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Hi, can anyone tell if a .300 BLK 16 inch barrel, with pistol length gas system would be compatible with the 5.56 M&P 16" carbine lower? Would different length gas system be ok with existing buffer weight ?
 

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Buffer weight would have to be changed along with some fitting .
I presume the same would be true when swapping a 7.62x39 upper onto a 5.56 lower?
So a heavier buffer spring and "plunger" (not sure what the correct name is) in the buffer tube?
What kind of other "fitting" would need to be done?
 

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The recoil buffer would be the primary change. Both it's weight and the spring tension / wire weight / design would be involved. I'd look to the upper manufacturer to provide advice.

When you switch uppers on an AR-15 platform, you must be aware of ATF regulations for short barrel rifles and for the difference between AR-15 lower receivers marked as rifles and those marked as pistols.

Since your S&W lower was shipped as and marked as a rifle, make sure that whatever you put on the top continues to qualify the firearm as a rifle. If it becomes a pistol or an AOW as a result of the change, it would likely be an illegal combination. Again, turn to the upper's manufacturer for their confirmation that it remains legal on a rifle lower receiver.
 

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Hi, can anyone tell if a .300 BLK 16 inch barrel, with pistol length gas system would be compatible with the 5.56 M&P 16" carbine lower? Would different length gas system be ok with existing buffer weight ?
Yes and Yes. Standard carbine buffer is fine.

In particular, the pistol length gas system with a 16in barrel is best for shooting subsonic ammo. If you do much switching between subs and full velocity ammo you might want to consider and adjustable gas block. Enjoy your new upper.
 

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I have watched a slo-mo video of my 7.62x39 upper on a standard PSA lower. Running the 3.0 standard AR buffer I got no bolt bounce. H2 buffers are a good thing with that round. I have a 6.1oz buffer in my 7.62x39 lower. I understand a standard .308 buffer spring might be an improvement too. I have some that I can try bought specifically for that purpose. I keep forgetting to try them.

If you are shopping for a x39 upper I whole heartedly recommend getting a midlength gas system. I have 3 x39 uppers. 2 16" and a 10.5" all middies. They soften the recoil nicely.
 
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