Im an LEO and building a Combat Oriented Patrol Rifle. Due to its role I want it light, maneuverable (I work in an urban environment) and reliable, especially for a two way range.

Due to departmental restrictions it can be no shorter then 16 inches. Therefore Im building the gun around a 14.5 inch Daniel Defense barrel with a permently pinned and welded Adams Arms VDI Manimal Extended Flash Hider, which would allow me to remove the gas block without cutting the barrel. I like there lightweight mid lentgh profile due to weight. They also have a  slightly heavier government profile 14.5 in carbine. The handguard would extended up until the muzzle device starts, so it would not have an effect on which gas system I use.

I have been reading a lot on Dwell time, and read a couple forums posts. My question is when would one choose carbine vs. Mid lentgh and vice versa for a 14.5, and how would that choice affect what BCG and Buffer to use (Im leaning towards BCM M16 Bolt Carrier Group). I am not planning to use an adjustable gas block or adjustable BCG. The goal is for an accuarte, reliable gun that shoots smooth for quick follow up shots.

Duty rounds will be .223 but I want it to run 5.56 with no problems. Some people on forums report issues with the 5.56 and 14.5 mid lentgh, however they didnt specify what BCG/Buffer they were using which may effect it. They weapon needs to be reliable, and two malfunctions on a qualification course would not allow it to be approved.

If you use an adjustable gas block it shouldn't matter. With AR's what you want is the longest distance between the gas port and the actual muzzle

If your next question is what ammo I would say XM556SBCT3.

What is your question exactly? If it’s carbine or mid length, both will work just fine.

https://aimsurplus.com/faxon-14-5-slim-perm-flash-hider-16-in-oal-gunner-223-5-56-barrel/

Here is a 14.5 Faxon light weight gunner profile with a pinned gas block and perm attached slim flash hider. This has everything you need without having to send the barrel off for work.

I just built another stick using a Spikes .625 CL LW 14.5 mid-length & pinned/welded an extended FH.  Runs great with all ammo I’ve tried (PMC .223, M193, Wolf, M855, 62-gr HP’s) using an H1 buffer & std. carbine spring.

Anecdotal but it’s not the first.  With a properly sized gas port mid-length works great in a 14.5”.

I would go with a 14.5 midlength, and Crane found the same thing.

In what applications would you choose one over the other?

This. And I have one. Very smooth, standard lo-pro GB, H buffer & standard carbine spring.

Any issues with weaker ammo? For the qualification course they use cheaper ammo. Also any issue running 5.56?

So you're recommending a pistol gas system?

Zero issues with anything remotely considered quality commercial ammo.  .223 pressure 75gr PPU, Federal Amer Eagle 223 (AE223) to 5.56 full-house M193.
FWIW, I run temperature ranges up here through all 4 seasons. I would not go a heavier buffer setup than that as I do not shoot to make pretty piles of brass - I want the fired case out of the frickin' gun. Even a standard carbine buffer/spring setup works fine; the H is just a touch smoother in terms of those little intangibles like 'flatter shooting' or 'recovery time of the sights' or whatever people call it.  Just my $.02.  14.5" mid-length semi-auto doesn't need anything fancy imo.

Thanks

My go-to carbine is a 14.5” Spike’s (FN) Optimal Profile barrel with a pinned Manimal flash hider, running with a standard carbine buffer and a Damage Industries chrome silicon spring.

It’s light, runs anything I feed it reliably, and has smooth, light recoil...I couldn’t ask for a better rifle really.

I've run a 14.5" Colt carbine-length barrel with H buffer on my work 6920 for years with no issues. I recently put a DD 14.5" LW mid-length on it after reading the benefits of mid's for the last couple of years (softer shooting, easier on the gun, etc.) and wanted to partake of the milk-and-honey myself...

I can't tell any difference.

Either can work fine. I personally have more faith in a 14.5'' carbine.

I only have 16'' barrels to compare mid length and carbine length with but they feel the same to me as well. Any difference in recoil that someone believes they notice between the two probably has more to do with other differences in the guns and less to do with gas length, IMO.