I ordered an action block yesterday, but just got to wondering will it work on a 9mm upper with the blass deflector in place?  Or will I need to remove/dremel some material out of the block to fit?

I just did a barrel swap from and A-1 9mm upper over to a flat top upper. I used the action block purchased from Bushmaster to do it. You do have to remove the 9mm brass deflector for the action block to fit. Most action blocks are not designed to accommodate 9mm uppers with big brass deflector. At first I was hesitant about removing the brass deflector, but once it was removed the action block fits perfectly. Reinstalling brass deflector, trap door, and spring was a simple affair that took about a minute. I wouldn’t bother using a dremel tool to remove material since the plastic on the block is very hard and it would probably take you longer to do it then to remove the brass deflector.

Sorry, I just asked the same question on the Build forum. What do you do to modify it? Do you just cut out a section that allows the deflector to pass through? Will it effect the block's use with a non 9mm upper?
Thanks!

I removed the door, spring and deflector and it was very easy.  Since I still don't have my action block in my hands, I re-installed everything to make sure I could do it and it was a breeze (it installed the same way it came out, from the rear).  Just be careful to not loose the C clip... it's really small.  I got a huge zip-lock freezer bag and put the whole upper in it and removed it inside the bag as to not loose the clip.

Thanks guys!

Don't use a Dremel.  All it will do is heat up the plastic and melt it into a mess.  Use a hack saw and plenty of elbow grease.

I removed the brass deflector and put on a regular door like a 5.56. No slinging of brass in my face.  It looks better this way to me.

Quoted: I removed the brass deflector and put on a regular door like a 5.56. No slinging of brass in my face.  It looks better this way to me.

That would look better.  Where does it throw the brass?