So I have a 12.5" I'm about to put a scope on, now a 20 MOA base is generally used for longer range right?  With the decrease in velocity from my shorter barrel would a canted base help me or hurt me- not planning on any LR stuff 0-300 yards is my plan

from what I've been told, It'll be negligible. The eleveated base just increases the number of vertical adjustments you could make

If your shooting no further than 500 or so you will be fine with out a canted base.

unless your optic has very limited travel I don't feel you'll have an issue.

On another note I very seldom find myself dialing elevation on my AR's.

I just use the reticle as I feel the AR is a fighting rifle and having to worry or forgetting if I dialed in elevation or worse say you dialed in elevation for 600 yards and never zero'd it out then your presented with a real life or death situation and miss your target closer distances. no bueno!

So all reticle for me. I can hit a 2/3 IPSC at 500 more often than not with a 14.5" shooting MK262 with a leupold 1.5-5 MK4

300 yards is a cake walk

Internal travel of the optic would help determine if it will be an issue.

Vortex Viper PST Gen II 1-6 is the optic I'll be using

For engagements out to 300yds with .223, you will have no technical need for a 20 MOA base/mount.  That's not to say you couldn't use one if you wanted, but you will never need it at those ranges.

With my 10.5" I've shot out to 500yds and not needed to have a MOA rail

No need for 300 yards.

Not needed, but I used one on this because it was $5 cheaper.