BARREL TWIST VS BULLET SPEED

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  • Last Post 14 November 2023
4895 posted this 11 November 2023

WHILE SHOOTING TARGET WITH CAST BULLET IS THERE A RELATIONSHIP BETWEEN BARREL TWIST AND BULLET SPEED? I'M SHOOTING 308 WITH 10" TWIST WITH A 180 GR BULLET AND 28 GR OF VARGET.WITH A VEL 1840. JUST WONDERING

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Ken Campbell Iowa posted this 11 November 2023

yes.  simplified a little, a bullet needs to spin pretty fast to stabilize it ...  the longer the bullet the faster it needs to spin to fly satisfactorily ...

spinning fast, it would be stabilized ... resisting being pushed off-axis by outside forces ....  whether going downrange or just whizzing around on a dinner plate ...  stability is from the rpm's, not from going across the cow pasture ...

but in our case, we spin it fast enough it by shooting fast enough ...  there are simple formulas but it is easier to just observe what others have found through experience .....

for instance your 180 gr, 30 caliber bullet out of a 10 inch twist is known to start being stabilized reasonably well enough at as low as 1400 fps velocity ...  and getting pretty steady at your 1840 fps.

just to keep ourselves humble, we need to note that although more faster is more stabile ... faster also magnifies any momentum error as the bullet leaves the muzzle ... darn these compromises !! ...

hope this helps,

ken, spinning but not stabile 

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pat i. posted this 11 November 2023

I'll probably get kick back on this but for 30 caliber rifles and gas checked bullets I like a rpm of around 130,000 give or take. But that's just me, and after checking what the better CBA match shooters are using if they know it or not. To find rpm take muzzle velocity times 720 divided by twist. In your case

1840x720÷10=132,500 rpm

I don't know if that answers the question you were asking but it's all I have to offer.

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Larry Gibson posted this 11 November 2023

pat i. gives a good answer.

 

Your 180 gr bullet, if it is a "normal cast bullet", is well stabilized at 1840 fps and in the zone for best accuracy out of a 10" twist.

LMG

Concealment is not cover.........

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malar posted this 12 November 2023

Does lead hardness affect this and what is the constant 720?

I'm shooting a 300 Blk with a 7" twist and would like to shoot faster than 1300fps.

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pat i. posted this 12 November 2023

720 is just the constant in the equation and is used across the board. Like I said I personally like to use somewhere around 130,000 rpm as an upper limit for 30 calibers and gas checked cast bullets but it isn't set in any kind of stone. Only way for you to find out if you can shoot faster is to try and see. Maybe it'll work for ya.

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ray h posted this 12 November 2023

Would the 130,000 rpm rule apply to 22 cal and 25 cal too?

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MP1886 posted this 12 November 2023

Would the 130,000 rpm rule apply to 22 cal and 25 cal too?

 

It's not a rule, it's generalization of what is a safe zone to be in. 

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9.3X62AL posted this 12 November 2023

Ray H--

IME it seems to do so.  I don't have a lot of experience with the 22s yet, but lots with the 243, 25/20 WCF, 250 Savage, and 25/35 WCF.  

The 243 has been a delight to work with for many years.  Its 1-10" twists have kept both RCBS 6mm-95-SP and Lyman #245496 between 1.0 and 1.5 MOA to 200 yards from 1600-2000 FPS.  The RCBS shows a 5%-10% smaller grouping potential overall between three rifles I have owned.  It remains my most tractable cast bullet varminter.  

The 25/20s and 250 Savage are/were able to run castings from 72-107 grains accurately and capably in their 1-14" twists 2000+ FPS.  The 25/20s weren't tried with the 107 grain bullets.  

The 25/35 (a Win 1894 flatband) had a 1-8" twist.  This was my first run-in with RPM/twist rate speed limits.  I had a nice NEI 117 grain FNGC that shot very well from the carbine--until its bullets ran past the 1600 FPS point.  Any faster than 1600 FPS and bullets didn't even shoot patterns.  There was no bore leading (sized at .258"), but they refused to 'Shoot'.  Dial back the coal, and accuracy returned.  

The 1-9" twist in my bolter 223 (Ruger 77RC) has been a lot more fussy than my 22 Hornet (1-14") and 22-250s (1-12" and 1-14") with the poured ones so far.  

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pat i. posted this 12 November 2023

This is a subject that's been beaten to death and fought over for years so I'll give my last opinion on the matter based on my personal experience. Not gonna defend or fight about it so don't bother. IMO the smaller than 30 calibers can get away with the bullet spinning faster and the larger than 30 caliber needs less rpm.. That's it.

