We get to see lots of different configurations of rifles when teaching Armorer Courses.  Many agencies seem to make their purchasing decisions based on what the local gun shop has to offer, or what their LE Sales Rep Company has to offer, or what a neighboring agency purchased, or what an Officer read in a gun magazine, or based on what looks cool (Cool Dude Index or Chicks Dig It factor), or many agencies local laws require it be put out to bid so the agency gets the lowest bid price rifle.

A question I often ask when teaching armorer courses is how long do agencies keep handguns, and the average answer is 7-10 years.  When I ask how long do agencies keep shoulder weapons, the answer is usually forever, which from my 3+ decades on Law Enforcement we have shoulder weapons currently in service that are 25-55 years old.  Taking into account that an agency will most likely have that shoulder weapon in service for decades, purchases should be based on supporting it for replacement of parts & service if necessary.

I see a lot of rifles in armorer classes that were bought on the lowest bid, where barrels were made o 4140 steel (Milspec is 4150), that had a parkerized finish on the inside/outside (Milspec is hard chrome lined).  These rifles worked great for several years, then one day they noticed that several rifles were shooting large groups with rounds tumbling at 15yds.  These rifles crossed my path in class, and we gave them a close inspection to find that there was no rifling left the last few inches of the barrel, which was causing the rounds to strike the baffles inside their suppressors causing major damage.

My suggestion is to look at the rifle as an investment, and invest wisely.  Do your homework.  If having to put put a bid, then write your bid specific with details on every piece of the rifle and get exactly what you want, and don't write bids generic or you may not get what you want.  If you want 4150 barrel steel that's chrome lined then specify it.  If you want a Carpenter 158 Steel Bolt that is high pressure tested and magnetic particle inspected then specify it.  If you want a single stage, Geissele SSA or Larue MBT two stage trigger, then put it in the bid, etc

I also see lots of agencies that purchase short barreled rifles (7"-10.5"), not realizing that effects on ballistics.  We spend a lot of time going through this when teaching Armorer Courses.  There are 3 types of ballistics that need to be take into account: Internal, External, and Terminal.  Internal is what happens inside the rifle.  External is what happens after the bullet leaves the barrel before it hits its intended target.  Terminal is what happens when the bullet hits its intended target.  All three of these ballistic issues have to be taken into account, and must work in conjunction with each other.

Internal ballistics: The ammunition needs to fit in the chamber, fire, bullet needs to travel down and out of the barrel, enough gas pressure needs to be created and properly cycled through the rifle to get it to function in all field conditions, and then the bullet has to working on proper conjunction to the barrel configuration so it can stabilize when it leaves the barrel.  There are many things that also have to be taken into account (But not limited to) like weather, altitude, .223 vs 5.56, suppressed/unsuppressed, etc.

External ballistics:  The bullet needs to stabilize so it can shoot the desired size group at the desired distance.  There are many things that also have to be taken into account like group size can be greatly effected at different distances, and there are also influences of weather, altitude, wind, etc.

Terminal ballistics:  I will refer you to do your own research and ask many of the Great Ballistic People in the field like Doc Roberts, Fackler, and also check the research done with the ammunition companies like Federal, Speer, Black Hills, Hornady, Winchester, Remington, etc.  For the Law Enforcement role, when the bullet hits its intended target, it must penetrate and cause trauma (Air in and blood out).  Generically we need to get a bullet to penetrate deep enough to reach vital organs, but then the other side of the coin is that Law Enforcement Administrators are also worried out over penetration where a bullet would leave its intended target and continue where it would then hit something you didn't want it to.

