Another rifling question - 1-in-4 twist for 30 cal

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benenglish

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I was perusing the sites of various barrel makers today and I noticed something that, to me, is pretty odd.

It's possible to buy a .308 bore barrel with a 1 turn in 4 inch twist. Such an animal is cataloged on the Pacnor web site as a standard item.

I don't have broad experience in the shooting sports so I'm unable to come up with a theory. The longest 30 cal bullet I know of is the 240 Sierra MK, and even when it's launched subsonic it'll stabilize in a 1-in-8. I can't imagine who needs a twist twice that fast. Since Pacnor shows it as a standard item in their list of "Barrels we make" (It's not a custom order piece) there must be more than just a few folks buying them.

Of what use is a 30-caliber 1-in-4 barrel? Who needs/what sport requires such a fast-twist? Is somebody out there swaging 500-grain VLDs or something?
 
It might be good for .30 cal pencils, but I can think of no practical reason for a 1 in 4 twist in anything. Some of the older 6.5 mm's used 1 in 6.5" twists for big long roundnose ~160 gr bullets. I guess that the high power guys who are using 90 gr SMK's in .223's are using twists that are in the 1 in 5.5 or 1 in 6 range. Makes my head hurt to think about it.
 
Al Nyhus said:
Think sub-sonic tactical rounds.....like the 300 Whisper. -Al
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I've worked with the 300 Whisper in handguns (never a rifle, tho) using the heaviest bullets at slow speed. With a 240 MK, a 1-in-8 twist is fast enough. Some folks like a little extra insurance and go with a 1-in-7 twist.

It looks like all or nearly all barrel manufacturers do 1-in-8 barrels and most do 1-in-7. That's all you need for the 300 Whisper.

But only Pacnor does a 1-in-4, at least that I can find cataloged as a standard item on the websites of every barrel maker whose name I can remember at the moment.

I'm curious enough about this that I fired off an email to Pacnor this morning. I'll report back if they reply.
 
With that kind of twist, bullets would often just blow up.

If you do the math, with the fast varmint calibers, bullets can easily hit 300,000 rpm.

When you go to the drastic extreme you are discussing, you can have bullets simply blow up.

Like, it is possible right now for .224 light bullets -- driven by a rifle like a Swift -- to simply self destruct a few feet out from the muzzle.

This is a case where you are "head tripping" over things you do not really understand.
 
Could it be a typo, websites are full of typos. Perhaps they meant 1X14"

Some BP barrels with extreme slow twists were given as one turn in so many feet. The Paradox guns could have one turn in twelve feet.
 
Montana Pete said:
This is a case where you are "head tripping" over things you do not really understand.
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Now Montana Pete from W'skansin there ya' go again! Just pickin'....


I believe the feller understands just fine, and that it's a good question to ask. (But then I'm just another headtripper who happens to own barrels in every twist imaginable :rolleyes: )



al
 
A resolution...sort of

alinwa said:
I believe the feller understands just fine, and that it's a good question to ask.
Click to expand...
I agree (with both your assertions:)) and that's why I asked.

Today I got an answer. Amy at Pacnor was a joy to talk to and perfectly willing to clue me in.

The answer is that Pacnor has no idea what those barrels might be used for.

According to Amy, "All we (Pacnor) know is..." that the Army Marksmanship Unit called them up and ordered some 1-in-4, 30 caliber barrel blanks. Now that the AMU paid the setup costs and Pacnor has made the button, Pacnor can supply the same bore/twist to anyone who wants one.

Pacnor has no idea why anyone would ever want one; the AMU felt no need to explain anything to Pacnor. But, thanks to the AMU order, Pacnor now has the manufacturing capability.

Your tax dollars at work, I s'pose. :cool:

I just wonder what those guys are up to. Montana Pete is correct to point out (at least, I think this is part of what he was saying) that bullets generally aren't strong enough to hold together if you get them up to the ridiculous RPMs that would be produced by a 1-in-4 barrel at "normal" rifle velocities. For that reason, Al Nyhus almost certainly had part of the answer when he said "Think subsonic..." And as ridiculous as it sounds, maybe I hit on part of the answer when I joked about "swaging 500-grain VLDs".

Maybe, in a few years, the AMU will tell us about it.

Till then, thanks to all for your thoughts and insights.
 
benenglish said:
The answer is that Pacnor has no idea what those barrels might be used for.
Click to expand...

Or maybe not. When it rains it pours. I just got this email from Pacnor:

This twist was made up for US Army research and development for a long
subsonic projectile called "the smart bullet"project.Its also been used for
240gr and above 300/221 fire ball subsonic loads.Regards,Chris​

While I don't know of any "and above" projectiles, clearly the horseshoe score for closest to the mark goes to Al Nyhus.:)

Again, thanks to all.
 
Probably not a simple copper jacketed lead bullet then...Might accept higher rpms?
 
The core material in the bullets built for the 1:4 twist 300 Whisper wasn't lead. And the reason for the 1:4 twist has as much to do with what happens when a bullet impacts an object as it does with stabilization.

A quiet sub-sonic round...tossing a heavy bullet...with a core material that minimizes pass through...and high rotational speed for increased hydrostatic shock.

Varmints come in all sizes......;) -Al
 
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