I'd just use a cross of two polys (making an X shape from the front) with a third poly perpendicular across the back near the muzzle.
Put an alpha additive muzzle flash texture on the two crossed polys, as the flash would appear from the side, like this :
On the third poly put an additive texture of a round muzzle flash, as it would appear straight on, like this :
Then make it visible for a frame or two, probably using the alpha setting for smooth on/off transition. You can rotate it randomly down the barrel-axis also to make each frame appear different.
Put an alpha additive muzzle flash texture on the two crossed polys, as the flash would appear from the side, like this :
On the third poly put an additive texture of a round muzzle flash, as it would appear straight on, like this :
Then make it visible for a frame or two, probably using the alpha setting for smooth on/off transition. You can rotate it randomly down the barrel-axis also to make each frame appear different.