The barrel on your std A4 is either .850 or .950 under the handguard, depending on the barrel.
I put an M5 RAS on one of mine,and rather than modify thebarrel, I took a little material off the inside of the front end of the RAS. There was a diagram here on ARFCOM some time ago that actually showed the material to be removed...but I don't think I still have a printed copy of it. Once you have both components in hand, I think you'll be able to see what needs to be done. If not, let me know and I'll see what I can do for you.