Sport shooting and tactical sports are obsessed with “getting the perfect platform”, and to tell you the truth, in limited sphere of sports you can get that. With the mild limitation “can you bullet make the falling plate fall with low hit?” type of thing. When you start thinking this through in wider world of hunting and warfare the effect on target comes much more pronounced consideration.
In this in mind I have collected a very generic table that approximates effects of barrel length to bullet weight to frangibility of bulled in distance. Light rounds like 5,56 NATO/ .223 rem do most damage not by temporary/permanent wound channel but by “exploding” or rather disintegrating (as no explosives are involved) in soft tissue within the target.
Many conventional 5.56 NATO projectiles (especially M193 and, under suitable conditions, M855) produce severe wounds because they yaw rapidly after entering tissue and may fragment if impact velocity is sufficiently high. Fragmentation greatly increases tissue disruption, but it is not the only terminal mechanism involved here. Again the bullet speed required for said explosive disintegrating varies with construction of bullet
So please with these grains of salt Here are my thought for the “Great barrel lenght” discourse.
The “perfect barrel” should be so long enough to allow all powder to burn and producing usefull pressure behind the bullet and to stabilize said bullet, be so short not to unvieldly and light but not to be so long that friction starts to slow down said bullet. SOMEWHAT achievable with tailored loads, but are on general terms maybe not worth the effort.
AR-15 Barrel Length vs. Muzzle Velocity Matrix (Imperial & Metric)
Note: Velocities represent standard 5.56 NATO pressure loads (e.g., 55gr M193, 62gr M855, 77gr MK262 / heavy match) measured at 3 meters (10 feet) from the muzzle.
| Barrel Length | 55 gr / 3.56 g FMJ | 62–63 gr / 4.0–4.1 g FMJ | 77 gr / 4.99 g OTM | Primary Operational Role |
| 10.5″ (267 mm) | 2,550 fps / 777 m/s (1,075 J) | 2,450 fps / 747 m/s (1,120 J) | 2,250 fps / 686 m/s (1,173 J) | CQB / Ultra-compact vehicle gun |
| 12.5″ (318 mm) | 2,750 fps / 838 m/s (1,253 J) | 2,620 fps / 799 m/s (1,280 J) | 2,400 fps / 732 m/s (1,336 J) | Balanced short-barrel rifle (SBR) |
| 13.7″ (348 mm) | 2,880 fps / 878 m/s (1,374 J) | 2,740 fps / 835 m/s (1,400 J) | 2,500 fps / 762 m/s (1,448 J) | Minimalist pin-and-weld build |
| 14.5″ (368 mm) | 2,950 fps / 899 m/s (1,441 J) | 2,820 fps / 860 m/s (1,482 J) | 2,570 fps / 783 m/s (1,531 J) | Military standard (M4 spec) |
| 16.0″ (406 mm) | 3,100 fps / 945 m/s (1,592 J) | 2,920 fps / 890 m/s (1,589 J) | 2,680 fps / 817 m/s (1,665 J) | Civilian legal standard / Do-it-all carbine |
| 18.0″ (457 mm) | 3,200 fps / 975 m/s (1,695 J) | 3,030 fps / 924 m/s (1,711 J) | 2,760 fps / 841 m/s (1,765 J) | SPR (Special Purpose Rifle) / DMR |
| 20.0″ (508 mm) | 3,250 fps / 991 m/s (1,749 J) | 3,100 fps / 945 m/s (1,791 J) | 2,820 fps / 860 m/s (1,843 J) | Full-length rifle (M16 spec) |
| 24.0″ (610 mm) | 3,350 fps / 1,021 m/s (1,859 J) | 3,200 fps / 975 m/s (1,908 J) | 2,910 fps / 887 m/s (1,962 J) | Long-range precision bench / Varmint |
AS can be seen from the table two inch, or 51mm jump from 10 inch barrel to 12,5 inch barrel gives quite of substantial jump in v0 speed, 200 feet per second but four inch (102mm) jump only gives you 100 feet per second increase with standard factory ammo. In fairness I guesstimate that speeds would in fact start dropping fast after 26 inch barrel. But that is not in scope of this writing.
