Why do the 9mm ARs use a longer buffer than a CAR or use quarters to make up the difference?
What will it hurt shooting a 9mm with a H3 or heavier CAR buffer?

Best I can tell, it's just to shorten the cycle stroke?  it needs less rearward travel to clear a 9mm case than a 223.  Doesn't seem to me that the longer stroke would damage anything, just slow the cycle rate?

I know the weight of the buffer is important in the 9mm since it's blow back operation instead of DI. But, if the buffers are the same weight just CAR vs 9mm length what damage could be done?

Someone that knows more can you enlighten me?

Thanks
SS33

to keep from breaking bolt hold open.

9x19mm vs 5.56x45mm.

The 9mm is a lot shorter.
If you don't limit bolt travel, you risk breaking bolt catches.

9mm bolt needs less distance to travel than a 5.56 bolt group.

If you leave the excess travel in there, you get excessive bolt velocity. Spring rate from a standard spring is too high for a 9mm already.

bolt carrier on both the 9mm and 223 is the same size. The travel would not over stress since it's cycling in the same carrier and buffer tube length.
The bolt carrier would cycle exactly the same in a 223 as a 9mm so I'm not seeing why there would be excessive force on any parts. ie the buffer pin or the bolt catch.

Only thing I can see is limiting the travel for quicker cycling.  The 9mm doesn't have a lighter spring. It works on blow back so you don't want a lighter spring that allows it to slam rearward too hard. That would cause more damage.

I originally thought that it was to protect parts like the buffer pin, but other than the extra weight from the heavier buffer and heavier spring the buffer pin would get the same pounding with 9mm and 223.

Am I missing something from the equation here?  The bolt carrier on both 9mm and 223 are the same length.  The only variable is the 9mm case vs 223 case and the blow back action vs DI.  Mechanical function pretty much is the same?  The 9mm brass can be ejected and cycled much shorter stroke than 223 brass.

I cycled a 9mm upper thru a 223 lower with the MEAN Endomag not changing the buffer out for 20 rounds  standard H3 CAR buffer.  That is what started me thinking about why the use of the 9mm buffer length and/or quarters or washers in the buffer tube is needed

The shorter stroke would limit the amount of inertia force created on the return stroke of the buffer and the heavier buffer that the 9mm tends to run could cause more force on the buffer pin.

9mm routinely break bolt catches.

You asked why the longer buffers.
They were developed to prevent breaking bolt catches.

The 9mm bolt is a LOT heavier than the 5.56. Combined with a heavy buffer, the force imparted to the bolt catch is significantly higher.
If you have full bolt travel, the spring compresses more, and imparts higher force.

The 5.56 bolt carrier group is longer in recoil with the bolt in the unlocked (extended) position.
The 9mm doesn't need that extra travel.

What are you referring to as the "buffer pin"?
If you mean the retainer, it is pushed out of the way when the upper is shut.

Bolt velocity is much higher causing buffer bounce back At a higher velocity, is my understanding because of blowback design.  Also why we any heavier buffers to slow this some

I use a hydraulic buffer to counteract this, along with felt recoil

The carrier on the .223 bolt and the bolt on the 9MM AR may be the same length - but the bolt catch on the .223 catches on the bolt, not the carrier.  On the 9MM AR the bolt catch catches the bolt (carrier length).

If bolt bolts travel into the buffer tube the same distance then the 9MM bolt move a greater distance from fully back to forward to the bolt catch - which allows it to accelerate to a higher speed vs. the .223 bolt and carrier assembly which makes for more impact/stress on the bolt catch.

I run a standard carbine buffer in one of my AR 9's and use a thick hard rubber spacer in the back of the buffer tube.  The other one runs a carbine H2 buffer with some steel washers in the back of the buffer tube.

The retainer pin was what I was talking about. I didn't know it gets depressed when you close the upper?
interesting. Never really checked that out.
Makes since now about the weight of the bolt carrier on the 9mm being heavier along with the heavier buffer.  I can see how it would pound the bolt catch especially since it catches in a different place than the 5.56.

Thanks for the info. I'm going to have to check out how the 2 catch on the bolt catch.  Never paid attention to that either.  I assumed the bolt catch caught the BC on 5.56, but never really investigated it.

Thanks