Considering how long it's been around.

I'd say the answer is: that depends.

If you consider the AR18 to be a M16 derivative, then yes. There are many rifles based off the AR18 base design, like the SA80, G36, SCAR, plus a number of others.

If you're looking at the piping of gas back into the BCG as a defining characteristic, then no. There aren't any rifles based on that idea that I can think of that were introduced after the M16.

AJ

I would say that the AR-18 moreso than the AR-10/15.  For the AR-10/15, those are your two main variants.  If you put a piston system in either one....you could say 4.  OR you could say those are more a derivative of the AR-18 system.

The AR-18 has spawned numerous systems....and the most interesting thing about that is that the AR-18 systems have proliferated in more recent times.  The AR-15 system has matured and benefited from material's sciences and engineering more than really evolving.  You could say that there is not really a need for the AR-15 system to evolve.  It pretty much is what it is and it does its job superbly.

Eugene Stoners AR16 /AR18 design spawned many children.  
From memory; the M17S, Taiwan's Type 65/86/91, Japan's Type 89,  Australia's Leader Dynamics T2 Mk5, England's SA-80 series, Singapore's SAR 80 series, the Masada/ACR, Germany's Hk SL8 & G36, the MPAR 5.56 and probably more.  

The AR15 reliable variants ? Canada, Denmark?, Korea, Philippines and other countries have produced AR15/M16 based rifles, carbines and automatic rifle variants under licenses. China makes unlicensed clones. Hundreds of countries use AR15/M16 types in police and military services. Hk and a boat load of others moved the AR's gas piston from inside the BCG to on top of the barrel and those run well too. Belt fed AR15's were a thing at one time too.

The take away here is Stoner made some design technology waves 55-60 years ago and we are still feeling the ripples today.

The basic problem with your entire post is assuming the AR18 started gas piston operation and Stone created any kind of new concept, that's just not true.

The FAL, AK47 and several others predate the AR18, and those are predated by others decades before.  The SA80, G36 and scar have more in common with the FAL and AK47 than the AR18.

Stoner isn't the visionary people claim he is.  He used old ideas to create a rifle, that's it. 


Edit: the SA80 is actually an AR18 design, but it didn't work well until HK fixed it.

I thnk what separates semi-auto rifles is the gas system that operates the rifle. I think what the OP is actually pointing to is the DI gas system of the AR15/M16. It doesn't seem this system has many proponents. Meaning, I can't think of any other major MSRs made in the last 50 to 60 years that are DI. Certainly no new/modern MSRs are using the DI gas system.

I own lots of ARs and this is AR15.com after all. But the truth is, DI is just not an aspirational gas system. It is a gas system to be avoided. If it was so wonderful, most rifles that followed the M16 would be patterned after it. Alas, history shows that is not the case.

I love 300BLK and love shooting it suppressed. But the major platform for it is currently the AR15. I HATE the tremendous amount of fouling that occurs even after one mag suppressed. A couple of hundred rounds, forget about it. It is a filthy operating system especially for shooting suppressed. For suppressed shooting in 223, I have switched over to Piston ARs. LWRC, Adams Arms. So much cleaner suppressed. Like a different rifle. In the AR world, there just really aren't any 300BLK Piston guns that are reliable. The CZ Bren 805 300BLK is shipping next month. I'm think an 11" pistol is in my future.

It wasn't my intention to say that the AR18 was the first gas piston operated rifle, because that's obviously wrong.

What I was trying to say is:
If a person considers the AR18 as a M16 derivative, then, to answer the OP's original question, the M16 did spawn many derivative designs.
If a person thinks that Stoner's design of the AR18 is not a M16 variant, then the M16 was pretty much a dead end, as far as small arms design goes.

There hasn't been much originality in small arms design since WWII*, pretty much every design has been using bits from different earlier designs assembled together.

AJ

*The only ones I can think of right now are telescoped bolt submachine guns and Benelli's inertia recoil operation.

The rotary "bolt" of Hk G11 rifle was as innovative as the 4.7mm case less round it fired.

You're right. I was thinking more along the terms of 'successful innovations'.

AJ