11.5 vs 14.5 inch AR15 barrel obviously have length differences. As far as ballistic performance goes in terms of only using 55 grain bullets for range use. The 14.5" AR15 barrel provides more muzzle velocity, and it provides the necessary barrel length requirement to do a pinned and welded configuration to be 16" length, which is legal without dealing with NFA laws.
11.5" barrel makes an AR15 into a short barrel rifle governed under the ATF's NFA laws.
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14.5 Inch Barrel For AR15
- Pros: Offers increased range and velocity, which could be advantageous for engagements beyond close quarters. A pinned and welded 14.5" barrel with at least 1.5" long muzzle device will make it a legal civilian rifle.
- Cons: Less ergonomic in close quarters combat (CQB) due to its length. Adding a suppressor could exacerbate its front-heaviness and increase its overall length, potentially making it cumbersome in tight spaces.
11.5 inch Barrel For AR15
- Pros: Enhances ergonomics and maneuverability in CQB even more than a 14.5" barrel, making it easier to handle and quicker to maneuver. It also provides more flexibility with suppressor usage without significantly compromising the rifle's balance.
- Cons: Reduced range and stopping power at distances beyond 300 yards, which might limit its effectiveness in longer-range engagements common in mixed urban and open environments.
Ballistic Performance
Effective Distance
For civilian use, the effective range is considered in terms of self-defense and hunting.
The minimum impact energy for self-defense is between 220 and 300 ft lbs, with 260 ft lbs being a practical threshold. Various 5.56 ammunition types can deliver this energy at distances significantly beyond 500 yards, making it effective for self-defense at typical engagement ranges.
For military applications
The US Army Technical Manual 9-1005-319-10 provides data for the M16A2 rifle and the M4/M4A1 carbine, indicating an effective range of 547 yards for carbines and 602 yards for rifles when engaging individual targets. For area targets, the effective range extends to 650 yards for carbines and 875 yards for rifles.
Twist Rate
Twist rates still matter. It controls the spin of the bullet for maximum accuracy. The most popular twist rate for 11.5" barrel is 1/7 for 55 grain bullets.
1:8 or 1:7 will serve you just fine for some heavier bullets like the 62 grain or 77 grain.
The same goes for 14.5" barrel. The extra 3" barrel length increases the bullet spin, and the 1:7 twist rate consistently delivers the performance among various USGI metrics.
However, if you don't shoot anything heavier than 55 grain, or shoot long distance, the 1:8 or 1:9 twist rate will do just fine.
Legality
Both 11.5" and 14.5" barrels are only legal under NFA rules. Pinned and welded 14.5" barrel with a muzzle device gives the barrel an overall 16" length.