55 Grain (M193 spec): Maximizes raw velocity. Ideal for standard range training and varmint shooting. Works best with 1:9″ to 1:7″ twist rates.
62 / 63 Grain (M855 / standard soft point): Offers improved wind deflection and structural integrity through barrier materials compared to 55gr. Optimized for 1:9″ to 1:7″ twist rates.
69 to 77 Grain (MK262 / Heavy match OTM): Retains kinetic energy and trajectory at extended ranges (300-600 yards). Requires a fast twist rate (1:7″ or 1:8″) to stabilize the longer projectile. Out of shorter barrels (10.5″–14.5″), heavy match bullets maintain lethal terminal mechanics better than lighter rounds that rely strictly on high speed.
Accuracy as such is not greatly affected by the barrel length provided that the bullet stabilizes reliably in said barrel. Velocity will be reduced, so cross winds and athmospferic phenomena will affect slower bullets for longer time and thus giving more pronounced effects, is of course in evidence on target. So groups tend to be bigger.
“European Union Legal” seems to be more of magazine capability than barrel lengt, In Finland rifle limit is 300mm so about 12 inch, and some of my competitors hawe such guns. I have not gone below 14,5 inch M5 Colt, and I maybe prefer the 16 inch Stag Arms, but that is just me. Both are also roe deer legal here (With suitable 3 round magazine).
And the all important effect on target
| Barrel Length | 55 gr M193 Threshold (~790 m/s) | 77 gr OTM Threshold (~640 m/s) |
| 10.5″ (267 mm) | 0–15 m (Immediate velocity drop below threshold) | 0–100 m |
| 12.5″ (318 mm) | 0–50 m | 0–160 m |
| 13.7″ (348 mm) | 0–80 m | 0–200 m |
| 14.5″ (368 mm) | 0–100 m | 0–230 m |
| 16.0″ (406 mm) | 0–140 m | 0–270 m |
| 18.0″ (457 mm) | 0–180 m | 0–310 m |
| 20.0″ (508 mm) | 0–200 m+ | 0–340 m |
| 24.0″ (610 mm) | 0–230 m+ | 0–380 m |
AS a military rifle I would not recommend going below 12,5 inch barrel lenght to get lethality on target. If said target is wearing say NIJ level IIIA vest, AR-15 SHOULD be able to penetrate said vest in all practical distances. and Level IV will stop the same round in all distances. So really we only need to worry about the terminal ballistics here.
But, as esteemed reader may know, that penetration issues were one of the major driving factors in adopting the Mk 7 spear and 6,8mm Fury ammunition. And that Ukrainian war shoved that russkies didn’t have the NIJ level IV protection after all has given life extension to 5,56 NATO round.
Key Takeaways: Velocity, Dwell Time, and Terminal Performance
- The Velocity Drop Curve: The gain in velocity per inch is not linear. Going from 10.5″ (267mm) to 14.5″(368mm) gains roughly 100 fps (30m/s) per inch. From 16″(406mm) to 20″(508mm), the return diminishes to around 30-40 fps (35 m/s) per inch. Beyond 20″, gains drop significantly while weapon length and weight increase substantially.
- Fragmentation Thresholds: M193 (55gr) requires roughly 2,600-2,700 fps (790 m/s) at impact to reliably fragment in soft tissue. Out of a 10.5″ barrel, that threshold is reached almost immediately past 50 yards, whereas a 20″ barrel maintains fragmentation velocity past 200 yards.
- The “Sweet Spot” Lengths:
- 12.5″ provides optimal dwell time and significantly increased velocity compared to a 10.5″, making it the preferred choice for short suppressor hosts.
- 16″ remains the most practical general-purpose setup, avoiding federal registration fees in many jurisdictions while maintaining fragmentation out past 150-200 yards with 55gr loads.
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