Twist rates and non-toxic bullets

Donnachaidh

Well-Known Member
Probably a silly question, but is it the weight or the length of a bullet which affects it's performance with different twist rates? I'm asking because non toxic bullets are generally lighter than a similar length lead bullet, and I'm trying to work out which weight should (in theory) suit my rifle; a 6.5x55 with a 1 in 8 inch twist.
 

Mossypaw

Well-Known Member
Looking forward to the replies, I was planning a similar post, .308 with 1 in 8 twist though. Are longer bullets more stable with a fast twist ?
 

srvet

Well-Known Member
Probably a silly question, but is it the weight or the length of a bullet which affects it's performance with different twist rates? I'm asking because non toxic bullets are generally lighter than a similar length lead bullet, and I'm trying to work out which weight should (in theory) suit my rifle; a 6.5x55 with a 1 in 8 inch twist.
I have shot 100gr and 120grain Barnes TTSX out of my 1/8.25 in 6.5. You will be fine with these weights. There is a bullet stability calculator on the Berger bullets site. I think you need bullet length, velocity and rate of twist for it
 

Swedish

Well-Known Member
Its the length. I also shoot 6.5x55 with a 24inch barrel and 8inch twist rate. I've recently had an email exchange with the mighty Gerard Schultz of GS Custom Bullets and he confirmed that length is the factor that determines bullet stability rather than weight. I had been using Lapua Mega 155gr bullets but after a recommendation from a mate have decided to try GS Custom for a copper bullet. It turns out that the most suitable bullet from them for my needs is a much lighter 110gr HV bullet (265110HV072) This large decrease in mass was the reason for my conversation with Gerard. The lighter GS custom bullet is also slightly longer than the Lapua Mega and that is what counts.
 

bluesako

Well-Known Member
i had a 17rem barrell in 1/10 twist it wouldnt stabilize 25 gr v max, they would tumble, but with 25gr bergers it would shoot great, i was told by a shooting buddy in the states its not the weight, its the length, and this guy is a top shooter reloader, bs.
 

Donnachaidh

Well-Known Member
Thanks for the replies; I believe I can get hold of some RWS Evolution Green which are 6.0 grams / 92.6 grains in 6.5x55, so sound very light in comparison to my usual 140grain lead ammunition but will give it a try.
 

stevenedwards

Well-Known Member
I use a 6.5x55 and have had no luck getting Hornady lead fee to shoot well but I have been able to load TTSX 120gr to group quite well but not as well as my usual Sierras
 

stevenedwards

Well-Known Member
Been worrying about this thread. Sorted out my twist rate for the 6.5x55 which is 1: 8.66, the barrel is 22 inches
The Berger stability calculator gives a value of 1.37, marginally unstable for 120gr TTSX. Is this why I can't replicate the accuracy I get with 120gr NBT, 1.73 stable and 125gr partitions 2.04 very stable. Both Berger and Barnes say I need a minimum of 1:8 for 120gr TTSX. The difference seems small but is it as the results seem to correspond with the prediction. The Hornady 120 GMX are all over the place, a pattern rather than group. I am trying to go lead free but it hasn't worked so far. Laurie where are you.
 

Jon Smith

Well-Known Member
It's the weight your stabilising, longer bullet will have more rifling contact to. Can't find own load data or BC's for Fox?
Wrong, never been about 'rifling contact', it is the bullet length - longer bullet needs more twist - it's a proportion relating to barrel bore and bullet length.
 

devonair

Well-Known Member
Very useful tool, thanks for posting.
I was contemplating making up some subsonic loads for my .308, 1:11"barrel.
165gn, 180gn and 200gn.
 

Edinburgh Rifles

Well-Known Member
Don't trust the twist rate calculator
It's an OK guide but not definitive
Lots of bullets it says won't stabilise can and vice versa from experience

6.5x55 already has a twist rate that can accommodate very very long bullets
We produce non lead, swede ammo from 100gr right through to 139gr which is one of the few in the sweight class on the market for the cartridge

On the whole most cartridges will accommodate similar weights to standard factory loads and maintain accuracy
Where it becomes tricky is in 224 1:14" and 243 1:10" attempting to maintain 50gr and 100gr stability

They are just too long
 

Rusty Gate

Well-Known Member
Wrong, never been about 'rifling contact', it is the bullet length - longer bullet needs more twist - it's a proportion relating to barrel bore and bullet length.
His question was “ lighter than a similar length lead bullet “ not longer, so you would be stabilising the weight.

Plenty of other helpful comments here for OP to find useful points of reference online etc.
 

Andy-shooter

Well-Known Member
For anyone who wanted to REALLY get into this kind of thing (and who is not bad at maths) this paper is very interesting.


