Suggesting the SAR or AR18 were weapons of choice is somewhat shortsighted. As noted, the Somalis had a variety of arms. What you were seeing were weapons of availability more so than weapons of choice.
The warlords often supplied weapons, ammo, food, and kot (sp?) to anyone willing to serve under them. The warlords supplied whatever was available to them via trade, sales, or capture. So there was hardly any sort of weapons standardization going on.
Now one thing that might be true and correspond with the weapons seen was that captured weapons were prized trophies. There is more status in having a trophy gun you took off somebody more than comes with a gun issued by your boss.
As for the murderous Somalis, don't be so quick to judge. They had made their wishes known long before that in regard to their sentiments to foreigners, especially Americans. There should be no surprise about what happened. A tremendous amount of the fault really has to go with the military planners who basically set up the operation just like the 20 before it and so the pattern of events was blatantly obvious and this is hardly a good thing for the US Soldiers. Next, they were limited in their rules of engagement. While a carrier was close by, they were not allowed to call in air support from fighters. There was a rescue contingency plan, but it was not adequate and the people engaged in the rescue has not been prepped for it. Too many of the soldiers in vehicles on the ground did not actually know how to navigate in the city even though they knew that roadways were often blocked and alternative routes would have to be used. The result there concerns the number of times the convoys got lost or had to back track. And lastly, some of the soldiers themselves treated the operation as a milk run, like previous milk runs, and they were not properly equipped for anything other than a milk run. This was shown in the movie very well when the soldier opted NOT to wear his 12 lb chicken plate and opted not to take water or full canteens of water. These guys had been shot at before and knew of other UN groups that had suffered casualties, and yet some of the folks rolled into the operation like it was a Sunday afternoon MILES scrimmage.
What the Somalis did was wrong in their treatment of our soldiers, but what they did is nothing new and nothing US soldiers haven't done over the years and you can darn sure count on that sort of treatment happening to unwanted armed invaders in the US. Heck, down here Bubba would hitch them up to the back of a truck and drag them down the road. The big difference with Somali is that it was not the aftermath that was shown, but the actual process of the mistreatment of the dead. And no doubt the footage was horrific, but the footage of such actions had a tremendous impact and effectivey forced Clinton into a change of American foreign policy.