Do you have weakness in your shoulder or arms? The AR platform (especially in carbine form) is very light. There should not be much of a problem with holding the rifle against your shoulder while you perform a mag change.
Out of curiosity, I just tried this with four rifles:
Colt Sporter with 20" barrel
Of those four, when I hold the Colt Sporter to my shoulder and drop my left hand, the barrel wanders down. The other rifles, even the relatively heavy M1, do not produce this effect and I can hold them to my shoulder with ease, one handed
It's because as I pull back with my right hand to sock the Colt rifle into my shoulder, my right hand, on the pistol grip, is lower than my shoulder, causing me to pull down as well
If I do not try to sock the butt into my shoulder, there's no problem doing it
As for strength, I have no problem whatsoever holding that same Colt Sporter out at full Arm's length, with just my right hand, as if the rifle were a pistol I would shoot one handed in the classic stance
It seems to me that this is a body mechanics issue that will vary from shooter to shooter