Does AR-15 Barrel Length Matter?
BY Herschel Smith
8 years ago
Tom McHale has a very good article at Ammoland on AR-15 rifle barrel lengths and whether it matters.
Remember that you have two competing effects on muzzle velocity. First, it’s advisable to get as much work out of expanding gases as feasible in order to increase muzzle velocity. Second, there is friction in the barrel, which is a detriment to the work being done by the expanding gases.
So there is a turnover point on the curve of barrel length versus muzzle velocity, where you no longer gain muzzle velocity with increased barrel length. So McHale performed some testing of barrel lengths, and this is what he came up with.
The difference between the 16″ and 18″ barrel is greater than the difference between the 18″ and 20″ barrel. But barrel lengths greater than 18″ doesn’t buy you much. McHale also has some data on the .300 Blackout round that looks interesting.
The one thing he didn’t give us is the effects of 14.5″ barrels (as with the M4), or pistol length barrels (e.g., 10″ barrels). I would like to see some test data on that, and unless persuaded otherwise I have to believe that the SpecOps trend to use shorter and shorter barrels lengths along with suppressors is adversely effecting muzzle velocity.
On December 19, 2016 at 9:20 am, wynn said:
the Spec Ops and others are using the Sierra 77 gr. bullet because of better lethality and downrange trajectory, although in the M4 it is some what slower. Sights can be adjusted.
Wynn
On February 21, 2017 at 9:57 pm, Paul X said:
“As you can see, these Aero Precision barrels are identical except for
length. They’re made in the same shop, from the same steel, using the
same techniques.”
Except, they aren’t identical. The only velocity-vs-length comparisons that are credible are those derived from cutting a single barrel shorter and shorter. When you do that, you won’t have anomalies like shorter barrels delivering higher velocity. There is far too much variation between different barrels to get good numbers the way the author did it. His 20-inch barrel was a slow one.
It is true that long barrels can deliver lower velocity than shorter ones, but you have to go very long to see this effect. There is a reason Palma shooters use 30″ barrels. Only with a cartridge like the .22LR will you see this cross-over point at a fairly short length (probably around 18 inches).
On March 20, 2018 at 11:28 pm, Georgiaboy61 said:
Interesting study… Paul X, good on you as you beat me to the punch concerning multiple versus one barrel used in the testing. There are indeed “fast” short barrels and “slow” long barrels and just about any variation within that one can imagine.
When considering the effects of short vs. medium vs. long barrels, it is well to consider the burn rate of the particular propellants used by various manufacturers, or for that matter, hand-loaders. A slower burning propellant will deliver more velocity from a longer barrel than a short one, whereas a faster-burning propellant will deliver more-velocity from a short barrel, all other things being equal, compared to a slower powder.
This variable is difficult to control for in field testing, since manufacturers customarily do not share information with their customers concerning which propellants were used to make a particular cartridge loading.
A degree of “reverse engineering” is possible, given ample time, curiosity and an accurate chronograph, but that’s asking a lot of most recreational shooters.
The barrel length & muzzle velocity debate also breaks down along the lines of ammunition design, specifically older, velocity-based designs such as the M193 and M855 – which depended on high MV’s to deliver adequate terminal performance – and newer, technologically more-advanced loads – such as the SOST Mk. 318 mod 0 62-grain round used by the Marine Corps and the 77-grain OTM load from Black Hills used by the Navy SEALS – which are more-velocity independent in their performance and thus may be used from SBRs and carbines with little reduction in performance.
High muzzle velocity is desirable – it reduces the effects of range-estimation errors and reliably delivers more KE on target than lower fps values do – but it is no longer the “be all, end all” of ammunition performance. The hi-tech loads being developed now mean that our troops going into harm’s way are no longer as dependent as in the past on larger, heavier, longer-barreled weapons as in the past.
Personally, I am old-school all the way and – if I was a young hard-charger going into battle – I would want as much KE on hand as possible, longer and heavier barrel or not. However, times change and so does the way people think about such things. Giving our best warriors more choices, more options, when outfitting for a mission – can’t be a bad thing.