My son has a S&W M&P10 that came with a carbine type adjustable stock. He pulled that off and installed a fixed stock using a standard rifle receiver extension, rifle length buffer and spring.

Come to find out that doesn't work because of the length of the M&P's BCG combined with the rifle length buffer, the action couldn't go far enough back to lock the bolt open. Switching to a carbine length buffer took care of that but I'm not sure if that was the correct or only change needed? I guess my question is this....

On a M&P10, if you want to run a fixed A1 length stock, what is the correct combination of receiver ext, buffer and spring and what should I be aiming for buffer weight wise (Standard, H, H2, etc.)?

This what he started with (16" barrel)...


But he's also replaced the gas block with a lo-pro GB and a longer mlok rail (don't know which brand for either of those parts?)

Is he using a AR-15 rifle length buffer? Those are around 5.9" long, a AR-10/LR-308 rifle buffer is around 5.2" long and weigh in the 5.4oz range. Springs are also AR-10/LR308 specific, but there are exceptions.
ETA: make sure he's using the short A1 screw and not a A2 screw.

At first he tried to use an AR-15 rifle length buffer but that was too long and didn't work. He now has installed...

AR-15 rifle length receiver extension (OAL ~10")
AR-15 carbine length buffer (3.25")
AR-15 rifle length buffer spring (~13" long)
The stock is a Colt "CS" A1 length stock using the proper length screw.

Using the carbine buffer with the rest of the parts listed took care of the BHO issue but from the research I've done, that buffer is too short for this setup? The M&P10 uses a lot of proprietary parts so you can't just go by generic "AR-10/LR308" parts. It appears the correct way to install this stock is to use an AR-15 rifle extension and then reuse the M&P10 factory spring and buffer that came with the gun plus the S&W A2 spacer which is ~2" long but I would love to hear from anybody who has an M&P10 with a fixed stock and what you did to get it to work properly? I would rather just order a proper length buffer for this setup but can't seem to get a straight answer as to what the proper length is?

SO NOT SHOOT YOUR RIFLE UNTIL YOU FIX THIS!

It is imperative that you use the correct length buffer, otherwise the bolt carrier gas key will impact the rear of the lower receiver, causing permanent damage and possible injury.

In your case, the rifle originally comes with an AR10 / A5 length receiver extension. With a 308 caliber rifle, this is designed to work with a standard length AR15 carbine buffer (not A5). When installing the fixed, rifle length receiver extension, you will either need to have a longer 308 rifle buffer (5.25") and 308 rifle length spring, or a spacer in the back of the receiver extension behind the original spring and buffer.

Spacer (to allow use of the original spring and buffer)
https://www.midwestgunworks.com/page/mgwi/prod/432270000


AR10 rifle buffer (5.25")
https://www.brownells.com/rifle-parts/receiver-parts/recoil-parts/recoil-buffers/308-ar-buffer-prod128046.aspx
You'll also need an AR10 rifle length spring.