7mm Remington Magnum (7 Rem Mag) Barrel Length Versus Velocity

7mm Remington Magnum (7 Rem Mag) Barrel Length Versus Velocity

Since its introduction in 1962, the 7mm Remington Magnum (7 Rem Mag) has developed a strong following, becoming the most popular metric hunting cartridge in the United States. Based on the 375 Holland and Holland, the 7 Remington Magnum case is shorter and fits in most commercially available long action receivers. It offers shooters a combination of velocity, sectional density and high ballistic coefficient in a package that has a manageable recoil impulse.

In the spirit of my last three 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, 300 Winchester Magnum: How Does Barrel Length Change Velocity- A 16″ 300 Win Mag?, and 308 Winchester / 7.62x51mm NATO: Barrel Length versus Velocity (28″ to 16.5″), I chambered a barrel for 7mm Remington Magnum and cut it back one inch at a time from 28″ to 20″.

The 7mm Remington Magnum is primarily a hunting cartridge, unlike the 223 Rem, 308 Winchester and 300 Winchester Magnum which are popular options for target shooting, tactical, and military applications. Because of this, the test will include three different hand loads as well as one factory offering.

WARNING: The loads shown are for informational purposes only. They are only safe in the rifle shown and may not be safe in yours. Consult appropriate load manuals prior to developing your own handloads. Rifleshooter.com and its authors, do not assume any responsibility, directly or indirectly for the safety of the readers attempting to follow any instructions or perform any of the tasks shown, or the use or misuse of any information contained herein, on this website.

The four different types of 7mm Remington Magnum ammunition tested were (left to right in photo above):

Federal Premium Vital Shock, 150 grain Sierra GameKing Boat Tail Soft Point (BTSP). Note: Sierra refers to this projectile as a Spitzer Boat Tail (SBT) in their catalog- for purposes of this post it will be referred to as a BTSP, the same designation given to it by the cartridge manufacturer.
160 grain Swift A-Frame, 66.0 grains Reloader 22, new Federal Nickel plated brass, Federal LRM primer, 3.29″ OAL. CAUTION: This load exceeds published maximums, only considered safe in the test gun.
165 grain Sierra GameKing Spitzer Boat Tail (SBT), 66.5 grains H4831SC, new Weatherby 7mm Remington Magnum brass, WLRM primer, 3.29″ OAL (Note: This bullet is not listed in the Sierra catalog, but is available from Federal in loaded ammunition, I purchased these in bulk 15 years ago). CAUTION: This load exceeds published maximums, only considered safe in the test gun
175 grain Sierra GameKing Spitzer Boat Tail (SBT), 60.0 grains Reloader 22, new Weatherby 7mm Remington Magnum brass, WLRM primer, 3.29″ OAL. CAUTION: This load is only considered safe in the test gun

Load #2, the 160 grain Swift A-Frame load, is my plains game hunting load for a Weatherby Custom Shop Mark V rifle I used in Africa back in 2001. Load #3, the 165 grain GameKing, is my friends hunting load for his Ultra-lightweight Weatherby Mark 5. Load #4 is a moderate load selected from a load manual using Reloader 22. Note the overall length (OAL) for each hand loaded cartridge was set to the SAMMI maximum of 3.29″. Charges were measured to .1 grain.

Loads #3 and #4 used Weatherby brand 7mm Remington Magnum brass- this isn’t a typo. Weatherby used to offer this brass a number of years ago and a friend managed to obtain some for his Mark 5 rifle chambered in 7mm Rem Mag- he sold the rifle and gave me the brass.

What kind of test rifle did you use?

The test rifle is a Remington 700 Magnum long action receiver in an Accuracy International AICS 2.0 chassis system with a Shilen 6-groove, 1:9 twist unturned stainless steel blank. The chassis is equipped with Viper Skins, giving it the appearance similar to the current AI AT chassis systems.

The rifle was built with the following components from Brownells:

The bolt was fluted by Kampfeld Custom.

The barrel was chambered for 7mm Remington Magnum with a SAMMI spec reamer from Dave Manson. Headspace was set at minimum. The first inch of the outside diameter of the barrel was turned concentric with the chamber, this is why you will notice the bright band of stainless steel in front of the rifle’s receiver.

Grooves were cut at one inch increments from 20″ to 28″ to be used as witness marks when cutting the barrel. Abrasive cloth was used to remove the patina from the outside of the barrel on alternating sections, creating the striped look above.

I stopped the test at the 20″ barrel length for three reasons:

1. Limited supply of ammunition and components

2. 7 Rem Mags are seldom seen with barrel lengths below 22″ (I haven’t seen many 16″ 7 Rem Mags around)

3. I plan on cutting off the chamber and turning the remainder of the barrel down to a sporter contour and chambering it in 7mm-08.

Test Protocol

Ballistic data was gathered using a Magnetospeed barrel mounted ballistic chronograph. At each barrel length, the rifle was fired from a bipod with rear bag. Five rounds of the Federal 150 grain load (load #1), four rounds of the 160 grain A-Frame (load #2) and 175 grain GameKing (load #4) and three rounds of the 165 grain GameKing (load #3) were fired at each barrel length. The number of rounds fired for the hand loads was limited by the components available. Average velocity and standard deviation was logged for each round. Since I would be gathering data on 36 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 and magazine were removed. The barrel was cut off using a cold saw. The test protocol was repeated for the next length. Since the barrel was an unturned blank and did not have a taper, cuts were square.

