308 Winchester / 7.62x51mm NATO: Barrel Length versus Velocity (28″ to 16.5″)

308 Winchester / 7.62x51mm NATO: Barrel Length versus Velocity (28″ to 16.5″)

Introduction

Released by Winchester in 1952 (and adopted by NATO in 1954 as the 7.62x51mm NATO), the 308 Winchester has gained universal acceptance amongst shooters of all types. Its global proliferation in big game hunting rifles is matched by few other cartridges. Nearly every target shooter had his teeth cut on a 308 Winchester. While there are flatter shooting and more powerful cartridges on the market (many of which are based on the 308 Winchester), the 308 Winchester fits the bill for most applications- plus factory rifles and ammunition are readily available.

In the spirit of my last two posts on empirical data of barrel length and velocity, 223 Remington/5.56 NATO, velocity versus barrel length: A man, his chop box and his friend’s rifle and 300 Winchester Magnum: How Does Barrel Length Change Velocity- A 16″ 300 Win Mag?, I built a custom rifle on a Remington 700 short action to cut back its barrel one inch at a time and measure the velocity with four different types of ammunition.

The rifle was built with the following parts ordered from Brownells:

Brownells also provided the four types of ammunition used in this test:

I did not true (blueprint) the action prior to threading and chambering the barrel. I also took the time to groove the barrel in one inch increments from 16.5″ to 28″ as a cutting guide. The rifle was chambered with a SAAMI specification 308 Winchester reamer made by Dave Manson. Headspace was measured at 1.633+ (.003″ over minimum).

What was the test protocol?

Ballistic data was gathered using a Magnetospeed barrel mounted ballistic chronograph. At each barrel length, the rifle was fired from a front rest with rear bags, with five rounds of each type of ammunition. Average velocity and standard deviation was logged for each round. Note: I also fired a 30 shots of the IMI Samson 7.62mm 150 grain FMJ with the 28″ and 16.5″ lengths- I’ll discuss why later. Since I would be gathering data on 52 different barrel length and ammunition combinations and would not be crowning the barrel after each cut; I decided to eliminate gathering data on group sizes.

Once data was gathered for each cartridge at a given barrel length, the rifle was cleared and the bolt was removed. The barrel was cut off using a cold saw. The test protocol was repeated for the next length.

Temperature was 47F.

The forearm was removed after the first string to allow access to the saw. Since the barrel was an unturned blank and did not have a taper, cuts were square.

All shooting was done from a bench, with a Sinclair rest and rear bag.

Results

308 Winchester Barrel length in inches versus Muzzle velocity in feet-per-second (ft/sec) Rifleshooter.com
Barrel length	Winchester 147 FMJ	IMI Samson 7.62 150 FMJ	Federal 168 Gold Medal	Winchester 180 PP
28	2965	2823	2706	2632
27	2962	2800	2697	2607
26	2955	2801	2673	2597
25	2917	2769	2659	2585
24	2909	2766	2635	2553
23	2877	2744	2618	2553
22	2837	2718	2597	2527
21	2807	2683	2580	2507
20	2804	2679	2565	2478
19	2757	2634	2532	2441
18	2739	2595	2523	2411
17	2707	2577	2481	2401
16.5	2682	2561	2466	2373
AVG velocity loss fps/inch	24.6	22.8	20.9	22.5

The data presented as a line chart

Cartridge specific information

Barrel length in inches versus Muzzle velocity for Winchester 147 Grain FMJ Rifleshooter.com
Barrel length inches	Winchester 147 FMJ	SD	Change ft/sec velocity	Change ft/sec velocity from 28″
28	2965	13.2
27	2962	10.1	-3	-3
26	2955	24.5	-7	-10
25	2917	14.1	-38	-48
24	2909	23	-8	-56
23	2877	18.1	-32	-88
22	2837	21.9	-40	-128
21	2807	21.3	-30	-158
20	2804	6.4	-3	-161
19	2757	31.2	-47	-208
18	2739	25.3	-18	-226
17	2707	30.5	-32	-258
16.5	2682	16	-25	-283

