Well that makes it fairly clear a rational argument isn't going to be of any use here.
Which put simply is enough to end my involvement with this.
We have agreed to assume it's somehow scaled down or otherwise managed.
and there is a massive difference between 8 inches and 18 inches....
A 'massive' difference? Maybe. But certainly not for the substance of this argument. For comparison the M24 sniper rifle used by the USMC, a decent compairson to the Desert Strike is 43 inches long, i.e. over twice as long as your character, and probably weights more too. That's not saying anything with regards to the recoil (which is a bitch) or handling that SoB while
flying. So you being 18 inches tall instead of 8 is basically a distinction without a difference. The problem with scaling it down is that scaling it down to a manageable size for a pixie would basically erase everything that makes the Desert Strike useful, specifically range, armor penetration and stopping power. That's simply a function of the amount of space the rifle takes up, and short of magic (of the literal or technological variety) there's no way around it.
However, if it works at your table and your GM is ok with it, then by all means ignore all of us and have a fraggin blast with it. Sounds like a crazy/fun character for a high end pink Mohawk campaign.