If you look at the bottom of a bolt carrier you will notice that there is a "ramp" from the firing pin to the bottom of the carrier. That is the surface that cocks the hammer. Since there is a "ramp" the hammmer is cocked gradually. On a 9mm bolt (since there is no gas system, the whole unit is the bolt), there is no "ramp". It is pretty much a 90 degree angle. A 9mm hammer is designed with a little different geometry to be easier on the hammer pin. The 90 degree angle was used by Colt to slow the rate of fire in a full auto submachine gun. With a complete bolt costing $150 and up depending on the brand, I wouldn't try to duplicate the angle of a rifle carrier with a Dremel tool. There are a couple of venders that can ramp the bolt for you. I had Joe Black at http://www.m60joe.com do two bolts for me. I also heard that Lee Minor at http://www.lrmfirearms.com/pages/507919/index.htm does bolt ramping. You can buy the a new Colt bolt already ramped with a tunsten weight instead of steel which SAW Sales calls "BOLT ASSY, 9MM SMG - IMPROVED MODIFIED COCKING ANGLE, TUNGSTEN WEIGHT" with a selling price of $395.
The question to ask yourself is how much effort do you want to do to switch from 9mm to 5.56? I have a Colt M16 RR. I'm not firmiliar with the semi auto 9mm hammer. If you ramp the bolt you don't have to change the hammer in your lower. To have someone ramp your bolt it could cost $50-100. A 9mm hammer for standard (.154") trigger pin is $15 from RRA + S&H. Maybe some else knows if a 9mm hammer will work with a 5.56 upper. If it doesn't then you have to change hammers when changing calibers.
For that matter the buffer. When I got my first 9mm converion upper, I was changing the buffer from an "H" buffer to the heavier 9mm buffer. The 9mm buffer was too heavy for my 5.56 uppers. The 9mm buffer slowed my 5.56 carrier so it would eject the spent round and strip a new round. But the carrier wouldn't go back quite far enough to cock the hammer. I bought a MGI rate reducing buffer. It makes all my caliber conversions run. It also smoothed out the recoil. Sight recovery with the MGI buffer was quicker even in semi.
Unless you buy a post ban barrel, everthing comes with a flash supressor if you buy a complete barrel. Since Colt was the one to come up with the 9mm set up they decided to have their 9mm barrels threaded to a different pitch so an armorer wouldn't thread a 5.56 NATO flash suppressor onto a 9mm barrel. The 5.56 has a smaller opening in the flash suppressor. The standard 5.56 thread is 1/2X28. Colt started using 1/2X36 for their 9mm barrel threads. Since Rock River Arms and Grarrison Manufacturing are Colt clones they use the same pitch. I'm sorry but I don't know what any other manufacurers use for a thread pitch. Since a 9X19 round has around 400 ft/lbs of energy and a 5.56X45 has around 1,100 ft/lbs of enery from a 16" barrels, muzzle rise should not be a big deal with the 9mm in semi auto.
One thing you might want to concider. Colt is the only manufacturer that I know of that makes a chromed lined barrel. Colt has "The" name, so you pay more. If you plan to shoot 9mm a lot (9X19 is much cheaper than .223 Remington) or you aren't so good about cleaning, I would recommend looking for a Colt barrel. If you are not going to use it much AND you are very good about cleaning and lubing then any one of the clones will be fine.
It is my understanding that a steel barrel in 5.56 is good for 8-10,000 rds. A chromed lined barrel is good for 20-25,000rds. 9mm moves at around 1/3 the velousity of 5.56. That would make for less wear. It is still a chunk of lead with some copper around it that slides on a surface of steel or chrome. The chrome is much harder. So it will last much longer. I think the bigger issue for most shooters is corrostion. I have to admit that a lot of the time it takes a while for me to clean my uppers after I have used them. Everything I have is chromed lined so I don't worry about it. Once you have a rust pit it will never go away. I'm not a big fan of Colt. I feel their stuff is over priced. They do make a good product but you pay "through the nose" for it. I would rather spend $250 for a barrel that I don't have to worry about than spend $175 on one that I know won't last half as long and I have to make sure it is lubed so it won't rust. This is my $.02 worth. Good luck with your build.
Scott