What most Law Enforcement Administrators don't understand or seem to take into account is that .223/5.56 rifle bullets act differently than handgun bullets.  Generically handgun bullets are short & fat, traveling either sonic or subsonic, and if you slow them down they generally will penetrate less distance.   It is just the opposite for .223/5.56 rifle bullets, in that they generically are long and skinny, traveling (optimally imho) 2800fps-3200fps, and if you slow a these rifle bullets down they penetrate deeper, and when traveling deeper they generally cause less trauma, and can also leave the intended target and now become a liability to anywhere else that bullet may go.  When it comes to barrel length, the shorter the length the less time the bullet stays inside of it which in turns will result in less gas pressure.  Less gas pressure pushing a bullet means that it will be traveling slower, resulting most times where the bullet will penetrate deeper and may not give the desired results of creating the needed trauma necessary to stop a threat.  Next you have to take into account that most .223 ammunition is a lower pressure than 5.56 ammunition.  Also keep in mind for Law Enforcement we are shooting hollow point or soft point rifle bullets for a duty round, and the Ammunition Manufacturers load these in .223 and generally not 5.56, so that means that you have a lower pressure duty round that is traveling slower.  Now combine a .223 duty round, with a short barrel, and you have a slower moving round, which can result in a bullet that travels deeper when it hits its intended target, and may not cause as much trauma.  I know that Federal, Speer, and Hornady offer some special short barrel loads that are loaded to higher pressures, but do not use these in longer barrels as they could be traveling too fast.

Other factors that come into play are things like weather, altitude, etc.  Check with your Agency's Designated Marksman/Snipers who keep data books, and they will see ballistic performance changes.  Generically in cold weather ammunition can lose velocity, and in hot weather it can gain velocity, this will effect external and terminal ballistics, and if the ammunition doesn't create enough gas pressure it could effect internal ballistics where the gun would short stroke resulting in fail to ejects or pick up the next round.

Other considerations that need to be take into account is the actual rifle & barrel configuration.  There has to be enough barrel length past the gas port (Dwell Time, which imho is optimally no less than 3.5") to get enough gas back into the gas tube & bolt carrier to get the gun to fully cycle properly for maximum reliability, this comes into play on both direct gas inpingement and piston configurations (We see a lot of piston set ups that can be fussier).  Too short of dwell time can result in a rifle that doesn't have enough gas pressure to reliably cycle, now combine that with a .223 duty load, cold weather, etc, and you can have issues.

When I get an Officer or Agency inquiring about shorter barrels, I always ask why do you want or think you need one?  The common answers are ease of getting them in/out of vehicles, not having to dip a muzzle when doing building clearing, adding a suppressor or possibly adding a suppressor, they look cool, etc.

What is optimum, depends upon the need.  There is a saying in that the Mission Drives The Gear.  IMHO I have to take into account my environment, and intended all around use for the Patrol Officer role or a SWAT Officer role, of having to employ the rifle from anywhere doing a high risk traffic stop (probably 25yds an closer), perimeter on a building (200yds and closer), hallways in a school (220yds in the high school where I live), shopping area parking lot like a Costco or Walmart (300yds and closer), rural farm driveway, or CQB building clearing.  I much prefer a 16" barrel, as it allows most ammunition to create enough gas pressure of getting the rifle to cycle reliably (Internal Ballistics).  The 16" barrel will generally create enough velocity on most ammunition to allow for good stability on most loads (External Ballistics), and the bullets generally will be traveling with enough velocity to penetrate deep enough to create trauma and reduce over penetration possibilities (Terminal Ballistics).

Where I may consider a shorter barrel would be specific applications like a Motor Officer needs to put rifle into a saddle bag, under cover plain clothes, SWAT work inside crawl spaces, or suppressing a rifle where overall length needs to be taken into account, and suppressing a rifle will also change how the rifle runs so you need to tune the rifle to the ammunition and suppressor for maximum reliability (Again something we cover extensively when teaching Armorer Courses).

So that is my quick $.02, which you can take or leave.

A suggestion is to get to a quality Armorer Course, whether from us or someone else.  This is where you will learn about how the rifle runs, how to maintain it, diagnose and handle repairs in house, learn about all the differences in parts, how to deal with timing issues and adding suppressors, etc.

CY6
Greg Sullivan "Sully"
SLR15 Rifles
TheDefensiveEdge.com
(763) 712-0123

I think Troy supplies parts for S and W M and P rifles. I think Troy's some kind of former LEO. Can't hurt to call