Is there a new cartridge that has come into existence since 1990 to present day that has made a huge difference in the rifle world ?
If there is one then in what way did it have an impact beyond what already existed ?

To name a few:

6.5 Creedmoor
6.5 PRC
300 PRC
7mm PRC
450 Bushmaster
300 Blackout

The first four opened new levels of chambering precision in factory rifles and are optimized for high BC bullets. The last two greatly broadened the horizons of America's favorite rifle.

🤦‍♂️

.577 GFY

The only ones on your list that have made a huge difference are the Creedmoor and maybe the Blackout.

For decades, the gun industry had a "big 6" group of cartridges that were virtually required chamberings when introducing a new rifle the 243 Winchester, 308 Winchester, 270 Winchester, 30 06, 7mm Remington Magnum and 300 Winchester Magnum. The 6.5 Creedmoor is the first cartridge to challenge this list in decades.

The Blackout is important due to its military applications which is always a good thing for success.

The PRC cartridges are still a small niche and will remainthat way, possibly going the way of the short magnums in a few years. They'll still be around but limited availability in factory rifles.

Would you say the 6.5cm is better than the tried and true 6.5x55 in the hands of a reloader ?

No.

It's not better than the 260 Remington either.

But it has made a difference in that manufacturers now consider it a must have chambering in their rifles.

Because of marketing has it made a difference correct ?
Im not saying these newer cartridges are not good and I do own a few of them but when it comes down to performance they dont do anything more than the older cartridges can be made to do

Ballistically, the last cartridge to make a huge difference was introduced in 1962 IMHO.

The Creedmoor was developed for target competition, unlike all the others I listed above which were developed for either military or hunting. Once it started taking off in the hunting community, it demonstrated to manufacturers what was possible with factory cartridges and rifles. Tighter chambers, primarily the necks, proper freebore and tighter twisted barrels make accuracy easier to obtain with factory loads and handloads.

The 300 Winchester Magnum was introduced 38 years after the 30 Super, aka the 300 H & H Magnum. Ballistically, the new 300 offered very little over the 30 Super, but it was a better, more compact package. Today, the 30 Super only exists to satisfy the nostalgic quirks of old farts - like me. The belted magnums introduced in the late fifties/ early sixties were originally referred to as short magnums. Nowadays, they're considered long cartridges and have that horrible belt on the case head.

The only 'newish' type rifle I own in a factory chambering would be the 280 AI, which has existed for decades as a wildcat. The rest are wildcats or cartridges which have been around for at least 60 years.

I myself was enamored with the ballistics of the 22 CM until a wise old man told me to look at the .224TTH
My lust for the 22CM quickly faded when I compared barrel life, bullet speeds and accuracy with these two
The .224TTH has been around since the 50’s mostly as a wildcat but has brass and dies available for reloading like the 280 AI
I own several of the newer cartridges and can say there is no noticeable difference in what I can make them all do when comparing bullet diameter and weight

I've been strongly considering a build in 22 250 AI with a 1:8 twist barrel. But the availability of high quality 22 Creedmoor brass and no fire forming is very enticing.

I looked at both of those as well

Tikka offers a 1:8 22-250.

I tried to find one and it just wasn’t available

Not interested in a Tikka.

I will say that I sat two identical Ruger SFAR rifles on the gun counter a few days ago. One in 308 and the other in 6.5CM. I left with the 308. Either would have been a good rifle though, and I load for both of them. I'm still an admirer of the classics too!

They are available, I could get you one right now

The 300 prc is a dead duck, the 6.5 will hang around in the shadows from now on. The 7 prc will remain as a standard cartridge from here on out.

Anyone remember the UltraMag cartridges coming from Remington ?
338
300
7mm
All were like a whale in the bathtub
I bought the 300 the year they were introduced and I still own it but havent shot it in so long I cant remember when I did

The Ultra Mags had very limited purpose and the 7mm UM burned up barrels under 1000 rounds. And they kicked like a mule without a brake or a 15 pound rifle. I've shot them and a 30 378 Weatherby and just never felt like I needed one.