Composure could even be called for when coming under fire from any Heavy Weapon; to my mind, assault cannon rounds, machine gun fire, and rocker/missile launchers are terrifying weapons, and this could be represented by a simple composure test.
Suppressive fire for sure (add a requirement that unless you succeed at a composure test (threshold whatever you want for your table) you MUST take the free action to drop prone or the interrupt action if you have no free action left. Until you succeed at your composure test (taken at the start of each round in a suppressed area, or if being fired at directly by a Heavy Weapon such as an assault cannon or rocket launcher and you are still alive) you may take no actions at all except movement and the take cover action. Something like that.
And yes, while rifling was invented a long time ago, advancements in miniaturization technology did not allow for revolutionary use of said technology until the 19th century.
Reaver; I said ammunition has developed, that doesn't mean "got bigger". While the standard assault rifle round currently used by NATO is indeed smaller than what it used to be (7.62mm vs 5.56mm), this doesn't mean that the rounds are the same as they were 100 years ago. Advancements in research methodology and manufacturing technology have allowed manufacturers to develop more powerful loads (i.e. projectiles impacting with higher kinetic energy), as well as ronds that have significantly higher accuracy potential.
Rounds and rifles consistently performing at sub-MOA levels only "just" came into production in the last 50 years, and when you look at specialized rounds such as those used for anti-matériel and sniper roles the differences are even bigger. While Zaytsev managed to score an impressive amount of kills with his trusty Mosin-Nagant, these days a 900m kill-shot is not considered a difficult maneuver with the right equipment. The invention of very-low-drag ammunition such as the relatively heavy .338 caliber round coupled with more efficient chemical loads results in effective ranges of 1500+ meters. While the furthest kill-shot in history is somewhere beyond 2000m, luck plays as much a part of shots taken at these distances as anything, but Lapua and Sierra both qualify their rounds out to 1500m, and wildcats will further increase the range and damage potential.
And while you may not consider optics part of the weapon, I will guarantee you that every soldier with a red-dot, milrad, or good old fashioned MOA sight does; unless you're a machine gunner, accuracy does matter and putting the other guy down first can be the literal difference between life and death. Optics are no longer a snipers tool (though we certainly rely on them more and more), and advancements in computer technology means my scope can now indicate wind speed at my location, as well as compensate for range and elevation differences, allowing me to place more accurate rounds on target faster.
Heavy weapons benefit from barrels constructed using new metallurgy techniques, allowing for longer bursts of sustained fire before barrel replacements are needed. Again, ammunition technology has leaped forward with the advent of high-explosive, armor-piercing, incendiary rounds (aka multipurpose, see Raufoss MK211) leaving infantry with the capability of disabling or even damaging lightly armored vehicles.
Then there's the manufacturing process itself. My old G3 was a solid piece of weaponry, but it needed constant care and maintenance. The G36 that replaced it, while less powerful at longer ranges due to the smaller caliber, was nearly maintenance free due to the way it was designed. It was also far easier to clean, making them more reliable in the field.
And while we haven't really seen much of the most recent technological advancements on the field itself, there certainly is a lot of possibilities. The G11 with it's caseless ammunition was mentioned, and while it itself was a bit of a failure marketwise several similar technologies are being worked on. Metal Storm is another concept that sounds scary as fuck to this former infantryman, as does the computer-controlled, directionally stabilized, "smart" rounds currently being researched.
Would a few hundred modern assault rifles in the hands of WW1 soldiers change the course of the war? No. Advanced weaponry is all good and well, but tactics and strategy wins wars, not the weapons themselves.