I would think you could fire it after conversion, so long as you did it right. The vents on the barrel shroud can be covered, but you'd want metal parts not plastic or else they might melt if you threw a lot of lead downrange. The only other issue is the magazine, but I'm certain with a little know-how and some precision welding one could cut the mag down and still make it function, albeit a few dozen rounds shy.
And removing the extra barrel length would be illegal in the US without a special license from BATF. The extra length was added specifically to meet the requirements of our federal law, which maintains any 'rifle' or 'shotgun' must have a minimum length of 16 inches from chamber to tip, otherwise it becomes a 'weapon of mass destruction.'