OzFclass

My question is, does changing twist rate necessitate a change in powder load? Example going from a 9T to a 8T need a reduction in powder charge owing to the extra resistance in the tighter twist.

Different barrels of the same twist don't all shoot the same load so i would imagine a different barrel with a different twist is likely going to sing to a different tune. It might not necessarily be a reduction though.
I have a 1:12 that takes 45.5 of 08 and a 1:10 that likes 45.6. Both shooting 155.5s

Tim L wrote:Different barrels of the same twist don't all shoot the same load so i would imagine a different barrel with a different twist is likely going to sing to a different tune. It might not necessarily be a reduction though.
I have a 1:12 that takes 45.5 of 08 and a 1:10 that likes 45.6. Both shooting 155.5s

What he said !

Testing in terms of FPS has shown that going from an 9 to an 8 twist (as an example) is minor in terms of FPS change (I would have to do some digging to find the actual numbers). So what the others said - its more about what the barrel likes.

Old Trev-39 wrote:My question is, does changing twist rate necessitate a change in powder load? Example going from a 9T to a 8T need a reduction in powder charge owing to the extra resistance in the tighter twist.

I believe that any change is lost in the "noise" of barrel to barrel variations in velocity with any given powder charge. In other words there are too many other variables to nail down a perceived velocity difference due to twist rate.
Keith H.

the thing to watch for might be an increase in pressure more so than a change in velocity as twist steepens.
bruce.

I thought as Bruce said about the pressure increase. I have previously ran 52.2 / 09 behind 175 M/K,s in 9T barrels in my .284W. I thought that I would ask the question as I have recently installed a 8T and have experienced a change in bolt lift, not exceptionally heavy but a bit of a nuisance. Will try .5 gr. reduction and see how that goes. Mind you it shot well with the 52.2 load.
Thanks for your comments.
Cheers,
Trevor.

Is the bore groove dimension exactly the same as the 9tw? I understand small changes here will have a much greater impact on pressure than twist.

It would be interesting to get very accurate muzzle V measurements.

On one hand, more energy is carried by a faster spinning bullet with the same MV, but on the other hand there is likely to be an increase in pressure perhaps also leading to higher MV.

Everything else needs to be absolutely identical which is of course unlikely.

Of course, best 'accuracy' MV may not now be what it used to be.

Peter Smith.

pjifl wrote:It would be interesting to get very accurate muzzle V measurements.

On one hand, more energy is carried by a faster spinning bullet with the same MV, but on the other hand there is likely to be an increase in pressure perhaps also leading to higher MV.

Everything else needs to be absolutely identical which is of course unlikely.

Of course, best 'accuracy' MV may not now be what it used to be.

Peter Smith.

"Noise"!!!!
Keith H.

Hi Peter,
Next time I visit Herberton can I run it over your Lab Radar? I am not to worried about MV as long as the accuracy is there. The load I was using was quite accurate. If the reduced charge is as accurate and reduces the bolt lift effort, I will be happy. I am not to worried about MV. Only need that for ballistics to calculate come ups.
Cheers,
Trevor.

Yes. I am more interested in MV out of curiosity. I could work out the linear and rotational Kinetic Energy changes but doubt that it will prove much.

Peter Smith.

For the most part twist doesn’t effect pressure so for all practical purposes won’t effect velocities. I’ll say it will effect it slightly but I’m talking a velocity variation of probably 20fps or even less.

We’ve made pressure test barrels in 308win for example in 12 twist and 8 twist and they used it for SAAMI pressure testing and as far as pressure and velocities go they gave apples apples.

What the actual bore and groove dimensions are in the giving barrel will effect your pressures and velocities more than anything else. Changing the groove size will have more of an effect than changing the bore size (bore size is the tops of the lands).

Later, Frank
Bartlein Barrels

Litz did a test on this a while ago and it only made about 5fps / 1" difference. A lot less than I was expecting

BATattack wrote:Litz did a test on this a while ago and it only made about 5fps / 1" difference. A lot less than I was expecting

That was a velocity test only. They where not pressure barrels and we did make and chamber all of the barrels at one time. Used the same chamber reamer etc....I would’ve liked the test to be done with more than one barrel per twist but I feel a honest effort was made to get decent and reliable data.

As I said before we’ve also made pressure and velocity test barrels for a couple of different ammo makers and no significant difference was noticed that I recall. So that would back up Litz’s test as well.