Temperature was 57F.

Results of experiment are found below:

Average velocities and standard deviation (SD) were calculated by the Magnetospeed barrel mounted ballistic chronograph.

As the barrel was cut from 28″ to 20″, average change in muzzle velocity for the four cartridges was 239.5 ft/sec. Average loss of velocity per inch was 26.6 ft/sec.

At 23″ in barrel length the rifle became noticeably louder. The 20″ barrel was unpleasant to shoot, the 165 grain GameKing load became very smokey, and the 175 grain GameKing load exhibited a lot of flash (I wrote “flash!!!” in my notes).

Individual cartridge data:

Load #1

The Federal 150 grain Vital Shock load lost 255 ft/sec as the barrel was cut from 28″ to 20″. The average loss of velocity was 28.3 ft/sec per inch of barrel. Significant loss of velocity occurred between 24″ and 23″, 60 ft/sec, and 23″ and 22″, 55 ft/sec. With this cartridge, the test rifle with a 22″ barrel, versus a 24″ barrel, lost 115 ft/sec (of 57.5 ft/sec per inch).

Load #2

The 160 grain A-Frame lost 219 ft/sec as the barrel was cut from 28″ to 20″ in length. Average velocity loss was 24.3 ft/sec per inch of barrel length. The largest losses in velocity occurred between 27″ and 26″ (44 ft/sec) and 21″ and 20″ (44 ft/sec). Between 28″ and 26″, the load lost 76 ft/sec (38 ft/sec per inch of barrel length), and between 22″ and 20″ the load lost 79 ft/sec (39.5 ft/sec per inch of barrel length). At 23″ this load became noticeably loud.

Load #3

The 165 grain GameKing lost 254 ft/sec as the barrel length decreased from 28″ to 20″. Average velocity loss was 28.2 ft/sec per inch of barrel length. Significant loss of velocity occurred between 21″ and 20″, 75 ft/sec, and between 24″ and 23″, 60 ft/sec. At 20″ this load produced a large amount of smoke.

Load #4

The 175 grain GameKing load lost 230 ft/sec of velocity as the barrel length was reduced from 28″ to 20″. Average loss of velocity was 25.6 ft/sec per inch of barrel length. Velocity loss of 105 ft/sec was measured between 21″ and 20″ of barrel length. This is the most significant reduction of velocity I have recorded in one inch of barrel length over four different cartridges. At 20″, the cartridge had a large amount of flash. Velocity loss over the rest of the barrel length was fairly consistent.

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

Since the 7mm Remington Magnum 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 (except the 165 SBT- I couldn’t find a BC for it) assuming a 8″ vital area. I selected an 8″ MPBR for comparison purposes, since this was the same size vital area used for the 300 Winchester Magnum , 223 Remington, and 308 Winchester 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.

Change in MPBR averaged 24.6 yards across the three cartridges.

Discussion

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 of three to 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. In my 308 experiment 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 dataRifleshooter.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

How do your results compare to QuickLOAD?

QuickLOAD is a ballistic modeling program designed to run on a PC. I run a Mac so I’ll I don’t own a copy. If you have QuickLOAD, and would like to run one of the cartridges above, please shoot me an email through the contact page so we can update this section of the post.

My graduate work dealt extensively with stochastic modeling 0f complex systems, we would often compare the models with an actual data set to see how closely they match. I’d be interested to see how this data set matches the modeled data set.

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.

What barrel length would you recommend for a 7 Rem Mag?

23″ or 24″. 24″ seems to be the the industry standard. Since the 7 Rem Mag is mostly a hunting cartridge and chambered in rifles meant to be carried in the field, weight and balance matter, and may figure into barrel length selection.

I am getting ready to build a custom 7 Rem Mag hunting rifle for a post and will most likely make it a 23″. This seems to give the best compromise of performance, weight and balance. Unlike the 308 and 223, you do seem to lose quite a bit as the rifle gets shooter and becomes significantly more unpleasant to shoot. So it appears, most of the hunting rifle manufacturers have been spot on with barrel lengths.

What are your thoughts on the 7 Rem Mag as a hunting rifle?

I’ve hunted with the 7 Rem Mag in North and South America as well as Africa. It is my favorite medium game cartridge and I like the mix of power, sectional density, and manageable recoil. While the 280 Ackley Improved (280AI) comes close to 7 Rem Mag performance (without a belted case), ammunition is harder to come by, giving the 7 Rem Mag a distinct advantage if traveling and your ammunition turns up missing.

While many hunters seem happy with the 140-150 grain loads, I prefer the heavier 160-175 grain loads. I’ve taken 4 species of game with the 160 A-Frame and found it too sturdily constructed for lighter game, often passing through without expansion. The non bonded, 160-175 grain offerings have always performed well. I would recommend 160 grain Sierra GameKing, 160 grain Nosler Partition, 165 grain Sierra GameKing, 175 grain Hornady Interlock, 175 grain Sierra GameKing and 175 grain Nosler Partition without reservation.