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ray h posted this 13 November 2023

9.3 -  Thanks for the 25 data. I only briefly worked a little with the Noe 80g in my CZ 25-20, it showed promise, I used a 10 tw.  I started a project years ago to build a switch barrel Mod 70 . I recently finished the 38-55 barrel. When I looked at the 25 cal barrel ( I want a 25-35 too ) I found I had ordered a 12 tw and started to try and sell it. But if you get those velocities with yours I should be ok with 100 to 120 gr at 1600+ with the 12 tw. I'm not looking for high velocity in cast bullets. I thought I got the 12 tw idea from this site but couldn't find where. Thanks,  Ray

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ray h posted this 13 November 2023

Pat- thanks for your reply, I sure didn't mean to stir the pot. When I read the 130,000 rpm it just seem like that  would make a good  goal  and maybe save some time working up accuracy loads. I've read a lot of Larry's and others info but it just didn't click till this thread. I read so much I go into data overload. Since my better half got sick this summer I haven't touch a round off since May. She has brain surgery tomorrow and recovery may be long and slow so no shooting till next spring. I'm going to have a bunch of time in the loading room. I want to have some loads ready to test but mainly cast enough bullets and get them sized properly and ready to load.   I mainly hunt ground squirrels and since May decided I want to shoot all cast bullets next spring if I get to go next year. I want to use 22 caliber (and my 25-20) on squirrels. Most of my shots are out to 225 yds  but not sure I can get the accuracy to cleanly kill one at that distance. If I could get a good load around 1800+ or so in the 22's I'd be happy. Most of my barrels are 12 tw in my 22 cal.  I always take 5000 rounds for a 10 day shoot so don't know how realistic it's going to be.

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pat i. posted this 13 November 2023

Ray you didn't stir the pot at all and I hope your wife does good in surgery and makes a full recovery. This subject just gets people rankled for some reason. Tom Gray was talking about bullets being twist limited long before any of the current players, me included, brought it up on the internet. There's really no rules and I just use my own experience and calculating what people in the CBA match game are doing. It seems to be the concensus even if it isn't known to the shooters themselves that there might he something to it. As for the smaller calibers I was shooting my 6.5 ppc with a 170 gr bullet in an 8 twist right around 1700 fps. That calculates to 153,000 rpm which lead me to believe smaller calibers can take more rpm before falling apart. And as I said for 30 caliber I believe around 130,000 is the upper limit but think around 125,000 might be better but as usual your mileage may differ. I'd like to see people put pen to paper and post what bullet rpms are giving them the best accuracy to either point to a possible correlation or call bs on the whole idea. Either works for me.

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MP1886 posted this 13 November 2023

I'm going to tell you all something about twist. I know much about the Barnes bullet company, their owners, and their friends. I also know the hurdles they had to jump to get their bullets to where they are today.  I also  know the engineer/scientist that solved one of their big problems with copper bullets.Why did Barnes eventually put grooves on their copper bullets? I said to reduce friction and reduce copper in the bore.  Well those are true, but the main reason was Barnes was having an accurate problem.  THE BULLET WAS TWISTING IN THE BORE before even leaving the barrel. That is hard to fathom because like me you're thinking a hard bullet like copper is twisting in the confines the bore and rifling????? How can that be.  Well it was and my friend is the one that found it and solved it. Don't ask me to explain it, it's over my head. I'll see if I can talk to him again and get in laymen language. I know what was done with the grooves and bands to prevent this and now I wonder if it can be applied to a cast Loverin bullet to push it much faster then we have been able to in the past. 

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Paul Pollard posted this 14 November 2023

Pat,

I looked through my notes and came up with some figures. 6PPC, 14 twist. Mountain Molds pointed bullet at 81.5 grains. Started at 32 clicks of LT30 (about 18 grains) at 2307 fps and 118,646 rpm. The groups here were around 3/4".

At 34 clicks (about 19 grains) 2436 fps and 125,280 rpm. These groups averaged just over 1/2".

With the Eagan bullet which is a truncated cone, it takes more velocity to stabilize. Usually a minimum of 2200 fps. I can shoot the Mountain bullet at 1700 fps without tipping. 

On the bullets I forgot to heat treat, the "groups" were 4 and 5 inches at 119,000 rpm.

 

 

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MP1886 posted this 14 November 2023

This is interesting Paul Pollard. Honestly I can't see how that 14 twist is stabilizing that bullet at all. Look at all the 6mm cartridge that have been made through the decades. Take the 243 Win. It was a huge success and Remington tried to cash in on it with the 244 Rem with a 12 twist and he had problems stabilizing bullets so then renamed it the 6mm Rem and changed the twist to 9 which was more then really needed.  I have a 6x45. I could have gotten it with an 8 twist or 9.  I took the 9 twist. I'm shooting a 72 grain bullet from MP moulds. My highest velocity is 2732 fps. That's 218,560 rpm and my accuracy in beating your 1/2 inch. So that right there blows up the rpm stuff.  In fact my velocities range from 2200 fps up to that 2732 fps all with the same accuracy. BTW the bullet allo is 50/50  WW/lead water dropped. Zero leading.  I don't shoot the rifle much anymore as it's not challenging to get cast to shoot because it does shoot cast about as well as jacketed.  Another BTW all the jacketed loads from it go into 1/4 inches. That includes, for the jacketed, 100 grain weight bullets.  If you really want to learn the real truth to rpm you talk to Bryan Litz.  

