I currently own a registered IMI Model B Uzi and had a registered IMI Model A that I sold.
I don't know where you guys are getting your information. I could be wrong but I'm pretty sure there were NEVER any semi-auto OPEN bolt uzi's imported in the US. Refer to the older Uzi article in the May 1998 issue of Small Arms Review which states this as well.
BOTH the Model A and B have a bolt blocking rail welded to the inside of receiver preventing you from just dropping in a SMG bolt. ConfederateTexan is incorrect, you can't just drop a SMG bolt in an Uzi and change the other parts to get it to run FA. You would have to either remove the bolt blocking rail or mill a slot in a SMG bolt. Which either operation would be seen as creating a MG in the eyes of the ATF.
Read Bardwell's links below that detail Uzi MG configurations addressed to the ATF.
www-2.cs.cmu.edu/afs/cs/user/wbardwel/public/nfalist/atf_letter46.txtwww-2.cs.cmu.edu/afs/cs/user/wbardwel/public/nfalist/atf_letter61.txtShaggy, you are incorrect regarding the trigger group pin sizes. BOTH the model A and B have the same trigger group pin hole sizes which is 9mm. A factory Uzi SMG is 8mm.
Now back to M4's original question on the differences:
On the model A, some say that the bolt blocking rail was not welded onto the receiver as "solidly" as it is on the Model B.
The Model A has the same front and rear sights that are used on the full size Uzi SMG which are calibrated for the shorter 10" SMG barrel. The Model B uses the newer style sights which are calibrated for the longer 16" barrel but note that they work great - just need to raise front sight up higher due to short barrel (I only use my 10" barrel since mine is a registered full auto). Also note that IMI no longer makes the full size Uzi for their own use (some are still in service of course but they are not making new ones for themselves - I know someone that works for IMI). But they do however still produce and deploy the MINI and Micro which both use the Model B style sites. The Model A sights are outdated.
Another difference is that the front sling swivel on the model B rotates a full 360 degrees while the model A does not (which is like the original SMG)
The bolt assemblies are different between the A and B:
The model B has a firing pin safety which is activated by a trip which hits the ejector to allow the firing pin to go forward.
The model A does not have this safety. So it doesn't have this trip in its bolt and in result also using a different collar that surrounds the firing pin.
Also VERY early model A's had fully supporting rim/lip on the bottom of the bolt face. Which would allow you to go FA with a FA trigger group installed. Later bolts had did not have this lip. If you notice the FACTORY FA MINI (currently IMI offers the MINI in CLOSED and OPEN bolt and the MICRO in ClOSED bolt only) and MICRO Uzi CLOSED and OPEN bolts all have this lip.
There are some registered receiver Model A's out there that were converted this way by using these rare bolts.