Barrel length in inches versus Muzzle velocity for IMI Samson 7.62mm 150 Grain FMJ Rifleshooter.com
Barrel length inches	Samson 7.62 147 FMJ	SD	Change ft/sec velocity	Change ft/sec velocity from 28″
28	2823	7.6
27	2800	21.6	-23	-23
26	2801	20.8	1	-22
25	2769	7.4	-32	-54
24	2766	15	-3	-57
23	2744	11.2	-22	-79
22	2718	12.6	-26	-105
21	2683	6.6	-35	-140
20	2679	15.9	-4	-144
19	2634	21.1	-45	-189
18	2595	20.3	-39	-228
17	2577	16.9	-18	-246
16.5	2561	28.6	-16	-262

Barrel length in inches versus Muzzle velocity for Federal Gold Medal 168 Grain Match BTHP Rifleshooter.com
Barrel length inches	Federal 168 Gold Medal	SD	Change ft/sec velocity	Change ft/sec velocity from 28″
28	2706	7.8
27	2697	15.6	-9	-9
26	2673	21.4	-24	-33
25	2659	13.1	-14	-47
24	2635	11.6	-24	-71
23	2618	21.2	-17	-88
22	2597	12	-21	-109
21	2580	6.4	-17	-126
20	2565	7.2	-15	-141
19	2532	7.4	-33	-174
18	2523	8.3	-9	-183
17	2481	8.7	-42	-225
16.5	2466	15.1	-15	-240

Barrel length in inches versus Muzzle velocity for Winchester 180 Grain Power-Point Ammunition Rifleshooter.com
Barrel length inches	Winchester 180 PP	SD	Change ft/sec velocity	Change ft/sec velocity from 28″
28	2632	16.4
27	2607	26.1	-25	-25
26	2597	10.6	-10	-35
25	2585	8.8	-12	-47
24	2553	28	-32	-79
23	2553	24.5	0	-79
22	2527	14.6	-26	-105
21	2507	28.8	-20	-125
20	2478	18.8	-29	-154
19	2441	38.3	-37	-191
18	2411	35.9	-30	-221
17	2401	20.3	-10	-231
16.5	2373	30.4	-28	-259

Cutting the barrel from 28″ to 16.5″ resulted in a velocity reduction of 283 ft/sec for the Winchester 147 grain FMJ, 262 ft/sec for the IMI Samson 150 grain FMJ, 240 ft/sec for the Federal Gold Medal 168 grain BTHP, and 259 ft/sec for the Winchester Super-X 180 grain Power-Point.

Average velocity loss from 28″ to 16.5″, was 24.6 ft/sec per inch for the Winchester 147 grain FMJ, 22.8 ft/sec per inch for the IMI Samson 150 grain FMJ, 20.9 ft/sec for the Federal Gold Medal 168 grain BTHP per inch, and 22.5 ft/sec per inch for the Winchester Super-X 180 grain Power-Point.

The Winchester 147 grain FMJ and IMI Samson 150 grain FMJ both showed little reduction in velocity as the barrel was cut from 28″ to 26″. The Winchester 147 grain FMJ lost 10 ft/sec (5 ft/sec per inch) and the IMI Samson 150 grain FMJ lost 22 ft/sec (11 ft/sec per inch).

2/1/16- Short barrel information can be found at:

308 Winchester/ 7.62x51mm NATO Short barrel length and velocity- A six inch 308 bolt gun?

1/24/15- Complete data sets for each cartridge can be found in the following posts:

308 Winchester Barrel Length and Velocity: Winchester 147 grain FMJ

7.62x51mm NATO (308 Win) Barrel Length and Velocity: IMI Samson 150 grain FMJ

308 Winchester Barrel Length and Velocity: Federal 168 grain Gold Medal BTHP

308 Winchester Barrel Length and Velocity: Winchester Super X 180 grain Power-Point

How did barrel length change exterior ballistics?

