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9/19/2017 7:27:10 PM
Posted: 3/31/2006 9:23:15 AM EDT
Of all the questions I see repeatedly asked and answered on this forum, this isn't one of them. My guess would initially be no or at least not enough to be measurable, but I was wondering if any of the chrono guys had tested this.

Surely someone out there has a couple of barrels from the same manufacturer, in the same length, in both 1x9 and 1x7 twist rates and knows the answer. Same ammo too, obviously.

I find this question interesting because it would settle once and for all whether a faster twist would be better for shooting all types and sizes of bullets. A lack of significant drawbacks (besides "over-stabilization" and lighter bullets flying apart) such as a decrease in MV would win me over to a 1x7 twist barrel to replace my 1x9 eventually. Faster MV would be even better.
Link Posted: 3/31/2006 9:27:00 AM EDT
[Last Edit: 3/31/2006 9:27:41 AM EDT by mr_wilson]
I suspect one of the reasons ya haven't seen this question arise around here is because no-one has developed a chronograph sensitve enough to detect the "miniscule" variations in MV between the two twist rates.

Mike
Link Posted: 3/31/2006 9:29:11 AM EDT
So the differences are always miniscule? Is this documented anywhere or speculation? I was leaning towards that answer but have no data to back it up.
Link Posted: 3/31/2006 9:33:32 AM EDT
Great question
Link Posted: 3/31/2006 10:25:47 AM EDT
I posed this question years ago too! I wanted to test a 1/12 vs. 1/7. No one seemed to know the answer then either.

I'm suspecting the difference is minuscule as stated.

Link Posted: 3/31/2006 10:56:33 AM EDT

It takes energy to spin that bullet. The faster you spin that bullet the more energy it takes. So yes, the faster the twist rate the slower the velocity. I don't know how much slower so Mr. Wilson could be right. The different could be negligable.

Shok
Link Posted: 3/31/2006 11:28:55 AM EDT
[Last Edit: 3/31/2006 11:29:51 AM EDT by Christian_Gun_Nut]
Funny you asked. I just happen to have some data on that. The 1:7 was just a tad slower on average, until you look at the 55gr WWB data. I think the issue is that I needed a larger sample than 5 rounds per gun get any proof of concept. one outlier in 5 rounds could really skew the data. However, the trend is interesting:

Hornady TAP,75 gr, 16 inches, 1 in 9 twist
2590.0
2552.0
2557.0
2520.0
Avg velocity 2554.8
max variance 70.0
Hornady TAP, 75 gr, 16 inches, 1 in 7 twist
2513.0
2519.0
2529.0
Avg velocity 2520.3
max variance 16.0

Black Hills Reman., 75 gr, 16 inches, 1 in 9 twist
2549.0
2512.0
2445.0
2492.0
Avg velocity 2499.5
max variance 104.0
Black Hills Reman., 75 gr, 16 inches, 1 in 7 twist
2478.0
2460.0
2492.0
Avg velocity 2476.7
max variance 32.0

IMI M-193, 55 gr, 16 inches, 1 in 9 twist
3084.0
3062.0
3084.0
3067.0
Avg velocity 3074.3
max variance 22.0
IMI M-193, 55 gr, 16 inches, 1 in 7 twist
3017.0
3026.0
3079.0
3060.0
3052.0
Avg velocity 3046.8
max variance 62.0

Looks real convincing until you get here.

Winchester White Box, 55 gr, 16 inches, 1 in 9 twist
2975.0
2998.0
3000.0
2987.0
Avg velocity 2990.0
max variance 25.0
Winchester White Box, 55 gr, 16 inches, 1 in 7 twist
3000.0
2992.0
3042.0
3038.0
3023.0
Avg velocity 3019.0
max variance 50.0

I think you would need to test at least 20 rounds per gun to get a true picture.
Link Posted: 3/31/2006 12:56:23 PM EDT

Originally Posted By Christian_Gun_Nut:
Funny you asked. I just happen to have some data on that. The 1:7 was just a tad slower on average, until you look at the 55gr WWB data. I think the issue is that I needed a larger sample than 5 rounds per gun get any proof of concept. one outlier in 5 rounds could really skew the data.



Not just that - you'd need to test multiple barrels - preferably from the same manufacturer and with the same number of rounds.

Things like variances in the chamber, bore, gas port and wear of these items all contribute to differences in MV.

For example I've got data from some Mk262Mod1 tests I ran a couple of years ago where a 1:8 twist 18" barrel (2820fps) had a higher MV than a 1:9 twist 20" barrel (2790fps). I used 15 round samples for each test. Both rifles were relatively new, the primary difference was the match chamber in the 18" barrel compared to the military chamber of the 20" barrel.
Link Posted: 3/31/2006 1:28:10 PM EDT
The problem is acquiring the data. You would have to have virtually all dimensions of the two rifle exactly the same for the data to be meaningful.

That said, what Qshock said sounds correct. If all things are equal (which is nearly impossible in the real world), the 1/7 seems like it should be slower, in theory.

In the real world, it is going to vary more barrel to barrel than twist to twist. So I guess that would mean it doesn't matter.
Link Posted: 3/31/2006 5:30:19 PM EDT
Makes sense. The higher grain bullets are longer which increase the surface area of bullet to barrel.
The more area, the more friction surface surface, the more energy lost due to said friction.

I would imagine with very accurate measurement you could detect that increased friction via barrel temperature after firing multiple rounds.

Link Posted: 3/31/2006 6:51:48 PM EDT
[Last Edit: 4/1/2006 12:12:59 AM EDT by usp8riot]
Also, a tighter twist maintains stability which in turn maintains velocity, am I correct? If that's the case, then a bullet coming out of a 1/7, for instance, will maintain a higher velocity throughout the trajectory than one coming out of the same hypothetical barrel with a 1/7 twist.

Correction: "same hypothetical barrel with a 1/12 twist"
Link Posted: 3/31/2006 7:59:13 PM EDT

Originally Posted By usp8riot:
Also, a tighter twist maintains stability which in turn maintains velocity, am I correct? If that's the case, then a bullet coming out of a 1/7, for instance, will maintain a higher velocity throughout the trajectory than one coming out of the same hypothetical barrel with a 1/7 twist.



It shouldn't. The big factor that effects velocity is the drag which translates into ballistic coeffecient. Unless the grooves left on the bullet from firing change the drag profile, the rotational rate shouldn't have an effect velocity.

The exit velocity depends on the pressure behind the bullet when it leaves the muzzel. All other things equal, the more pressure you have, the more velocity you have.
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