M&P 10 Carbine Receiver Extension...

[30 cal slut]

...is a smidge longer than a milspec m4 receiver extension.

anyone try swapping the factory tube out with a milspec one?

[Clint]

What is a smidge?
3/4"?

I haven't handled the MP10, but it probably uses the longer AR10/A5 length RE.

If the buffer length is the same ~3.25" as the AR15 uses, the RE must be longer to produce the travel needed for the 308 cartridge.

Why are you looking to swap it out?

[Joe R.]

What Clint said. The gun runs fine with the stock receiver extension. After market mil-spec stocks fit fine, they just don't look like they collapse fully since the tube is longer.