Author Topic: Relationship between twist rate / bullet weight / barrel length  (Read 448 times)

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Offline Hat_man

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Relationship between twist rate / bullet weight / barrel length
« on: October 07, 2018, 05:49:19 PM »
Hello everyone.

Let me start by saying I am already learning so much in the rimfire section and have received so much great advice.  I don't feel like the dumb new guy asking silly questions.  Secondly, I DON'T  have a center fire rifle (yet lol) am not in the market for one (yet).  Until this broken foot heals, all I can do is read and try and learn new things.  If this is the wrong section for this then please move to the correct place.

I was reading about twist rates and how they interact with bullet weight.  It seems slower twist rates work better for lighter bullets but tighter twists can shoot both lighter and heavier bullets within reason.

In general, I think this applies.....

1:16 = 30-40  gr
1:12 = 40-50  gr
1:10 = 40-60  gr
1:8   = 50-75  gr
1:7   = 60-80+gr

How does barrel length play into all this?

For these questions let's use a CZ 527.  I pick this because at one time it had a 1:12 twist and now is 1:9 as a standard.  I have seen barrel lengths from 18.5" up to 25.5" with the 1:9 twist and 24" (maybe others) with the 1:12. I am assuming the actions/triggers to be the same. Accuracy? Speed? Trajectory? Etc?

First, what is the difference be between a 527 with a 24" barrel and the 1:12 twist and the same 24" barrel with the 1:9 twist?

Second, what is the difference between a 527 with an 18" barrel and a 527 with a 25.5" barrel?   (I think it was a Euro Varmint I saw this on)  Both with the 9:1 twist.

Third, does a slower twist in a longer barrel give more speed to a heavier (45-60 gr) round? Or slower twist in a longer barrel characteristics in general.

Someday I'll be in the market for a new gun, but for now ill be happy with learning and shooting my little BRNO Model 2 rim fire. 

Thanks for the education.

If nice guys finish last, are you willing to pay the price to finish first?

Offline M1A4ME

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Re: Relationship between twist rate / bullet weight / barrel length
« Reply #1 on: October 07, 2018, 08:02:11 PM »
For a given caliber - the heavier the bullet, the longer it has to be.  The longer it is the faster it has to spin to maintain stability.  That's why you see the barrel twist increase (1 turn in 9" is faster than 1 turn in 12" and so on) over the years as they attempted to make a short/intermediate range varmint cartridge shoot farther and hit harder by going to heavier and heavier bullets.

If, if, you could push the heavy bullets fast enough the barrel twist might be more flexible - meaning a slower twist might to okay with a longer heavier bullet.

Some bullets (.224 diameter in the lighter weights = meant for varmint hunting) should not be loaded/shot through the faster twist barrels.  At one time Hornady and Sierra both had warnings in their reloading manuals telling you not to use certain bullets in 1 turn in 9" barrels.

I used to use the thin jacketed 45 grain bullets in my M700 Remington (1 turn in 12" barrel) and they worked great.  I tried some of them in my Ruger Mini14 (1 turn in 9" barrel).  Some of the would explode short of the 100 yd. target.  All of them would leave a highly visible swirling vapor trail a few yards out from the muzzle.  Some of them would fly apart prior to hitting the 100 yd. target and the ones that made it to the target were all over the paper.

I can't tell you that a fast twist barrel shoots light bullets just as good as a slow twist barrel, I haven't tried it.  Maybe some day.
Better to have it and not need it, than to need it and not have it.  So, if you see me walking the dogs with my SIG 556R, its okay.