Difference between .270 & 7mm bullet weights/Twist rates

**umshiniwam** · 23-02-2010, 11:00

Hi,

the difference between the .277" & .284" calibres is so tiny (0.007 inches = 0.1778 millimetres) yet the 7mm calibres can stabilise projectiles from 100 - 175 grains and the .270 is limited to 100 - 150 grains (and a Nosler 160 gr).

I presume that this is this due to the difference in twist rate (1 in 10 for the .270 & 1 in 9 for the 7mm calibres)? Can the .270 actually stabilise bullets of 175 grains? Does the 1 inch variance in twist rate make such a difference?

PS: The .30-06 also has a 1 in 10 twist rate ut can stabilise projectiles from 110 - 220 grains.

**Webster** · 23-02-2010, 22:42

This is a very interesting question. Rhino also make a 160gr .270 bullet and many guys are happy with it for the bush - not sure what twist they have in their rifles. Mine is a 1:10 and it struggles to stabilise 150gr bullets. This is not a problem because I believe the .270 is at its best with a 130gr bullet. 7mm is awesome too!

**obasch** · 24-02-2010, 06:43

Originally Posted by umshiniwam

Hi,

yet the 7mm calibres can stabilise projectiles from 100 - 175 grains and the .270 is limited to 100 - 150 grains (and a Nosler 160 gr).

PS: The .30-06 also has a 1 in 10 twist rate ut can stabilise projectiles from 110 - 220 grains.

Although the bullet selection for the 7mm range from 100gr to 190gr (Claw) and the 30-06 from 110-220, and even the 55gr Accellerator, it is unlikely that any one rifle will stabilise the whole spectrum of bullet weights.

**Arnold** · 24-02-2010, 08:56

I was also impressed by the variety of .30 cal bullets, now I use only one type bullet in my 308W ...

My next rifle might be a 270W, since Sierra makes a nice 135gr matchking. In 7mm you have to go up to 168gr for a comparable matchking and then you can't drive it at the same speed :(

That does not matter that much when hunting though and then the 7mm with its heavy bullets will have the advantage.

**Webster** · 24-02-2010, 09:06

Arnold - what you say makes enormous sense and I have been saying it for a while. It is all very well having a wide range of bullets to choose from but there is a downside too. You have to rezero each time and your gun might not stabilise the heavier or lighter bullets at the end of the spectrum on offer. A .270 with a 130gr or a 308 with a 155gr or a 243 with a 100gr bullet etc etc is a far better prospect and you can get to know your gun really well. Too much of a variety can be a mess!

**umshiniwam** · 24-02-2010, 15:04

Agree about the use of a single weight bullet loading for an individual rifle.

I am interested on the ballistic/technical aspect of my question though

**Sean Murphy** · 24-02-2010, 15:21

I think the greater variety of 7mm bullets is due to there being a greater number of cartridges available in 7mm with a wider range of twist rates.

As bullet diameter increases, less twist is required to stabilise the bullet as long as bullet length remains the same. Therefore, a .30 caliber with 1:10 twist will be able to stabilise longer bullets than a .270 with 1:10 twist. The longer, larger diameter bullets in the larger caliber will naturally be much heavier than the thinner, shorter bullets. This is a simplistic view of twist and stability as velocity also comes into play but the basic rules apply.

Sean.