On top is a brand-spanking-new DSArms SA58 Voyager with a 16" barrel.
On bottom is Jasmine, a FNified SA58 Standard with a 21" barrel.
There are many detail differences, some even apparent in this photo.
Losing 5" of barrel and using aluminum for the lower and top cover saves over a pound of weight (10.9 vs 9.8 loaded).
In GURPS terms, though the 16" barrel costs you damage and range. 6d pi and 800/3,500 for the 16" barrel vs 7d and 1,000/4,200 for the 21" barrel. For the record, 18" gives 7d-1 pi and 900/3,900 (no word on weight).