One of the problems is that a lot of the firearm innovation has legal limits.
There is small things that can change such as the increased use of optics mentioned by Justin.
Small additions of improved accessories.
People do not want electric guns for a number of reasons, not least because gun grabbers will jump at the chance to create all kinds of mandated circuitry to limit users, require a print to fire etc (sounds good, but imagine some thug just stabbed you and the blood pouring out of you is between your hand and the sensor or you grip it wrong preventing the weapon from working.)
States have already passed law requiring smart guns if they are created. That essentialy means only a fool or someone that wants to ban guns would create a gun that could easily implement "smart gun" technology.
Ammunition has major potential increases in effectiveness. A .22 with an explosive payload can be as effective as a .45 with far less recoil. An older grandmother with arthritis and low bone densitiy could be as well protected as anyone else, but such rounds are illegal.
Very reliable projectiles that use a host of various mechnical principles can be created (though more expensive per round) which vhastly exceed the effective stopping power of the best defensive hollowpoint. But laws forbid it.
Laws also limit the materials that can be used as in the metals that can be used, and various other limitations. Meaning just experiementing and designing is severely hampered.
The civilian community has long made many of the innovations, but the same community is limited to .50 and under caliber, no select fire, certain types of projectiles and projectile materials, certain firearm dimension, etc
So the majority of improvements and innovations for everyone, military and civilian alike come in the form of things civilians can use and experiment with, greatly limiting the scope of improvements for everyone when that market has major limitations.
If you can't use full auto you can't easily and accurately test how reliable your design holds up to certain types of use and where it needs improvements under such use for example.
The limitations are numerous and it moves the improvement out of the great garages and small shops of the nation that have created much of our current firearm designs in the past, and keeps it at large facilities that already have licenses and are in the business.
Individuals such as John Moses Browning would not have tinkered and gone on to design what are now great designs and became foundations for many later designs if he had the restrictions we have now.
Some of his first designs he experimented with just to learn were full auto for example. Where he studied the self loading principles that gave him great insight in future design. Since full auto actions can be much simplier than semi-auto it allowed quicker progress for someone tinkering. In fact full auto allows someone to see how well an action flows better than the stop and starts of semi auto. You couldn't design a new engine you couldn't start nearly as well as one you can start up and adjust to a smooth idle. Yet that is what you have to do with firearms.
John Browning didn't have to worry if the dimensions or action on some randomly assembled thing he was tinkering with violated the NFA. He had freedom to just initialy learn and later allow creativity to shape the outcome of projects.
In fact we even have laws today that a firearm must look like a firearm. But what exactly does a firearm look like (it is explained in the law)? They did not have thier current appearance until long after thier invention. They were originaly straight poles before.
So someone cannot even make a new shaped firearm, or a firearm that attaches or is operated by the user in a different way.
To be a rifle or shotgun it has to be designed to be fired from the shoulder. To legaly be a pistol it has to be a weapon that is designed to be fired from one hand.
Combined with the fact that it must have a specific appearance as defined by law the legal limits essentialy demand a firearm remains what a firearm currently is.
There is maximum pressure limits firearms can reach, so combined with that maximum legal limit of .50 bore, the pressure in .50 caliber is the upper limit of power that can be put behind a projectile.
In pistols designs they cannot add improved additional grips, collapsible second grips, or some way to make it more versatile, numerous exotic or unique designs would still fall under the legal restrictions.
They cannot easily make compact all purpose modular packages. Designs that can be a pistol, a rifle, etc. There is a huge potential in between the size of most pistols and the legal 26"+ requirement for stocked firearms.
Not just in traditional designs, but in unique innovations. However due to those size limitations there is little incentive to venture too far from existing designs. You gain nothing by creating a radical handy design if you then need to stick long components just to make it a minimum length. You can't make designs that can adjust to the needs of the user at will, because then they are in possessiong of a SBR by having the components that can be illegaly assembled.
While many people may be under the impression the military or security forces create innovations, most throughout history actualy come from the civilian market.
It is a market more favorable to radical changes that depart from the norm, where sales of something radical are still possible. Where a successful concept can then grow. It is only then adopted by military and security forces after it has proven itself, which it can only do if it had a market to begin with. The big agencies usualy stick with things already proven, and order large numbers.
That means the small numbers of sales of various new designs needs to be sold to the civilian market to exist long enough to become popular They cannot go from new with no market to being ordered in bulk, they have to start somewhere.
However with the civilian legal limitations that situation greatly diminishes innovations that may have otherwise been the next greatest leap in design.
If a guy like John Browning were to sit in his garage he couldn't just be creative, as more than likely a violation of law would occur. He would first have to think of the law, and design within the limitations of the law.
The most intuitive designs would likely be of illegal dimensions.
You also can't improve the rate of fire, that is set by law. You cannot improve the power, that is set by law. You cannot improve the effectiveness of projectiles, that is set by law. You cannot make a firearm that does not look like a firearm, the look of a firearm is described and set by law.
Once you understand all the limitations put on creativity you realize why most technology used today is little different than designs of over 100 years ago. Why designs rapidly increased in a flood of design from the late 1800s to the early 1900s and then improvements slowed to a mere trickle for almost 100 years.
100 years ago every mechanicly inclined individual with a garage, workshop, or workspace and some tools could tinker without worry of violating any law. If a new idea popped into thier head they could just implement it.
If others liked thier design it could then be sold, without prior intent and licensing, if it failed to show promise they could just tinker some more.
It could be then casualy be shipped by mail anywhere in the nation, so even without local interest if someone someplace liked the design it could survive long enough to catch on.
During design it could be shipped anywhere to have work done, be looked at or have modifications performed by someone more capable, friend family, casual associate etc. Today you wouldn't ship it casualy, you would have to first justify the need because of the added hassle and expense.
Today it does require prior intent, in the form of licensing to do many things, and legal limitations on innovation and design themselves exist.
Firearm innovation is more hampered than innovation of most other products. Because of that most innovation is left in the hands of large existing companies. The entities most likely to stick with what already works and sells, not waste large amounts of profit in innovative designing of what could become failures.