We ran the 147, 150 and 168 grain loads for 28″, 24″, 20″ and 16.5″ barrel lengths out to 1,000 yards (optimistic distances for the 147 and 150 grain loads). Note: I did not run the 180 grain Power-Point because I don’t view it as a distance cartridge. These graphs assume a 1.75″ height of the optic over the bore and weather conditions of 59F.

Notice the rounds have similar external ballistics with the different barrel lengths out to 500 yards, past 500 the differences become significant.

How did barrel length affect Maximum Point Blank Range (MPBR)?

Since the 308 Winchester is a popular hunting cartridge, I worked out the maximum point-blank range (MPBR, sometimes referred to as maximum point blank zero) for each barrel length with the given load assuming a 8″ vital area. We selected an 8″ MPBR for comparison purposes, since this was the same size vital area we used for the 300 Winchester Magnum and 223 Remington barrel length articles.

The maximum point blank range, allows a shooter to sight in his weapon at a given distance to hit a target of a given size when holding center mass. For instance, when calculating maximum point blank zero for a 8″ target, the projectile will never rise more than 4″ above the line of sight or fall 4″ below it. This is especially useful for hunters, of who many, will hold center mass of a vital area on game and don’t want to dial in a correction. My calculations assume a 1.75″ sight over bore height.

308 Winchester Maximum Point Blank Range (MPBR) in yards for 8” Target
Barrel length	Win 147 FMJ	IMI Samson 150 FMJ	Federal GM 168 BTHP	Win 180 Power-Point
28”	325	311	299	288
16.5”	296	284	274	261
Change in MPBR	-29	-27	-25	-27

Note the change in MPBR averages 27 yards with an 11.5 inch change in barrel length.

Closing

What are possible sources of error in your experiment?

Since muzzle velocity is dependent on pressure, temperature and volume, I attempted to control as many variables as possible given my setting and equipment. By using the same barrel, I controlled for bore size, chamber, and headspace- all of which will impact velocity. Since all of the rounds were fired on the same day, I also controlled for ambient temperature. I did not control for barrel temperature. The barrel did heat up during firing. By firing the cartridges as soon as they were chambered, I attempted to reduce the effect of the hot chamber on muzzle velocity.

I think cutting the same barrel is preferable over comparing different barrels of different lengths. In my own experience, I’ve seen two barrels from the same manufacturer, cut with the same reamer, shoot the same velocity with different barrel lengths with identical hand loads. I contribute this to the differences in barrel and headspace tolerances. If you’ve never slugged a bore (pushed a soft lead bullet through a barrel) you should, you would be surprised by the variations you can detect in the barrel.

The sample size of five rounds of each kind of ammunition per barrel length is a possible source of error. However, testing indicates it may not be as much as initially thought. I fired 30 rounds of IMI Samson 150 grain FMJ at 28″ and 16.5″ and recorded the results. Comparing the data from the 30 shot strings (28″ 2824 and 16.5″ 2555) to the 5 shot strings (28″ 2823 and 16.5″ 2561) I found a loss of 269 ft/sec (23.4 ft/sec per inch) as the barrel was cut. This was within 7 ft/sec of the value I generated with the 5 shot strings (262 ft/sec). Velocity loss per inch of barrel was .6 ft/sec away (22.8 ft/sec) from the value calculated with 5 shot strings.

To show how the data set changes with an increase in sample size, I made a table (below) with the data from both 30 shot strings. The “shot” column represents the shot number in the respective string. “28” barrel ft/sec” and “16.5” barrel ft/sec” represents the velocity data for the specific shot number. “AVG 28″ ft/sec” and “AVG 16.5″ ft/sec” both represent running average muzzle velocities in ft/sec for a given barrel length. “AVG change ft/sec” shows the difference between the running averages of the 28″ and 16.5″ barrels. “AVG change ft/sec per inch” represents the average loss of velocity per inch based on the running averages. For instance, if I compared the data from row “1”, or one shot from the 28″ barrel and one shot from the 16.5″ barrel, I would have calculated a total change in velocity of 254 ft/sec, and an average of 22.1 ft/sec per inch. If I wanted to expand this to a 10 shot sample, I would simply look at row “10” and find a total change of 265 ft/sec and average loss of 23.0 ft/sec per inch of barrel. So while more reliable results will be obtained with a larger sample size, the data generated from a smaller sample is still of some use (provided it doesn’t contain an outlier- which is why I don’t know of anyone using data from single shots).

