I need to get you through an Armorer Course, as we cover this extensively. I will try to put some of it here in print.
There are basically 3 types of ballistics that you you need to consider as it pertains to the AR15 / M16 weapons system: Internal, External, and Terminal. Internal is what happens inside of the firearm. External is what happens after the bullet leaves the firearm, before it hits its intended target. Terminal is what happens after it hits its intended target.
Internal: What you need here is ammunition that fits, fires, and can reliably function the firearm. The ammunition much feed into the chamber, and allow the action to lock into battery. As it pertains to the feeding cycle, the cartridge must reliably strip off of the magazine and go up the feed ramps. Once it goes up the feed ramps, it must be able to reliably chamber and allow the action to lock into battery. So in order to do this the shape and length of the bullet, overall length of cartridge, depth of seating of the bullet, must reliably fit into a magazine, strip off the magazine and have the correct angle as it goes up the feed ramps, fit into the chamber, mate correctly with the throat / lead / freebore so the bolt can reliably lock into battery with the barrel extension.
Then once the round of ammunition is fired, the gas pressure inside the cartridge must create enough gas pressure to push the bullet down the barrel, and as the bullet passes by the gas port the gas pressure goes through the gas port, and there must be enough gas pressure created to reliably cycle the action rearward so the bolt carrier assembly unlocks, extracts the empty cartridge, and that the bolt carrier assembly travels rearward enough to eject the empty cartridge, and the hammer gets recocked, that the action spring gets compressed enough to create enough pressure so the bolt carrier assembly can start the feeding / chambering / locking cycle all over again.
External: The bullet needs to leave the end of the barrel and stabilize, so it can shoot the desired group at whatever distance you need it to. The external ballistics also have to work in harmony with internal ballistics. The bullet weight and shape, along with the velocity the bullet is traveling, has to be in harmony with what's going on inside the barrel. Many people just go by bullet weight and match it to the twist rate of the barrel, which is a good place to start imho. If you shoot a lot of different loads, you will find that some bullets in the same weight will shoot tighter groups, and some won't, this can a lot of times be attributed to the velocity of the cartridge, aerodynamic correctness of the bullet shape, but can also be attribute to the bullet shape of how much of that bullet actually makes contact with the the actual barrel rifling. There are other variables of some of these (but not limited to): barrel crown, free bore, chamber machining, rifling machining, barrel finish, deformation of the bullet as it passes down, erosion, bullet velocity, etc.
Terminal: This is what happens when the bullet hits its intended target. If you are shooting paper or steel targets its one thing, if you are shooting to create trauma it is another thing. From my mind of the Law Enforcement perspective, we want the bullet to create trauma (air in, and blood out). This is where the opinions based on actual facts from Ballistic Professionals people like Dr Roberts, Martin Fackler as to what specific loads and bullets do and don't do should be taken into account. Along with considerations given to what the bullet and ammuntion manufacturers develop for ammunition (I am lucky to have a great one close by, that I can get opinions on).
Terminal ballistics of creating trauma is based on many variables like bullet shape, design, materials, stabilization, velocity, etc. In the case of the .223/5.56, the bullet has to be moving fast enough to create the desired trauma you want. Many of the .223/5.56 load development has been done based on 20" barrels due to the history of the AR15 / M16 weapons system. Those loads were developed to be reliable for internal, external, and the terminal ballistics that the AR15 / M16 weapons system needed. Now we take into account that many of us in Law Enforcement are using shorter barreled rifles like the M4 in 14.5"-16", and shorter barrels of 10"-11.5", and we shoot many of the same loads that were designed for 20" barrels, and what we see is that the bullets are moving slower in velocity due to the shorter barrel length. In generic terms, the shorter the barrel, the less velocity of the load. This slower velocity will generally cause a bullet to penetrate further (Look at Ballistic Gel Tests). This further penetration can also result in less trauma. Now take into account that most of the duty loads that we use in Law Enforcement are a hollow point of soft point of some sort, and these are generally .223 and not 5.56. The .223 loads are generally developed with less gas pressure, as compared to 5.56 which is higher pressure. To compensate for the short barreled rifle issues, some manufacturers are now offering special short barreled loads (Federal, Speer, Winchester, etc).
Now to put the whole picture together. You probably won't see a huge difference in accuracy between a short barrel vs long barrel. What you will find is that the shorter barreled rifles will lose terminal ballistic performance at longer distances, where the longer barreled rifles will allow better terminal performance at longer distances due to the higher velocity on the bullet. Do we see a gain in velocity when a suppressor is added, in simple answer yes, it's minimal, and we don't really see any change in external or terminal performance.
Where we do occasionally see a difference on accuracy is barrel harmonics of adding a suppressor. A barrel has a harmonic vibration when you fire a round, kind of acting like a tuning fork. Sometimes when a suppressor is added you might see where the groups open up for accuracy, sometimes groups tighten up, sometimes there is no real change, and sometimes the grouping is similar but there is a definite zeroing shift (We see this often on lightweight barrels).
Where the issue of short barrels vs longer, suppressed versus non-suppressed on traditional baffle suppressors, is what is the timing of the rifle of getting it to run reliably. When using baffle type suppressors, the barrel/chamber pressure stays higher longer, resulting in the fired casing staying swelled longer, so you will most likely need to slow the bolt carrier from unlocking a fraction of a second longer (Done with heavier buffers, stiffer springs, regulating gas, etc), so the empty casing unswells and can be reliably extracted.
IMHO: If going to be using the rifle suppressed and unsuppressed, and lives depend upon it, then don't go under 11.5" for reliability of cycling, and get a short barreled load. The hot and cold weather, altitude, and .223/5.56 all have effects on how the rifle runs, and how the ammunition performs (internally, externally, and terminally).
I could go on longer, but it's hard to put into written format without sounding like I'm going in circles. And if I sound like I'm talking in circles, then ask and I will help clarify my explanations.
Hope this helps some.
Greg Sullivan "Sully"