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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.
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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.