Would I encounter any major problems by shortening a 50 BMG barrel to 16" and installing the 6" brake?  I know that accuracy and velocity would suffer, but this would be just a "fun" rifle.  Looking for the "cool" effect.

Shoulder dislocation comes to mind, other than that..............................................

I believe that shortening the barrel would reduce felt recoil, as there would be more powder burned outside of the barrel.  Therefore, velocity and energy would be reduced.  I have experienced with sawed-off shotguns.

It may be a stretch on my part, but sawed off shotguns and .50 cal rifles are a little different.  The amount of weight reduction due to trimming the barrel will likely be significant and felt recoil will increase with a lighter rifle.

As an example, I've shot an M82A1 and recoil was stout.  The rifle weighs 28.5 lb, a good portion of that being the 29" barrel.  Now take about half of that barrel length and weight off, while shooting the same round.  Your felt recoil will be more, likely significantly more.  This effect would be worse with a bolt action .50.

Your shoulder and your rifle.  Have a ball.

Why are you wanting to shorten your .50 barrel in the first place?  Of course, accuracy and energy would suffer, from the shortened barrel.  My main concern and the key to being able to shoot the .50 BMG weapon from the shoulder, without breaking your collar bone, is the the Muzzle Brake. here
I don't know what muzzle brake you are thinking of using, but the fact that you referred to it as a '6" brake', kinda concerned me.  Save yourself a lot of pain and an emergency room bill, and use a pre-manufactured brake, designed for the .50 BMG rifles.  If you don't like the Aramlite design (which with the lighter rifle, I would go with the best brake available, especially if my shoulder was on the line here), there are several other designs you can look into, like the "shark gill" design, that will make the recoil managable.However

The main problem with cutting the barrel down is the taper of the barrel.  Normally the muzzle is the narrowest part of the barrel, with the barrel increasing in diameter as it progresses back towards the chamber (I have seen some .50's with bull barrels on them, that appear to have  little if any taper at all).  The problem you may find is that depending on how drastic the taper is, your muzzle brake may not fit the shortened barrel, unless you turn down the new tip, to accomidate the brake.  If you don't have access to a lathe, I suggest getting a machine shop to turn down the barrel for you, as you want don't want to get the brake off center, otherwise you could end up with the bullet striking the brake.  I hope you aren't planning to just hacksaw the barrel off.  Again, if you are not a machinist, I would have a machine shop shorten the barrel so it will be neat and even.  It might cost you $30 to $50 bucks to have it done, but the end result will be worth it.  I have seen very short .50's, so other than the fitting of the muzzle brake, there shouldn't be any other problems with the shortening of barrel.  If I am not mistaken, there was a production .50 BMG rifle out that sported a 16" or shorter barrel.  It was a Maadi-Griffin, only with a very short barrel www.maadigriffin.com.  I saw one being packed around at the Knob Creek National Machine Gun Shoot and Gun Show, about 2 or 3 years ago.

One other problem you might fine, is with a shortened barrel, you are going to be a lot closer to the muzzle and muzzle brake, which means you will probably take a beating from the back blast (pressure wave) from the muzzle brake, each time you fire it.  You will have a lot of unburned propellant existing the brake as well, which could make it somewhat painful

Hope this helps!

Be careful and have a blast! (no pun intended)

Thanks guys.  You have successfully scared the heebeejeebies out of me.  I am definitely reconsidering it now.

Sorry, didn't mean to scare you, just didn't want you to get hurt.  Check out the Maadi-Griffin link I put in, and it will show you their short version of their .50's.  It is very short, and I haven't heard of anyone dying from shooting it.  You might even contact them about the "shorty" version and ask them what the recoil is like with it.  Your theory about a large amount of the propellant being expelled before it can burn and thereby reducing the recoil, might have some merit.  You will still be awfully close to the muzzle, and with all that unburned propellant gets outside the barrle before it ignites, you should have one hell of a fireball... up close and personal!

Besides... broken bones and flash burns will heal... eventuallyhe

Famous last words:
"Hey guys, check this out!"

But seriously Maadi-Griffin hasn't produced a rifle or parts in years.
You can still buy one of the plan books though.

Yes, I am aware of Maadi-Griffin's production status, but I am telling you, I saw the shorty version of they weapon at the Knob Creek, KY Machinegun Shoot either 2 or 3 years ago.  I merely put the link to their site so you could see it... it is in the scrolling slide show.  I do have a plan book from them, that I bought about 5 years ago.  I just haven't had time to build one yet.  I will be using an Armalite muzzle brake on it, should I put one together.  Maadi-Griffin ran into some BATF problems that ended up closing them down.

I can't find the pic but I saw a nice chopped M82A1, barrel was shortened but I forgot how much.

I saw a short barrelled .50 that incorporated titanium in the design but saw that every time the guy fired it, the backblast really pounded him.  If he wore a hat, it would be blown off with every shot.

Depending on the powder, it can exhibit a reduction in recoil.  Snubby .44's and 45-70 pistols are suprisingly mild as not all the powder is burned.

Why cut it down?  Because you can.  Different is always cool.