Bushmaster XM-15 M4 twist barrel?

Question

I recently purchased a Bushmaster XM-15 M4 Type Carbine and it says there is a 1 in 9" twist barrel. What exactly does that mean, what is it used for and how is that twist performed?

Any other useful information on this gun that I may need would be great too.

Thank you.

Answer 1

Instead of me explainig to you or trying to impress you with my smartness just take a look here.... Its the .223 cartridge guide. There is a ton of info on twist rates and its relation to bullet weight etc, etc, etc,,,,

Have a look and SAVE this page....

http://www.6mmbr.com/223Rem.html

Answer 2

Twist rate is the measurement of the riflings in your barrel. Your rifle has a 1 in 9 twist rate meaning that as the bullet enters the riflings it begins to spin, at nine inches in the barrel the bullet has made one revolution. If you have a 18 inch long barrel then the bullet will make two complete revolutions before it exits the barrel. The twist rate affects the accuracy and trajectory of the round as well. If you have a copy of the Speer reloading manual it has some good information on this item. Different manufactures may also use different twist rates in their weapons. Also some older rifles may have a different rate than a newer one.  

Answer 3

twist ratio is a spec that that describes the way your barrel is rifled.

1:9 means for every 9" inches of barrel length, the grooves in your barrel will have one full turn. this twist spins the bullet and stabilizes it in flight.. like a spiraling football

You can machine just about any ratio you like... but standard twists are calculated based on bullet weight, length and velocity.

Every caliber and bullet weight, and powder charge would have an optimum twist rate based on calculations.

So .."they" (manufacturers) pick a standard twist... and we shop for the load our rifle likes ..or tinker with handloads that favor our guns.

AR's are a great example for this discussion... Old Mil spec was 1:7.. for 55-62 grain bullets. 1:9 should produce better accuracy with bullets in the 70 grain weight range.

That being said... most AR's, in whatever combination of variables out there, will mechanically out shoot your or my ability to hold the sights on target.

If that's confusing enough ... two identical rifles will shoot the same rounds to a different point of impact... your rifle may be more accurate with a specific brand, bullet or powder charge than mine