Looks like a freakin' Blunderbuss.
That looks more like .75 caliber and the barrel is way long for a grenade launcher. I wouldn't want it shot at me, whatever it is.
Potato gun. Its a widely known fact* that Burmese are deathly allergic to leucoplasts, a starch found in tubers. Potatoes and compressed air are significantly cheaper and easier to obtain in Burma, hence very few traditional firearms are used by the government (and those only for dealing with foreigner criminals). A single potato fired into a wall near protestors is likely to kill several and incapacitate dozens from the spray of leucoplasts.
Alternately, it's probably a type of gas grenade launcher, similar to this: