Huh?
People who are in the business of making entertaining movies aren’t necessarily involved in weapons design. If you want a normal-sized weapon to have a lot more effect, all it takes is to have a friend in the Special Effects department.
For the time being, if you want a more powerful weapon than what is commonly carried, it will need to be bigger.
Wanted to point out (as suggested also by Lemur866 and Enipla) that small arms evolution over the last few centuries has made the individual infantryman more “powerful” in the sense of increasing his/her effectiveness, not in having a bigger gun.
In fact, the opposite has generally occurred - developments favoring better rate of fire and lightweight weapons.
A 19th century musket fired a 69 or 75 caliber ball, and might be supported en masse by a 6 pound cannon or two.
In the middle of the 20th century armies used 7+mm ammunition in fairly heavy and mid-range battle rifles like the M1 Garand, Mosin-Nagant, and K98k.
Compare to an infantry squad today with M4’s firing 5.56 caliber and perhaps an M249 or a few 30mm grenade launchers. Emphasis on mobility in shorter-range combat.
With the prevalence of body armor and CQB-type situations as the world population increases, I’d expect more stuff like the FN P90 using 5.7x28 rounds (these are designed explicitly to defeat armor**) in the future as already mentioned in this thread. Note again the small size of the weapon.
I thought Heckler and Koch had a fantastic future infantry rifle in the G11 a few years back. Caseless 4.7mm ammo so you can carry more (in 50rd magazines!!), delayed recoil in 3rd burst, and the whole thing is fairly smooth on the outside so it can be decontaminated easily (we can expect NBC situations to be the rule and not the exception on a modern battlefield). Also, no pesky reciprocating bolt.
***So, unlike what Hollywood would have us believe, I see military small arms continuing to shrink in the future, with the public spin-off copies and derivatives following suit as they always have.
**Note - only certain bullets and loadings do this (not generally available to civilians or non-LEO).
I just want to say 3 (possibly 4 words) for a powerful gun: Boys Anti-Tank Rifle. I’ve always wanted to shoot a rifle that was as long as I am tall.
Anyway, following up on Thrasymachus comment immediately previous, probably the most powerful military weapon today is the AK-47. It’s so light that even children carry and fire it, it’s cheap to manufacture, and it is probably used by every armed insurrection in the world. Carried en masse by para-militaries, faux militaries, warlords’ minions, rebels, criminal gangs that call themselves militaries, it’s a weapon used en masse. To quote Mao: Power comes from the barrel of a gun. The more guns in more hands is more powerful than a few hyper-powerful guns in the hands of a few professionals.
Hmm, do railguns kick ? 'Cause I could see a portable railgun happening somewhere around the time we master miniaturized cold fusion. So, noonish, thereabouts :).
The projectile has to “push” against something.
Mass goes forward, mass goes back. No exceptions.