Twist Rate calculator something seems wrong

[ShooterJ]

I was playing around with the Berger Twist Rate Stability calculator (http://www.bergerbullets.com/twist-rate-calculator/)

Something seems wrong here. I put in the stats for a 32gr .204 vmax with a 1/12 twist barrel at 4100 fps. It says the bullet will have marginal stability. Most .204s are 1/12 and most shoot the 32gr.

The 40gr come up even worse on it.

Maybe the bullet length is wrong? BC makes no diff on the calculator just MV and length do.
Is this correct?

[Bill K]

Have not played with that program, but I would say they need to upgrade or do something with it, as most any 20 cal 1/12 twist caliber will stabilize the 32 grain bullet very well. I would think it should read Comfortable stability. Bill K

[Rick in Oregon]

Yep, sumthin's wrong okay. And I'm thinking it's with Berger's calculator, as why would virtually every rifle manufacturer who makes rifles in 204R put 12 twist barrels on them?

I'll agree that the 11 twist is probably the most useful in this caliber, but pure speed was the goal when this caliber first came out, and the factories assumed we'd all want as much speed as possible so they went with 12 twist for 32gr bullets. Time has proven them wrong, as most shooters want good BC's for long range which require heavier bullets (39/40gr) which also requires a faster twist for best performance. Most here will agree that 40gr bullets in the 204R is a hit and miss proposition using a 12 twist barrel.

Bottom line: There are many twist calculators on the net, and they all do not agree with one another.

[Bill K]

Just out of curiosity I pulled that system up and ran some figures. I did note that they only offer the 40 grain bullet and not the 32 in their choice under 20 caliber. and if you show the 40 in the top window, but type in the 32 in the side windows, you get unstable. But if you type the 40 grain in both, then you get the good stability showing. If you are going to stick with the 1/12 twist and want to shoot the heavier bullet i.e. 40, just use and try the 39 grain BK Sierra and you should get fine results. But as Rick mentioned, most all of us have re-barreled and use the 1/11 twist which will allow you to shoot any of them, very well. Bill K

[ShooterJ]

I wish they would just let people know at what rpm range the bullet is stable at. Thats the bottom line the rpms.

My .204 bolt came in 1/10 twist. I wonder since Thompson was late to the .204 game if they did not test 1/12, 1/11 and 1/10 to see what stabilizes the best.

The Honady SP 45gr are actually shorter than the 40s and might shoot like those 39gr bergers do.

[ShooterJ]

Just out of curiosity I pulled that system up and ran some figures. I did note that they only offer the 40 grain bullet and not the 32 in their choice under 20 caliber. and if you show the 40 in the top window, but type in the 32 in the side windows, you get unstable. But if you type the 40 grain in both, then you get the good stability showing. If you are going to stick with the 1/12 twist and want to shoot the heavier bullet i.e. 40, just use and try the 39 grain BK Sierra and you should get fine results. But as Rick mentioned, most all of us have re-barreled and use the 1/11 twist which will allow you to shoot any of them, very well. Bill K

I just did what you said keeping their 40g and even upping the velocity and its still marginal.

[Bill K]

Has to be the site, as I get Comfortable stability. Maybe try another program or just re-barrel with a 1/11, like 90% of us did. Bill K

[ShooterJ]

I found a better twist calculater. The Berger one did not change with bullet weight adjustments.
JBM calulator has inputs for plastic tip, weight etc and they all change the outcome.
http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi
ie. Items in GREEN are good. yellow is marginal red unstable.

[ShooterJ]

I fooled around with the calculator to see what bullet rpm the 32 and 40 need.
rpm = MV x 720/twist rate in inches

32gr are stable at 4000 fps in 1/12 = 240,000 rpm
40gr are longer so need more rpm to achieve the same stability as the 32gr.

most of the 40s don't shoot at 4000fps more like 3700-3800 fps. So the rpms are going to be less than 240,000 (around 222,000)
while they probably need 250,000 rpm.

Looking at the 3 examples the 1/10 twist shows up as stable and is at 266,000 rpm.

[Bill K]

Now that one does look much better, for sure. Bill K