As stated above, unless you enjoy constantly switching stuff out a dedicated lower is the way to go.
Your buffer options, carbine with spacer, standard 9mm length, and my fav bk 9mm hydro.
My form 1
As has been stated I have a dedicated glock lower. Form 1 SBR.
Also built a colt style 16 inch gun and gave it to my daughter.
The colt mags suck to load, with out a loader I would say it is almost impossible. I can load 32 rounds in a glock mag without a loader. When I shoot (once a month) USPSA my pistol holds 22 rounds and I load those mags all day without a loader. I can not get more than 7 rounds in a colt mag without a loader.
Both the colt and glock mags are ugly.
I have not run hollow points through the colt gun but I can say that 9mm AR's have issues with feeding hollow points. My glock gun did and took alot of work to feed them reliably. I believe this is due to the sharp angle the round must traverse to get into the chamber.
Also i shoot with a lot of guys who shoot PPC and most do not have or have removed the BHO feature because again 9mm ARs are not known for reliable BHO. I have seen many fail.
Also I have 150 pmags, taking 6 and marking them 9mm will be no big deal.
This mag insert puts the round at the rear of the mag this does 2 things. One the BHO will be activated exactly like an AR is supposed to be. With the round to the rear it also give the round 1.1 inches to rise into the chamber instead of about 3/8s of an inch. This is a much gentler angle and I am hoping will make hollow points more reliable.
I will not be swapping, but will build a new gun if they work as advertised.
Also i like to try new stuff. If it works, i will bet you see the end of the colt conversion kits and possibly the glock lowers also.
Ps. This will make a 9mm AR an even better trainer, as the muscle memory to reload will be the same. Reloading those long ass colt/glock mags is not as smooth.