Range Report Twist rates and MV

[gstaylorg]

I have done a fair bit of experimenting with the effect of barrel length on MV and typically the numbers I have recorded fall in the expected 15-25 fps per inch barrel length range for .308.

What I'm interested in finding out now is whether anyone has any information regarding how <span style="font-style: italic">twist rate</span> affects muzzle velocity. As a specific example, if you went from a 1:11.25 twist to 1:10 twist in an otherwise identical barrel (manufacturer, length, etc.) how much (if any) would you expect the faster twist might reduce the muzzle velocity with a given load?

 

[Graham]

Re: Twist rates and MV

It doesn't. Not noticeably. Meaning mot measurably - literally, you can't measure it.

 

[MitchAlsup]

Re: Twist rates and MV

After the bullet has engraved itself on the lands, the difference in friction between one twist rate and another is basically unmeasurable.

 

[gstaylorg]

Re: Twist rates and MV

OK, thanks for your replies, that is very informative.

Here's my situation in greater detail. I have a new .308 GAP with a 27" Bartlein 5R 1:9 twist barrel that I had built with a specific projectile in mind. In my initial characterization of this rifle, I found that the avg. MV (determined from 2 x 10 shot strings) is 2737 fps using Applied Ballistics 175 gr Tactical load. I have two other GAPs with 22" and 24" (respectively) Bartlein 5R 1:11.25 twist barrels. The MVs for this same load from those rifles is 2670 fps and 2726 fps, respectively. I had expected that the MV for this load from the new rifle would be in the 2775-2800 fps range, due to the extra barrel length compared to my other two GAPs. However, I'm only measuring an increase of ~10 fps.

If it's not due to a decrease in MV because of greater friction with the faster 1:9 twist rate, the only other thing that comes to mind is that I've reached a point where powder burn has been completed and therefore I'm not seeing a significant increase in MV, even with the longer barrel. In the last few days, I read a post (I think possibly from Graham or Greg Langelius) about barrel length and complete powder burn, that mentioned 26" as a barrel length. I understand that "completeness" of powder burn with regard to barrel length totally depends on the type powder used. My question is why am I not getting more than a 10 fps increase in MV from the increased barrel length, using the identical ammunition load?

 

[tobias]

Re: Twist rates and MV

When you go from a 1:11.25 twist to 1:10 twist the muzzle velocity is 99.957% of the original value (for the Sierra MK HPBT 168 grns.) As the others already stated there is practically no change.
There is one barrel length that leads to max. MV (with a given powder type). When you go to a greater length friction slows the bullet down. Maybe you just reached the perfect length.

 

[MitchAlsup]

Re: Twist rates and MV

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: gstaylorg</div><div class="ubbcode-body">OK, thanks for your replies, that is very informative.

Here's my situation in greater detail. I have a new .308 GAP with a 27" Bartlein 5R 1:9 twist barrel that I had built with a specific projectile in mind. In my initial characterization of this rifle, I found that the avg. MV (determined from 2 x 10 shot strings) is 2737 fps using Applied Ballistics 175 gr Tactical load. I have two other GAPs with 22" and 24" (respectively) Bartlein 5R 1:11.25 twist barrels. The MVs for this same load from those rifles is 2670 fps and 2726 fps, respectively. I had expected that the MV for this load from the new rifle would be in the 2775-2800 fps range, due to the extra barrel length compared to my other two GAPs. However, I'm only measuring an increase of ~10 fps.</div></div>

You might give a slower burning poweder a try; something like RL17 or RL19.

I pretty much agree with Greg (or whomever) indicated that a 308 is "done" by 24". That is the powder has burned, created all the pressure it can, and is not really pushing on the bullet in the last few inches of the barrel. Similarly a 223/5.56 is done by 20".

In order to push the bullet later in the barrel, one needs a slower burning powder, and often a bit more of it. As always, start low and work your way up.

Many of the PALMA shooters with 30" barrels are using some of these slower burning powders.

 

[gstaylorg]

Re: Twist rates and MV

Thanks for the info, I'm happy to hear that. If the issue was the fast twist rate, that would obviously be harder, if not impossible, to rectify. Burn rate for this barrel length should be much simpler to optimize.