308 Winchester/ 7.62x51mm NATO Comparison of velocity data Rifleshooter.com
Shot	28″ barrel ft/sec	AVG 28″ barrel ft/sec	16.5″ barrel ft/sec	AVG 16.5″ barrel ft/sec	AVG change ft/sec	AVG change ft/sec per inch
1	2835	2835	2581	2581	254	22.1
2	2814	2825	2533	2557	268	23.3
3	2821	2823	2541	2552	272	23.6
4	2823	2823	2551	2552	272	23.6
5	2824	2823	2601	2561	262	22.8
6	2834	2825	2572	2563	262	22.8
7	2811	2823	2587	2570	252	21.9
8	2816	2822	2546	2564	258	22.5
9	2821	2822	2545	2562	260	22.6
10	2827	2823	2520	2558	265	23.0
11	2835	2824	2584	2560	264	22.9
12	2820	2823	2592	2563	261	22.7
13	2825	2824	2554	2562	261	22.7
14	2820	2823	2551	2561	262	22.8
15	2842	2825	2585	2563	262	22.8
16	2833	2825	2573	2564	262	22.7
17	2825	2825	2540	2562	263	22.9
18	2813	2824	2492	2558	266	23.1
19	2791	2823	2550	2558	265	23.0
20	2797	2821	2546	2557	264	23.0
21	2836	2822	2567	2558	264	23.0
22	2850	2823	2541	2557	266	23.2
23	2826	2823	2559	2557	266	23.2
24	2842	2824	2478	2554	271	23.5
25	2838	2825	2537	2553	272	23.6
26	2831	2825	2569	2554	271	23.6
27	2842	2826	2601	2555	270	23.5
28	2833	2826	2534	2555	271	23.6
29	2796	2825	2578	2555	269	23.4
30	2810	2824	2536	2555	270	23.4

Why did you pick these cartridges?

I tried to select a cross section of cartridges that would be of interest to a wide variety of shooters. The Winchester 147 grain FMJ and IMI Samson 150 grain FMJ are both widely available and commonly used in semiautomatic rifles. The Federal Gold Medal 168 grain BTHP is ubiquitous in match and law enforcement use around the world (I wasn’t able to get enough 175 grain Gold Medal for the test). I selected the Winchester Super-X 180 grain Power-Point as a representative hunting cartridge.

How did your findings compare to a reloading manual?

They were fairly close. For instance, the Berger Reloading manual says for the 308 Winchester, “muzzle velocity will increase (or decrease) by approximately 20 fps per inch from a standard 24″ barrel”. My data showed an average change of 27 ft/sec per inch of barrel.

Did you shoot any groups?

No, I did not. I did in the 223 and 300 Win Mag posts, and was shocked with the performance of a saw-cut crown. Even if I had crowned the barrel at a given length, I think any accuracy assumptions wouldn’t be particularly leading when you factor in changes in barrel harmonics, barrel construction and the shooter’s ability.

Why didn’t you crown the barrel?

Time. My lathe is a two hour round trip to the range. Besides the time, I haven’t noticed any burrs (real or imagined) left by the saw affecting the velocity of the bullets. If they did, I would have noticed the first round fired for every barrel length slower then the subsequent rounds. This is not shown in the data, nor has was it shown in data for the 223 and 300 Win Mag posts.

Why didn’t you discuss the difference between 308 Winchester and 7.62×51 mm NATO?

