So I have a 20" A3 upper in gov't profile that I like, I just don't use it much since it is so long to my tasks.  To mix it up a little, I was thinking about getting it cut down to make a dissipator upper.  My concern was the gas timing with such as short amount of barrel after the gas port.  I starting researching and it looks like many of the 16" dissi's have a carbine or mid-length gas system and a non-functioning FSB to keep the timing good.

So my questions are these?
-Are there gas timing problems with a 16" barrel and a rifle length gas system?
-If there is a problem with 16", what's the shortest I could go to maintain 100% reliability?

Yes, you could open up the gas port to make it work.
The problem is the really short dwell time that the bullet is still trapping the gas between the gas port and the end of the barrel.
Everything has to be happening during that last two inches which means it has to happen fast, and that makes it happen hard.
You can make it work, but it won't be 'right', and it wont be as much fun, or as reliable over as broad a range of conditions as a proper set-up would be.
1.) Keep the upper as is, get another as a dissy.




2.) Get a dissy, and sell this upper.
I recommend #1.
ETA: Nothing is 100%.  As you get closer to the ragged edge - your reliability drops off quickly.  It'll start being less reliable as the temp goes down, and as it gets fouled and dry while you're shooting it.  Different brands and types of ammo wont all work as they should with predictable results.
Add all those factors together, and they end up giving you a quirky weapon.





 

How about if I had it cut to 18"?  I'm assuming this would work as SPR's and such are 18".  I'm just having the itch to do something unique and fun with it.

It's been done. IIRC the builder said he had to open the gas port to 0.10" (yes one tenth of an inch) but it worked great and was very soft shooting. Go for it. If you decide on 18" you'll have no issues at all. Rifle length is the best gas system length for an 18" barrel anyway. Maybe have to open up the port one step, but I don't think so.

I've got a 20" 5.45 with rifle gas that I was thinking about cutting to 18 as well.  My FFL - who has a small machine shop - said he'd be happy to do it for me if I really wanted to, but he'd do everything he could to talk me out of it first.



He talked me out of it.



You lose 2 inches.  That's not really enough to make a big difference in 'looks', and you lose muzzle velocity.



By the time you disassemble it, cut it, thread it, crown it, ream the port a few thousandths, and reassemble it, it'd be close enough to just buying another barrel.



And by the time you have another barrel - you're half way to a 2nd upper anyway.  

I 'm doing the same tihing with a BM heavy weight post ban 20 inch barrel.  Ihad it cut to 16.5 inches.  I found a few threads on the net that mentioned enlarging the port to about .10 and I think that is what I did (its been a while).  My build is from used parts I had.  Only thing I need is a bolt and lower parts kit.  I hope it works out for you.

i've done three 20" barrels cut down to 16.5, and enlarged the gas hole with a 1/16" drill bit, if i remember correctly.  i only shoot brass, but havent had any problems with any of them.  i also don't rely on them as my SHTF gun, and can't say i've shot them much in the cold.    i'm actually working on another one now, but i do the home brew setup of using the bench bandsaw, a brownells target crowning tool, and a drill bit to do all the work.

Thanks for all the replies.  I think I decided to leave it as is.  18" versus 20" isn't that noticeable anyways and I don't want to take a gamble with enlarging the gas port size.  I think I would be better off putting the money towards something else.  I just wish there were more options for the butt plates on A2 stocks.

Adco did mine.

Softest AR I ever fired.