9MM x 19 Barrel Twist and Bullet Weight?


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BlindJustice
July 24, 2008, 09:46 PM
I read a brief mention once about the CZ 75B and variants in
9MM Luger having a higher rate of twist 1 in 9.7 inches & it handles
heavier bullet weights like 147 gr. better than the regular barrel twist.

What's the 'standard' twist for service pistols with 124 gr. bullet weight
being the issue round?

Randall

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Gun Slinger
July 25, 2008, 12:13 PM
BlindJustice,

It is typical for most European (esp. German, Austrian) manufactured 9mm pistols to have a twist rate of 1 turn in 9.84 in. (~25 cm). Glocks, CZs, XDs and HKs have this rate of twist, IIRC.

The second and third generation S&W steel/aluminum framed semi-autos (3900s and 5900s) had a twist rate of 1 turn in 16 in. and I believe that the current crop of the S&W polymer framed M&P series also have this same rate of twist.

Typically, heavier (therefore longer) bullets require a faster rate of twist (spin) in order to stabilize them and lighter (shorter) bullets require less spin imparted to them from the rifling in order to achieve the gyroscopic stabilization necessary for balanced flight.

You can also use the Greenhill Formula to determine the minimum required rate of spin for a particular bullet weight (and length).

Since you may not have those lengths available to you, I've provided them from pulled FMJ ammunition (S&B 115 gr. and 124 gr.) and from pulled Hornady 147 gr. XTP "CQ" JHPs below:

"L" in inches for common, various 9mm bullets:

S&B 115 gr. FMJ: 0.590"

S&B 124 gr. FMJ: 0.608"

Hornady 147 gr. XTP JHP: 0.657"


The Greenhill Formula is:

150 (D) L = length of minimum required twist in expressed in inches.

where D is the diameter in decimal inches of the bullet and L is the length of the bullet in decimal inches.

so for example:


A 9mm 147 gr. JHP having a diameter of 0.3545" and a length of 0.657" will need at least:


150 (0.3545") 0.657" = 1 turn in 28.6918 inches in order to be properly stabilized.



So you can see that even the longest 9mm bullets can/will be adequately stabilized by even the rifling pitch of 1:16 of the S&W autoloaders.

The 124 gr. FMJ would need a twist rate of at least 1 turn per 31.00 inches of travel in order to achieve proper gyroscopic stabilization.

Hope this helps you out,

mainmech48
July 25, 2008, 01:20 PM
As a practical matter, while all of the 9x19's I currently own have their own preferences they all shoot at least acceptably well with all three of the most common weights.

I've never had a "keyhole" or other sign of gross instability with any. The biggest difference is in POI relative to POA and most of that is in elevation.

The fixed sights on two of mine are regulated for 115 gr. loads and one for 124 gr. Either put 147 gr. slugs close enough to where it's pointed to still be acceptable for SD at short range, say to 10 or 15 yds.

Other pistols may vary, but in mine it seems to be pretty much a non-issue.

kartoffel
July 25, 2008, 02:13 PM
Steyr M9's have a 205mm (1:9.8 in.) twist. Steyr M40's and M357's have a 406mm (1:16 in.) twist. Both work fine.

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