Should this tank have the muzzel break on it and got left off from the makers
When you say makers, I'm not sure if you mean 21C or the manufacturers of the actual AFV..?? The M18 Hellcat was equipped with the 76mm M1 series tank gun which was also found on later models of the Sherman. The gun was designed to be used with a muzzle brake, but apparently some Hellcats didn't come off the assembly line (or at the depot..??) with one installed, in these cases the barrel, which had a threaded end, was fitted with a sleeve to protect the threads from damage, rust and dirt accumulation, etc. This sleeve presumably is what is represented on the 21C model of the Hellcat, and I have no idea why they (21C) didn't put a muzzle brake on. To the best of my knowledge there was no M18 "variant" that had a different nomenclature based on being equipped with or without a muzzle brake, but I don't have a really comprehensive reference on the M18 available to me to confirm this. For what it's worth, most pictures I've seen of real M18's show it equipped with a muzzle brake, but a significant number don't have it, but why that is, I have no clue. Here is a link to info on the gun..
http://en.wikipedia.org/wiki/76_mm_gun_M1
Here is a pic of the real tank pictured on the cover of the Squadron book, firing at a target in Wiesloch Germany in 1945..
http://en.wikipedia.org/wiki/Image:M18- ... 450401.jpg
Here is another page with some good info and pics of the M18...
http://afvdb.50megs.com/usa/m18hellcat.html
Gen. George S. Patton Jr., 28th Regimental Colonel, 3rd Armored Cavalry Regiment, U.S. Army, "Blood and Steel"