Why are 9mm and 38.37 twist ratios different?

Share your experience in caring for your favorite Rossi with other members or ask the question that you cannot find the answer to!

Why are 9mm and 38.37 twist ratios different?

Postby dalek » 24 Mar 2020 05

I have seen the same bullets listed to both calibers, or being not very far off (356 vs 357/358) from each other in diameter. However, 1:10 seems to be the standard twist ratio for 9x19 rounds while for 357/38 special the ratio ranges between 1:20. Rossi M92 being in the later group. I have been told in kitty school that bullet weight and length determines twist ratio for a given diameter. Am I missing something?
dalek
 
Posts: 130
Joined: 22 Nov 2015
Has thanked: 1 time
Been thanked: 30 times
My Rossi Choice: Ranch Hand
Location: In the mudness

Re: Why are 9mm and 38.37 twist ratios different?

Postby Daryll » 24 Mar 2020 06

I think you've answered your own question there... bullet weight...

9mm is normally 115 or 124 gns, and quite a short bullet.... .38/.357 is normally 158gn and above...
Daryll
 
Posts: 9
Joined: 27 Mar 2019
Has thanked: 0 time
Been thanked: 2 times
My Rossi Choice: M92
Location: Cambridgeshire UK

Re: Why are 9mm and 38.37 twist ratios different?

Postby Ohio3Wheels » 24 Mar 2020 10

Yep, bullet weight as reflected in the length. And it's all black magic anyway :D

Make smoke,
Curt... makin' smoke and raising my carbon foot print one cartridge at a time Image
Ohio3Wheels
Supporter
Supporter
 
Posts: 1405
Joined: 31 Jul 2014
Has thanked: 212 times
Been thanked: 269 times
My Rossi Choice: M92
Location: Dayton Oh

Re: Why are 9mm and 38.37 twist ratios different?

Postby Nashville Stage » 24 Mar 2020 12

That's a great question, and I'm not really sure of the answer. The calculations for spin-stabilizing a bullet seem to include equal parts art, science, and specific application.

For example, the M16 was initially designed with a twist rate of 1:14", which was *just* fast enough to keep the bullet stable in flight through air, but allowed it to tumble as soon as it struck an enemy soldier.

However, the twist was increased slightly to 1:12" soon after. When used in very cold weather conditions, the bullets had a lower velocity and therefore had a slower spin, making the bullet start to tumble sooner and lose accuracy.

When the standard bullet changed from M192 to the much longer & less dense M855, the twist rate increased again to 1:7"

I'm amazed at how bullets remain intact from the massive forces that are placed on them, and not just acceleration, either. If you run the calculations, a .223 bullet that's fired from a rifle at 3,000 feet/second through a 1:7" twist barrel is rotating at over 300,000 RPM!
Nashville Stage
Supporter
Supporter
 
Posts: 104
Joined: 08 Feb 2018
Has thanked: 5 times
Been thanked: 56 times
My Rossi Choice: M92
Location: Nashville, TN

Re: Why are 9mm and 38.37 twist ratios different?

Postby dalek » 24 Mar 2020 22

Daryll wrote:I think you've answered your own question there... bullet weight...

9mm is normally 115 or 124 gns, and quite a short bullet.... .38/.357 is normally 158gn and above...


But the so-called defense rounds, such as Remington HTP .357 Magnum 110 Grain SJHP and Remington Golden Saber .357 Magnum 125 Grain JHP, have lighter bullets. So we are in the same domain as common 9mm (115) and NATO (124)

Nashville Stage wrote:THowever, the twist was increased slightly to 1:12" soon after. When used in very cold weather conditions, the bullets had a lower velocity and therefore had a slower spin, making the bullet start to tumble sooner and lose accuracy.

When the standard bullet changed from M192 to the much longer & less dense M855, the twist rate increased again to 1:7"


So, if we assume the Rossi M92 was designed to shoot 158gr and heavier bullets, shouldn't its twist ration be higher than the 1:20, being closer to the 1:10 from 9mm?
dalek
 
Posts: 130
Joined: 22 Nov 2015
Has thanked: 1 time
Been thanked: 30 times
My Rossi Choice: Ranch Hand
Location: In the mudness

Re: Why are 9mm and 38.37 twist ratios different?

Postby Ranch Dog » 25 Mar 2020 04

The current catalog does not list the rate of twist for the 92, but it has always been 1:30.
Michael
Image
User avatar
Ranch Dog
Site Admin
Site Admin
 
Posts: 9174
Joined: 23 Jan 2012
Has thanked: 1672 times
Been thanked: 2177 times
My Rossi Choice: Rio Grande
Location: Cuero, TX

Re: Why are 9mm and 38.37 twist ratios different?

Postby GasGuzzler » 25 Mar 2020 05

Remington HTP .357 Magnum 110 Grain SJHP and Remington Golden Saber .357 Magnum 125 Grain JHP are an insult to the .357 cartridge. They are also moving faster than a 9X19.
__________________________________________________________________________________________
I've always been crazy but it's kept me from going insane.
User avatar
GasGuzzler
Supporter
Supporter
 
Posts: 2226
Joined: 02 Nov 2015
Has thanked: 218 times
Been thanked: 304 times
My Rossi Choice: Revolver
Location: Cooke County, TX


Return to DIY Gunsmithing, Projects, & Tips!

Who is online

Users browsing this forum: No registered users and 1 guest

  • Advertisement