It is outside the scope of this post. There are plenty of good sources of information on this topic. The IMI Samson 150 grain FMJ is a 7.62x51mm NATO spec cartridge.

What’s your thought on short barrel 308 Winchesters?

I like them- in some ways you don’t lose a lot. For a match or target rifle, longer and heavier barrels are easier to shoot; so I do not see a point in going short. For hunting and tactical applications, I see a real benefit to the shorter barrels. If you take a look at the series posts (below) on my 16.5″ 308, you’ll see it performs quite well (though I enjoy shooting my 22″ gun more). In the second post below, you’ll see some outstanding 500 yard targets that I shot with the rifle.

Super Short Precision Rifles: Is there such thing as a 16.5″ .308 Tactical Bolt Action Rifle?

Short Rifle, Long Range: Testing our 16.5″ 308 Remington 700 out to 635 yards,

16.5 Inch 308 Winchester Precision Rifle: Summary of a Short Rifle

My 16.5″ 308 Winchester (above) is a fun little rifle.

My 22″ 308 Winchester (above) is my favorite rifle to shoot.

How did you measure your headspace within .001″? I have a go and a no-go gauge, that’s it.

I use a Forster Match Rife Headspace Gauge Kit. It includes gauges in .001″ increments.

Are you aware of any similar studies?

I’ve referenced a few other 223 Remington studies in my prior posts. This is what I’ve been able to find on 308 Winchester:

In “Barrel length and the precision rifle” the author makes some claims about 308 Winchester and 300 Win Mag which aren’t supported by a data set. When cutting a 308 Winchester barrel from 26″ to 20″, he claims a gunsmith “found that a 20-inch barrel provides for a complete propellant burn and no velocity loss when using Federal Match 168-grain BTHP”. I think he may be misquoting his source. Maybe he meant negligible?

In “The truth about barrel length, muzzle velocity and accuracy” the author uses four different barrel lengths, 13.5″, 16″, 18″ and 26.5″, and eight different cartridges. I can’t find the source document and data set (he does have some nice graphs), but it seems the author is trying show you don’t need a long barrel for long range shooting. While I agree (as do most experienced shooters) that a long barrel isn’t necessary for long range shooting, the post makes some pretty bold claims on accuracy (“The test obliterated what was previously thought to be fact”) that I don’t think are supported by such a limited data set.

“Effect of Barrel Length on the Muzzle Velocity and Report from a Mosin-Nagant 7.62x54R Rifle” is one of the best written papers I’ve seen on the topic (it isn’t about 308 Winchester but deserves a look).

How much did muzzle blast change?

The rifle with a shorter barrel was louder. This was noticeable from 19 inches and below. Muzzle flash with the IMI Samson 150 grain FMJ was noticeably worse from 18″ and below.

So you built a custom rifle just to cut it apart?

Yes, I guess I did. I got the longest blank I could and went from there. Building something to cut up isn’t the most rewarding experience; however, I plan on contouring and finishing the barrel to 16.25″ (that’s why I stopped at 16.5″).

UPDATE 12/30/14: I tapered the barrel and installed a brake. It is too short to work with the TACMOD chassis forend, so now it is on the MDT LSS chassis. The bottom picture is the first group I shot with the rifle, 5-shots, .785″ including cold bore. Here are some pics:

Why are you destroying a rifle? Wasting bullets?

I get this every so often. I am cutting up a barrel. The rifle is fine. I will actually contour what is left and use it on a custom build. I think data sets are great. No guessing, no models, no arbitrary numbers. I think the value of this data is worth the time, effort and expense. I post it here, for free, so other shooters can learn from it.

I see you upgraded from a hacksaw?

Yes, I did for the 223 post. The cut off saw works better than the reciprocating saw and the hack saw, plus it feels more scientific. I may wear a lab coat to kick things up another notch next time.

Do you like the TACMOD chassis system?

Yes, I am very impressed. Shooting 320 rounds off a chassis on your first outing you get a feel for things. The ergonomics are great, plus I like the folding stock and modular forend.