Your best bet is to survey the last 100 years of self loading rifle and pistol design to ensure what has been designed wasn't already tried and discarded. Some things can be tried again with new technology to make them affordable, or actually work at a different level of dynamics - but the field has largely been covered. There have been hundreds, if not thousands of engineers searching for new designs for a very long time. And there are hundreds, if not thousands of law clerks who will do that search to prove the concept never previously existed and is definably "unique" under the law.
If there is any question of it, and money is involved, then prepare to defend it.
After that - you need a working prototype that has already gone thru some development and has basic reliability to demonstrate the new principle and it's uniqueness. You need a patent lawyer, a machininst on retainer, a range to shoot on, and a ready source of ammo. Not being knowledgeable about any of it means depending on others. Others won't do it for funsies, they want money.
As for the features, an AR with cutaway mag well to take AK mags, and an adjustable gas regulator can accomplish what's been mentioned. That could be done with an MGI Hydra lower, ASA non reciprocating handle upper, and aftermarket gas port valve. As for it being long action - the trend for the last one hundred years is to make it shorter.
Ideas are unfortunately, a dime a dozen. What makes the money is bringing an idea to the market as a reliable product thru existing channels of distribution, or direct to the public. Doing that makes the money - the patent rights and their value are completely dependent on that potential alone.
All said and done, this rifle would have to offer a magnitude of performance not currently available. That really means being able to get hits on target substantially more often than what is currently done. Actions don't get it done - the human operator does, and what has done the largest increase in hits on target in the last twenty years is red dot optics. That's been the #1 improvement in combat rifles, and driven most of the changes we see in the M16 making it into the current M4. Whether an action has it's motive operation located on the barrel or in the bolt carrier is basically moot - either will work. Putting bullets on target is the real problem, and doing so reliably out to 500 yards the goal.
Entirely why the 7.62x39 isn't the best choice. Effectively, it could be improved, but ballistically, it's a .30 in a short case, underpowered and dynamically limited. The adoption of it as the basic cartridge reflects poorly on the design concept, simply because there are so many better ones out there. And they are in current use, which can't be said for the X39 and modern armies.
Eliminating the brass case is the next quantum leap in firearms, and we're likely a decade from it - just because it takes time to change the infrastructure more than simply produce one. There are already hundreds of working prototypes already owned by the government, and getting it to work means adding 40% more ammunition to the soldiers load - with corresponding increase in hits. The additional advantages of reducing battlefield noise, signature, logistics, and recycling salvage for the military are incentives in their own right. It highlights again that the major improvements in firearms aren't in actions, but related to hit probability. Increase that, and the mavens of marketing will beat a path to your door.
I promise not to steal your ideas.