Section 8 deals with the Mcoy model for gyroscopic stability but as can be seen in section 9, there is a bullet geometry element to the statistical stability as well... basically some bullets are inherently more stable than others because the center or aerodynamic pressure is close to or behind the center of mass (like and airgun pellet)

Dust off your A-levels and enjoy...
 

Laurie

Well-Known Member
In simple gyroscopic stability terms, bullet length (in relation to calibre) is the primary factor determining the spin rate needed for a given shape ('form') of bullet. Weight does have an effect, but actually in the opposite direction from what most people believe - if you have two say 1.3-inch OAL BT bullets in 0.264, but one weighs 108gn and the other 140gn, the heavier example is fully stabilised with a slightly slower spin / twist rate. The convention about such and such a weight needs an x or y twist rate in a calibre never was accurate but had some force to it when all bullets had similar shapes and a heavier model also meant it was a longer model. We have such a large range of types and shapes today that using weight as a guide can be very misleading.

On the twist rate calculators which all use the Millers formulae, note that the equations in it are 1) designed for bullets of a particular density, ie that obtained with a 'normal' lead or lead core type. They won't be 100% accurate for all copper bullets with a much lower density. (still gives a useful pointer though). ........ and 2) the Miller calculator is set up for boat-tailed bullets and is inaccurate for flat-base designs. A BT tail section bullet needs more spin for full stabilisation. As a rough guide add one inch to whatever the calculator says so that if it says you need 10-inch twist, 11 will be fine. (If you go on the Berger website stability calculator section, this notice / link appears in bold red at the top of the page:

» CLICK HERE FOR AN IMPORTANT NOTE ABOUT FLAT BASE SG CALCULATIONS

and there is then a short notice saying the calculator is no good for them with a second link to a table of twist rates required for Berger's own small range of flat-base models.)

Of course, this raises issues of just what is a BT design as many short-range / hunting models are officially BT but really only have a vestigial version, little other than a small bevel that aids bullet seating and has little effect on the aerodynamics. Miller will likely overstate the need here too.

With all-copper designs being less dense and longer and with longer bearing surfaces that generate more friction / pressure for a bullet weight (but this now overcome in Barnes banded / grooved designs), the 'traditional' rule and still a good one is to drop one weight category from whatever you'd use in a lead-core bullet, 140gn 6.5 reducing to 130 or even 120gn. The other thing to do is look at what the manufacturer recommends of course. ,............. so he gets out his copy of Barnes Reloading Manual #4. ............ and can he find recommended twists, well no, but I'm sure I saw it there somewhere. Online isn't much more helpful either.
 

Edinburgh Rifles

Well-Known Member
The "drop a weight class" idea with many non lead bullet manufacturers was to maintain a higher terminal velocity to aid expansion which was proving to be unreliable in the higher BC bullets of old.
The other tactics to aid expansion was to drive them faster as per Hornady GMX in Superformance form, to tip them as per the move from TSX to TTSX (which ironically has little to do with improving expansion unless combined with a large meplat, larger hollow point, thinner jacket (cup and core) or softer material construction.

There is no real stability limitation on the majority of non lead bullets as very few of the cartridges are close to weight/length/twist stability (apart from 243, 222 in Deer legal form)

non lead is available and stable in :
.222 (1:14") 45gr
.222 (1:12") 50gr
.223 - 50-60gr
.243 - 80gr
.25-06 - 100gr
260/6.5x?? - 139gr
7mm 145-150gr
308/300wm - 130-165-180gr
8mm 165gr-180gr
.338 - 220gr
9.3mm 250gr

The driving bands are more often than not of a removed material variety, 3-5 relief cuts in the shank(or stamped in the case of Barnes)
This has been significantly improved with designs like the Hasler relief bands, the shank is lower than the bands
These bands are the only points in contact with the barrel.
Velocities are increased over predicted when compared to like for like charge and bullet weights

for example:
Quickload data for 130gr Fox Bullet, 110gr TTSX and 130gr GMX ,270 were within 40fps of actual perceived Muzzle velocity.

With Hasler the 112gr actual velocity was 235fps above the predicted velocity in a standard 24" barrel.
 

Treedave

Well-Known Member
Ive been using non toxic monolithic bullets for a while now. Having tried various factory offerings and reloaded them myself, I’ve ended up using GSCustom.
They’ve data galore and plains game experience to back it up. They have the drive band technology and are used in a ‘light for calibre’ format.
There are those that say fast & light is not the way to use non toxic bullets, and I’m sure there are designs that don’t need it or benefit from it. I know that a 95 gr gscustom bullet from a 260 rem pushed at about 3000 fps gives 5” of drop at 250m on a 160m zero. I also know that the terminal performance is more than satisfactory on the largest of our red deer.

For those that are interested, a snippet of the data about what and how to use them
 

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