twist vs bullet weight

Rick65Cat

Rick65Cat

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Just a quick question...
Whats the formula? Is it *more* twist per inch needed for heavier bullets? Or is it more twist for lighter bullets? I can never remember when I read it in the posts here. (Gonna write it down)

I'm thinking of a Savage Mod. 12FV in .223 or .204 but want to go in the 32 to 40Gr. bullet range. I just want to be sure the gun I buy is going to work with my bullet choice.
 
From Shilen's website:

.204/20
- 9" for VLD bullets
- 10" for bullets up to 50gr.
- 12" for bullets up to 40gr.

.222 RF
- 10"* 60 gr. bullets Ratchet rifled 4 groove
- 14"* Twist for pistol barrels
- 15"* Special twist for rifle barrels
- 16" Standard twist for rifle barrels
- 16"* Ratchet rifled 4 groove
- 17"* Special twist for rifle barrels

.224 CF
- 7" for bullets heavier than 80gr.
- 8" for bullets up to 80gr.
- 8"* Ratchet rifled 4 groove
- 9" for bullets up to 70gr.
- 12: for bullets up to 63gr.
- 14" for bullets up to 55gr.
- 14"* Ratchet rifled 4 groove
- 15"* for bullets up to 55gr. driven 4,100 pfs or more
- 16:* for bullets up to 55gr. driven 4,300 fps or more
 
Its about length and speed:

GREENHILL FORMULA

FOR VELOCITIES UNDER 2,800 FPS
twist rate = (150 * bullet diameter squared) / bullet length
bullet length = (150 * bullet diameter squared) / twist rate

FOR VELOCITIES OVER 2,800 FPS
twist rate = (180 * bullet diameter squared) / bullet length
bullet length = (180 * bullet diameter squared) / twist rate

This is an excellent page that puts this into context and has info on many bullet makes

ht tp://stevespages.com/page8e.htm
 
Longer bullets are not alway heavier, i.e. Lapua .30 154gr. Scenar, which requires a quicker twist than 155gr. Sierra MK. EBV4 website is informative. Length in relation to diameter is more important than weight.

Regards,

Peter
 
dan belisle said:
Not always, which is why I said "longer". - dan
Click to expand...

I agree...

I made a mistake while experimenting making jacketed .17 caliber bullets back in 1967 ... while waiting for some pure lead wire to arrive I used a core made of 50/50 solder which is considerably lighter than lead... so I had a light bullet but long for it's length... It grouped about 6 inch groups at 100 yards and every bullet had gone through the paper sideways... later the same bullet only with a pure lead core and being the right weight for the length of bullet shot 1/2 inch groups...
 
Here's a basic formula:

Twist = 150 X D²/L

Where: D = bullet diameter in inches L= bullet length in inches

To see how this works out, assume you bought a .222 Remington rifle and you measured the twist. The twist was 1 in 14. You have two .224 bullets you want to use, the 70-grain Speer SPS and the 50-grain Hornady SX. The Speer bullet measures .812 inches in length and the Hornady measures .520 inches. Using the formula above we calculate the following twist rate:

Speer 70-grain: 1 in 9, Hornady 50-grain: 1 in 14

These calculations show that the 50-grain bullet will be stabilized, but the 70-grain won’t. Sure enough, when you try these bullets out, the 50-grain shoots ¾ MOA while the 70-grain won’t group on the paper at 50 yards. Twist is important!


The Greenhill forrmula is another one which can be downloaded.

Good luck - SD
 
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