I own 8 AR-15’s. Two are chambered for the standard 5.56 NATO M195 cartridge used in Vietnam and it’s subsequent iterations. One is chambered in .22 LR, the common squirrel cartridge. One is chambered in .300 Blackout, which fires a 125 grain bullet at an initial muzzle velocity of 2230 fps, with 1360 ft lbs. of energy at the muzzle. One is chambered in .458 SOCOM using a 250 grain bullet at 2150 fps and 2150 ft lbs. at the muzzle, and another in .50 Beowulf 300 grain bullet at 1870 fps, 2330 ft lbs. at the muzzle. One is chambered in 6.8 Wolverine cartridge using a 90 grain bullet at 2750 fps with 1510 ft lbs at the muzzle. The last is chambered for the .300 Ham’R using a 125 grain bullet at 2600 fps with 1765 ft lbs of energy at the muzzle. There are .410 shotgun upper receiver conversions that are available too.
Compare this with the G.I. M195 Vietnam era cartridge firing a 55 grain bullet at 3,250 fps with a muzzle energy of just under 1300 fps. With the exception of the .22 LR round and my 5.56, they are all more powerful then the Vietnam era cartridge. The only limiting factor as to what cartridges can cycle through the AR-15’s action is the length of the cartridge (max 45 mm) due to the size of the mag well in the AR-15. (The AR-10 can accommodate cartridges 51 mm in length) But if you are willing to forgoe semi or full auto firing, I have seen bolt action single shot upper receiver conversions that allow a .50 Browning Machine Gun cartridge to be used with the lower receiver as the trigger control and stock group.
But that is PRECISELY why Eugene Stoner’s brilliant design is arguably the most versatile firearm ever conceived. I assembled each and every one of these myself from parts starting with a standard Mil-Spec AR-15 receiver, assembled the barrels into a standard stripped Mil-Spec upper receiver. Once the receivers are assembled, they can be installed on ANY Mil-Spec lower receiver by simply pushing out two captive pins. They all use the same STANAG magazines (the .458 and the .50 Beowulf can only hold 10 rounds though) and the same bolt and bolt carrier. (The .22 LR requires a dedicated bolt carrier and barrel and a different magazine)
I believe that there are approximately 175 different rifle and pistol cartridges to which the AR-15 and AR-10 platforms can be readily and easily adapted to with common hand tools. (The only specialized tool needed is a armorer’s wrench)
The 2nd amendment is about a militia. A militia is a body of citizens organized for military purposes and bearing weapons supplied by themselves and of the kind in common military use at the time. The RKBA IS about “weapons of war.” If we let turds like most leftist and RINO gun grabbers define the 2nd amendment as being for plugging bunny rabbits, we have negated it’s purpose altogether. The framers of the constitution did not write it to ensure your ability to hunt and blast paper targets. Most states require a license to hunt anyway.
The fact that a rifle firing an intermediate power “assault rifle” cartridge is being transmogrified into a gun grabber “powerful” notion should have NO BEARING whatsoever in the right of the citizen militia to possess it in order to maintain the “security of a FREE state”!!!
I’m in the same boat as you. Currently have a couple 5.56 AR’s, a 300 Blackout, a couple 9mm’s, and the most fun, a dedicated .22lr SBR with a suppressor. Also have parts ready for a 7.62x39 build and an AR10. They are the Lego set for adults. On my radar is a 450 Bushmaster build, and one of the 6.5. Current mission is aquiring “standard capacity” magazines for the ones I already have, and one’s on my radar, since WA state’s “hi cap” ban goes into effect July 1.