I was wondering, what the velocity of a .45 ACP would be in a AR15 16" barrel. I know the 230gr. FMJ`s velocity is about 850 fps in a 4" to 5" barrel. I don`t think it will be much faster in the 16" barrel, but I would like to know how much faster. Thank you for any help that you can give me.

Hah! Something that I can answer... I have built a bunch of .45 ACP Rifles. Keep in mind, the .45 is designed to be used in 5" barrels and rarely anything over 7" -- I have found, by actually cutting off a bit at a time, that anything over 10" is detremental, especially in blowback designs.

The big problem is that the powder has fully burned off by the time the bullet is only 8" or so down the bore... the friction of the barrel actually starts to decrease the bullet speed in longer barrels... Or, the case starts to extract too soon.

I have seen factory loads get up to 1200+ from a carbine with a 10" barrel. Some people buy those cheesy 5.5" flash hiders and open them up to use on the .45 barrels, not a bad choice I guess for a non NFA weapon.

The .45 in a rifle supresses very well and makes a nice compact rifle -- one of my favorite rifles is my wife's chopped Marlin Camp Carbine... Short, light and bewry bewry quiet.

Dean Grennel talks about .45 carbines in his book as well...

I'll get back to you on this one. I'm looking for a cheap chrony to find the answer to this very question.
FWIW, I shot 2 rounds of Texas ammo 230gr 45 Super thru my Oly 11.5" bbl 45 ACP this last Thursday.
Shooting steel plates at 50yds, the "ding" occured a LOT sooner than with 230gr Fiocchi ball. Not a very scientific method of measuring velocity, but it was the best I could do.
The 45 Super was noticeably hotter than the Fiocchi ball ammo. I can't wait to see what it does to a whitetail this fall.  

If you're a handloader, you can do much better than anything you'll get in a factory load, by using slower burning powders.

Now it's been over 20 years since I sold off my M1 Thompson, but recall using 2400 powder, 240 grain Kieth style lead bullets, and getting 1200+ fps out of a 10 1/2 inch barrel  Check the loading manuals for .45 Auto Rim data---especially some of the really old manuals from the 1970s and 80s.  Recent .45AR loading data has been totally pussified; at least the last charts I looked at a couple of years ago.  Handloading with a slower burning powder will let you use the extra inches of your barrel productively [:D]

In my 16-1/2" .445 SuperMag Contender Carbine barrel, .44 specials (near equivalent to a .45ACP) sound like they're pulling a vaccuum by the time they exit. Sort of a "hollow-pipe" sound with the low report.

One cautionary note about load data from ‘old’ handloading manuals.

Powder makers make subtle changes in powder through the years but retain the old name for consumer loyalty.

Thus the 2400 of circa 1930, is not the same 2400 you buy today!!!

Workup slowly and watch for pressure signs. And keep in mind that if you are shot in the chest with a 230gr .45 will 50 or 100fps really make any difference in your life expectancy???