April 24th, 2016

.308 Win Barrel Chop Test: How Velocity Changes with Length

rifleshooter.com barrel .308 win cut-down test saw ammo GMM velocity magnetospeed

With barrels, one wonders “Can a little more length provide a meaningful velocity gain?” To answer that question, Rifleshooter.com performed an interesting test, cutting a .308 Win barrel from 28″ all the way down to 16.5″. The cuts were made in one-inch intervals with a rotary saw. At each cut length, velocity was measured with a Magnetospeed chronograph. To make the test even more interesting, four different types of .308 Win factory ammunition were chronographed at each barrel length.

rifleshooter.com barrel .308 win cut-down test saw ammo GMM velocity magnetospeed

READ RifleShooter.com .308 Win Barrel Cut-Down Test Article.

Test Barrel Lost 22.7 FPS Per Inch (.308 Win Chambering)
How much velocity do you think was lost, on average, for each 1″ reduction in barrel length? The answer may surprise you. With a barrel reduction from 28″ to 16.5″, the average speed loss of the four types of .308 ammo was 261 fps total. That works out to an average loss of 22.7 fps per inch. This chart shows velocity changes for all four ammo varieties:

rifleshooter.com barrel .308 win cut-down test saw ammo GMM velocity magnetospeed

Summary of Findings: The average velocity loss per inch, for all four ammo types combined, was 22.7 FPS. By ammo type, the average loss per inch was: 24.6 (Win 147 FMJ), 22.8 (IMI 150 FMJ), 20.9 (Fed GMM 168gr), and 22.5 (Win 180PP).

Interestingly, these numbers jive pretty well with estimates found in reloading manuals. The testers observed: “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’.”

How the Test Was Done
The testers described their procedure as follows: “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 were logged for each round. Since we would be gathering data on 52 different barrel length and ammunition combinations and would not be crowning the barrel after each cut, we 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 47° F.”

rifleshooter.com barrel .308 win cut-down test saw ammo GMM velocity magnetospeed

CLICK HERE to Read the Rifleshooter.com Test. This includes detailed charts with inch-by-inch velocity numbers, multiple line charts, and complete data sets for each type of ammo. Rifleshooter.com also offers ballistics graphs showing trajectories with different barrel lengths. All in all, this was a very thorough test by the folks at RifleShooter.com.

Much Different Results with 6mmBR and a Longer Barrel
The results from Rifleshooter.com’s .308 barrel cut-down test are quite different than the results we recorded some years ago with a barrel chambered for the 6mmBR cartridge. When we cut our 6mmBR barrel down from 33″ to 28″, we only lost about 8 FPS per inch. Obviously this is a different cartridge type, but also our 6mmBR barrel end length was 5″ longer than Rifleshooter.com’s .308 Win start length. Velocity loss can be more extreme with shorter barrel lengths (and bigger cartridges). Powder burn rates can also make a difference.

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April 24th, 2016

Tubb T7T Two-Stage Trigger for Remington-Type Actions

David Tubb two-stage 2-stage T7T trigger Rem 700 Remington clone high power

You probably know by now that David Tubb has introduced a new two-stage trigger for Remington 700 rifles, the Model T7T. Priced at $385.00, the new T7T Trigger is a true two-stage design: first and second stages are independently adjustable for both weight and feel. Overall (combined stage) trigger pull weight can be adjusted from just under 1 pound to slightly over 3.5 pounds.

If you are interested in the 7ZT, watch these two videos. The first shows how to adjust pull-weights for both stages. The second video shows how to modify your stock to provide clearance for the T7T.

This video shows how to adjust the new Tubb T7T two-stage trigger for Remington 700s:

The new T7T is a quality product. We’ve received good feedback from “early adopters” who tell us the two-stage function works well. While installation of the T7T may require minor modifications to your action, most gun owners with basic mechanical skills can install the T7T by themselves. The T7T is made using stainless steel and aluminum for maximum corrosion resistance. Note, installation of the T7T trigger (either Right-Hand or Left-Hand) also requires a companion T7T bolt release, offered by DavidTubb.com for $10.00.

David Tubb two-stage 2-stage T7T trigger Rem 700 Remington clone high power

This video shows how to modify your stock to fit the T7T trigger:

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