Sorry to stir the pot. 

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MP1886 posted this 14 November 2023

Ray you didn't stir the pot at all and I hope your wife does good in surgery and makes a full recovery. This subject just gets people rankled for some reason. Tom Gray was talking about bullets being twist limited long before any of the current players, me included, brought it up on the internet. There's really no rules and I just use my own experience and calculating what people in the CBA match game are doing. It seems to be the concensus even if it isn't known to the shooters themselves that there might he something to it. As for the smaller calibers I was shooting my 6.5 ppc with a 170 gr bullet in an 8 twist right around 1700 fps. That calculates to 153,000 rpm which lead me to believe smaller calibers can take more rpm before falling apart. And as I said for 30 caliber I believe around 130,000 is the upper limit but think around 125,000 might be better but as usual your mileage may differ. I'd like to see people put pen to paper and post what bullet rpms are giving them the best accuracy to either point to a possible correlation or call bs on the whole idea. Either works for me.
For me the best person for explaining this is Pat I.  This sentence here "There's really no rules and I just use my own experience and calculating what people in the CBA match game are doing." is one of the best explanations that there is.  THERE ARE NO RULES especially since LG's threshold "stuff" is an elastic band that can be stretched.  That means you can really stretch and bend the hell out of it by changes things. It's almost like there is an infinite rpm limit. Notice I said "almost".  

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9.3X62AL posted this 14 November 2023

That's interesting to learn about the Barnes all-copper bullets.  In California we are limited to lead-free bullets for hunting, so Barnes have been my go-to hunting bullets for 15 years.  Try as I might, they just don't shoot as accurately in my rifles and handguns as the core-and-jacket Noslers do.  They are hunting-accurate for deer, and their expansion is excellent--but they aren't quite Nosler Partitions yet.  

In the casting realm, Rotometals offers a non-toxic casting alloy that behaves a lot like lead alloys at the pot and in a gun barrel.  It is NOT CHEAP, but for the rifles in calibers that Barnes doesn't cater to it will keep you hunting.  So far this metal has held up to 1-9" twists at 1700 FPS and shot 1.5-1.8 MOA at 50 and 100 yards.  Interestingly, this metal has roughly the same specific gravity of the Barnes copper bullets.    

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MP1886 posted this 14 November 2023

9.3x62AL  yes I found it very interesting.  My friend is just fascinating to talk too. We're on the phone every week.  I think it was I that got him started on the BARNE's bullet.  Not only does he know the Barnes family his dad did too. The grooves aren't all the same size. You can't notice it, but the walls of the grooves aren't straight vertically. The groove is actually tigther at the bottom of the groove and he explained the machinery that can do that. I forget how he/they figured out the bullet was twisting in the confines of the bore and groove.  That's a tough one to envision. I said "so from the front of the bullet to the rear, it's twisted"? The reply was yes. Right away my mind went to a long cast Loverin bullet.  Those do have a speed limit and the faster you push them the less accurate they become. I'm going to bring that up with him and maybe we can design a better mould and bullet, BUT we will never get the bullet to drop from the mould because of that tapered walled groove!  I'll see what he says.  Still though might explain why many people said those long cast bullets for the 6.5 Swede were twisting, maybe they really were. 

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pat i. posted this 14 November 2023

Pat,

I looked through my notes and came up with some figures. 6PPC, 14 twist. Mountain Molds pointed bullet at 81.5 grains. Started at 32 clicks of LT30 (about 18 grains) at 2307 fps and 118,646 rpm. The groups here were around 3/4".

At 34 clicks (about 19 grains) 2436 fps and 125,280 rpm. These groups averaged just over 1/2".

With the Eagan bullet which is a truncated cone, it takes more velocity to stabilize. Usually a minimum of 2200 fps. I can shoot the Mountain bullet at 1700 fps without tipping. 

On the bullets I forgot to heat treat, the "groups" were 4 and 5 inches at 119,000 rpm.

 

 

Thanks Paul. I hope other people reply with some stats. Now people have to remember that when I give my opinion about bullets being twist limited I'm talking about for the best accuracy. I had my 06 plain base mental frustration out today. I ran the velocity up to 1600 fps. Granted the bullet came out of the barrel but shot 3 and 4 inch groups. Not that this means anything but rpm at the most accurate load is what I'm talking about which you provided.

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