Okay, I know I am going to get some smart comments about this question, but if I dont ask the question then I wont know the answer.
How significant is the 4" difference between a 10.5" and 14.5" barrel. Beside for the obvious answer that the barrel is 4" longer. Will bullets of equal weight and size have a significant increase in velocity? Will accuracy be significantly increased? Will range be significantly increased? Additional information of the previous type is what I am looking for in an answer. What do I mean by significant. How about an increase of 25-35%. If this is unrealistic increase let me know. 
You should probably post this question in the NFA section, but here are some velocities that I’ve measured from the barrels you mention (10.5" LMT & 14.5" Colt):
77gr MK262:
10.5" = 2293fps
14.5" = 2579fps
62gr XM855
10.5" = 2610fps
14.5" = 2940fps
55gr XM193
10.5" = 2745fps
14.5" = 3091fps
Velocities are 10-shot averages. Chrony was 10’ from the muzzle. As you can see, the differences are fairly significant.
Rich has set you up well with the velocities, where that really comes into play is in the wounding potential of the round when it hits soft tissue.
When the .223 round falls under 2650-2700 FPS it fails to yaw after hitting soft tissue and just makes a .22 hole rather then achieving it’s full wounding potential. Of course this is also dependant on bullet type, but that’s the short answer.
Thanks for the information. I will submit the question to the NFA thread and see if I get more answers.
Okay, I know I am going to get some smart comments about this question, but if I dont ask the question then I wont know the answer.
How significant is the 4" difference between a 10.5" and 14.5" barrel. Beside for the obvious answer that the barrel is 4" longer. Will bullets of equal weight and size have a significant increase in velocity? Will accuracy be significantly increased? Will range be significantly increased? Additional information of the previous type is what I am looking for in an answer. What do I mean by significant. How about an increase of 25-35%. If this is unrealistic increase let me know.
As I posted in your other thread:
Here are some velocities that I’ve measured from the barrels you mention (10.5" LMT & 14.5" Colt):
77gr MK262:
10.5" = 2293fps
14.5" = 2579fps
62gr XM855
10.5" = 2610fps
14.5" = 2940fps
55gr XM193
10.5" = 2745fps
14.5" = 3091fps
Velocities are 10-shot averages. Chrony was 10’ from the muzzle. As you can see, the differences are fairly significant.
And while we’re at it, you need to ealize that the terminal effects from a 16 " is aound 150 yards or so. Anything shorter just loses more expansion effectiveness.
Choose wisely.
So from the velocity chart it would make sense for me to get a 14.5"bbl in order to maintain sufficient velocity to achieve a better chance of a good wound. Thanks for your information, it was very helpful.
So for best results I should shoot at targets around 150 yds or more to achieve the best expansion?
Don’t put too much importance on the fragmentation range. It’s certainly something you should consider, but the 5.56x45 can easily produce fatal wounds without fragmenting if you put the bullet in the right place. The fact that members of this forum have used 10.5" guns effectively at 300yds is a testament to that fact. I’m not saying you should consider a 10.5" rifle to be the best choice for 300yd work, but it’s certainly capable if you’re proficient with the platform.
However, if fragmentation is your main concern with SBR’s, then you should stick with the heavier bullets since they retain their fragmentation properties at lower velocities. For example, the 75gr TAP round will fragment around 2250fps compared to the typical 2700fps threshold quoted for M193/M855.
With a mid length barrel like an M4 or 16", that is what you can expect with a 55gn. to 75gn JHP. Of couese there are some nuances, but this is a good rule to follow. Remember, these are “Close Quarters Battle” CQB weapons. Anything farther than that is a good reason to excape and evade IMHO.
With a 20-24" barrel, your fragmentation could be around 200- 250 yards.
You can learn a lot from reading the tacked “Ammo FAQ” about terminal velocoties with various weights and barrel lengths.
Aside from the ballistic benefits it’s a little easier to make a 14.5 run than a 10.5. You have to stick to high-quality parts (which you should anyway) when you get to barrels in the 10.5 range.