6.8 SPC and Twist Rate

[carnaby]

I posted here in my industry forum on the topic, but I'm repeating here in the general section because this information is important. Here's the rub: Twist rate makes ZERO difference to barrel performance. My earlier theoretical analysis appears to be supported and I expect that twist rates equal to or slower than 1:4 will see no appreciable difference in pressure or muzzle velocity.

I may have a heavy target barrel made to get the final word on accuracy, but so far I haven't seen any difference with 16" 4-groove barrels in 1:7 vs 1:11 twist.

The numbers (mv in average of several shots):

16" 6.8 BSP

SSA 115 OTM: 2492 fps

SSA 110 TSX: 2547 fps

SSA 85 TSX TL: 3010 fps


16" 6.8 SPC II 1:11"

SSA 115 OTM: 2493 fps

SSA 110 TSX: 2575 fps

I had exactly 5 rounds of the 85 TSX TL and didn't have any left for the SPC II 1:11" twist. I'll get those next time. I didn't measure accuracy closely but I was shooting about 1.25" to 1.5" at 105 yards with everything. Neither barrel showed better accuracy than the other. The brass looked exactly the same. With a suppressor or without, didn't make any difference.

More testing will follow with the SSA Tactical loads

[ron169]

I didn't see it listed, what equipment did you use to test pressure?

[rjmmfl]

How many barrels did you test? If it's only one of each, statistically speaking, you can't draw any such conclusion - one way or the other. Given the magnitude of variability between even supposedly identical barrels, one of the barrels could easily be at one end of the population distribution, and the other one at the other end.

Although I also suspect that the magnitude of pressure increase due to twist is much smaller than the contribution from other variables: lead length, land:groove ratio....

[saddler]

Interesting - will have to see what other data comes to light

MAY make it easier to upgrade earlier SAAMI spec 6.8's to SPECII for use with a wider range of ammo

[carnaby]

I didn't see it listed, what equipment did you use to test pressure?

Pressure was tested at SSA with a Wiseman 1:7" twist pressure barrel etc. I don't have the data from this, just their notes that pressure was safe.

[carnaby]

How many barrels did you test? If it's only one of each, statistically speaking, you can't draw any such conclusion - one way or the other. Given the magnitude of variability between even supposedly identical barrels, one of the barrels could easily be at one end of the population distribution, and the other one at the other end.

One of each. This was basic range testing. The barrels are made with nominally identical chambers, land/groove dimensions etc, just different twist rates. I didn't use any particular process to select these barrels. It's just as likely that either was at one far end of the spectrum as the other and for this basic testing this was not important. The fact remains that if twist rate had a significant effect, then a statistical survey would not be required, and therefore was not performed.

Although I also suspect that the magnitude of pressure increase due to twist is much smaller than the contribution from other variables: lead length, land:groove ratio....

Agreed. In fact there's no reason to think twist rate will have a measurable effect until twist is about 1:5 or faster, and it shouldn't have a significant effect until 1:3" or so. The fact is that so little energy goes into spinning the bullet 1:4" or slower that bullet velocity and pressure will not be affected.

[HTR]

One of each. This was basic range testing. The barrels are made with nominally identical chambers, land/groove dimensions etc, just different twist rates. I didn't use any particular process to select these barrels. It's just as likely that either was at one far end of the spectrum as the other and for this basic testing this was not important. The fact remains that if twist rate had a significant effect, then a statistical survey would not be required, and therefore was not performed.



Agreed. In fact there's no reason to think twist rate will have a measurable effect until twist is about 1:5 or faster, and it shouldn't have a significant effect until 1:3" or so. The fact is that so little energy goes into spinning the bullet 1:4" or slower that bullet velocity and pressure will not be affected.

I wouldn't be surprised if, given a similar rifling profile, the increase in twist rate has no effect on pressure.

[sneedb82]

So, as a non-scientist, I would assume that the leading cause in pressure issues before is the freebore, and that has been essentially eliminated due to the SPCII style chamber (or better)?



Ben: In working with the 9.5 twist barrels from M1S, the issue seen with them, after being reamed to the correct chamber specifications, would be land to groove ratio, rather than twist?

[carnaby]

So, as a non-scientist, I would assume that the leading cause in pressure issues before is the freebore, and that has been essentially eliminated due to the SPCII style chamber (or better)?

More or less, yep.

Ben: In working with the 9.5 twist barrels from M1S, the issue seen with them, after being reamed to the correct chamber specifications, would be land to groove ratio, rather than twist?

Right, and the bore cross section area, whatever that turns out to be.

[walt214]

One of each. This was basic range testing. The barrels are made with nominally identical chambers, land/groove dimensions etc, just different twist rates. I didn't use any particular process to select these barrels. It's just as likely that either was at one far end of the spectrum as the other and for this basic testing this was not important. The fact remains that if twist rate had a significant effect, then a statistical survey would not be required, and therefore was not performed.



Agreed. In fact there's no reason to think twist rate will have a measurable effect until twist is about 1:5 or faster, and it shouldn't have a significant effect until 1:3" or so. The fact is that so little energy goes into spinning the bullet 1:4" or slower that bullet velocity and pressure will not be affected.

Thank you for the test and the info.