Modern firearms can be difficult to do well in an RPG system, and many rule systems fail dismally when it comes to trying to replicate them. This is especially true of systems which have fantasy roots.
Basically, firearms can be very deadly, and any system which replicates this and also has any amount of firearm combat is going to lead to characters with very short life expectancies. Yags tries to be realistic in how it handles such weapons, but also tries to provide a couple of get-out clauses to enable PCs to survive a bit longer.
It should be noted however that Yags does assume that there is nothing special about hero PCs. In it's aim of Cinematic Realism, it allows skilled characters to avoid being hit (or even shot at) as long as they act intelligently, whilst throwing in the risk that if they do get shot, then they're probably going down.
As everybody knows, Imperial Stormtroopers couldn't hit a barn door at twenty paces, despite apparently being highly trained troops. Bad guys in many Hollywood epics are also often trained at the same school, enabling heroes to escape despite being the target of many fully automatic assault weapons.
To some extent however, this isn't as necessarily unrealistic as it seems (there are a number of resources available on the internet which go into some detail on just how ineffective guns can be at anything other than point blank range when used in real combat), and Yags focuses on two core point in order to give heroes a fighting chance.
Firstly, automatic weapons are horrendously inaccurate, especially at anything more than short range. They are good at suppressing an area, and forcing the enemy to keep their head down (you probably won't get hit, but most people aren't willing to take the risk), but not actually effective at taking the enemy out. Many modern wargames actually get this right, with 'pinned' or 'suppression' results a lot more common than actual casualties.
Yags has two rules in particular which limit firearm effectiveness in a real fight - initiative and cover.
Clever use of cover enables a character to dodge a bad guy's reflexes, rather than their bullets. The rules for cover are deliberately quite detailed, allowing a number of options depending on the type of risk PCs wish to take.