gun choice is so personal... if you're not completely constrained by budget then you can find just about every famous gun from the last 30 years. With an emphasis on m4/16 variants and AKs.
It's all up to what you like. Remember most guns short or long can and will shoot roughly the same distances as long it's of reasonable quality. (ie not walmart junk) So barrel length is not as much a concern for accuracy. Just consider size of battery pack (and where it goes on the gun), availability of magazines and ease of carry of those magazines.