I'm learning the hard way about how AR's work and are assembled.
I have a M&P 10 rifle, bought an UBR stick off the ad listings, added an adjustable gas block on my next trip out. After fighting what I thought were gas issues, I've figured my buffer throw I'll call it is off after trying to also figure out why my bolt stopped locking back.
When I swapped my OEM adjustable stock out, I used the OEM buffer and spring. The UBR buffer tube is about .5" shorter than the stock tube. There is not an alternative length buffer tube available from Magpul. The stock buffer is 3.25" long.
I guess my choices are simply go back to the OEM stock or swap the buffer out, I saw one 2.5" long on Brownell's. Agree?