Clarification on twist rates and their effect on boolit accuracy/velocity

[BHuij]

Hopefully I've posted this in the right subforum.

I'm trying to get my head wrapped around the relationship between twist rates, projectile diameter/composition/mass, and accuracy. I've done a bunch of reading and I think I have the basics down, but I came here to get clarification and correction if I've got something wrong.

Here's what I think I've learned:

-A smaller diameter projectile must spin faster to stabilize. Spinning too fast or too slow will cause inaccuracy
-A heavier projectile must spin faster to stabilize.
-A harder and smoother projectile (i.e. j-word vs cast lead) can spin faster before destabilizing and losing accuracy (hence we can't push boolits as fast as we can push bullets)

As an example, I have a Mosin Nagant I recently started reloading for. I'm shooting Lee 185 grain round nose GCs out of it. The 1-10 barrel twist means I hit maximum accuracy with this projectile as I approach 1800 FPS. Some informal load workup testing has confirmed this. I'm not actually chronoing, but it's easy to graph a line between the minimum load velocity and maximum load velocity in my literature, and sure enough, my most accurate load comes at about 1730 FPS.

Supposedly the accuracy falloff from pushing this boolit faster comes from the fact that WW alloy lead destabilizes as it spins faster than whatever RPM is achieved at 1800FPS. However, if I decrease the mass of the projectile, i.e. by going to a 155gr mold, I have to spin even slower to properly stabilize, which translates to an even lighter load becoming necessary (maybe somewhere in the neighborhood of 1500 FPS or so?).

Of course, factory ammo comes out at well over 2000 FPS to my knowledge. It certainly kicks like a mule compared to the mouse fart 23.1 grain load of IMR 4227 I'm getting best accuracy out of with my homebrew boolits.

Basically, due to the 1-10 twist of the rifle, if I want to shoot cast rather than buying j-word ammo, I'm basically capped at 1800 FPS unless I want to sacrifice considerable accuracy. Theoretically I could go to a heavier projectile (200 grains +?) if I wanted to be able to shoot faster, but I don't see an extra 15 or 20 grains of mass getting me even up to 2000FPS before my accuracy craps out. Is this all correct?

The reason I ask is because I'm a fairly bad shot with a rifle, and even casting and reloading for my MN, I'm still spending close to $15 to shoot 50 rounds every time I go to the range. I'm looking to get a rifle that I can reload for dirt cheap, where I can go and shoot hundreds of rounds for practice while keeping costs way down. 22LR is an obvious choice for economy (minus reloadability), but I'd like something I can get out a little further so I can practice shooting at 200 yards + for cheap.

I've looked into several small-caliber centerfire rifles, like .17 Hornet, .22 Hornet, etc. Something where the lead and powder cost per round become almost negligible and the price to shoot comes down to barely more than a primer. Unfortunately, it looks like most of these calibers are made to be well over 3000 FPS, often into the 4000+ FPS territory. If I can't push my MN past 1800 FPS, how fast can I reasonably expect to push a lead .17 projectile before it destabilizes? I've even heard stories of smaller projectiles spinning so fast that they completely disintegrate a few yards out of the muzzle.

I'd like to find something that costs pennies per round to shoot, which indicates small caliber. But also something where I'm not neutering the round's speed or accuracy by shooting lead instead of buying factory projectiles. Kinda seems like if that's what I want, I need a long barrel with a very slow twist, which probably puts me into "super expensive custom rifle" territory, defeating the purpose of trying to shoot cheap.

Any and all feedback is welcome. Thanks in advance!

[William Yanda]

Isn't that why some add copper to toughen the alloy?

[BHuij]

I have heard of this being done, however, my Lee 20 lb. production pot will not melt copper as far as I know :) Lead melts around 620 F, wheel weights seem to actually melt a little sooner, but copper doesn't melt until nearly 2000 F.

If it was possible to alloy the two in a cast iron pot and make ingots of copper/lead alloy, I don't even know if those would melt in my production pot.

[Silverboolit]

IIRC, that it is the length of the projectile rather than the diameter. Longer bullets require a faster twist to stabilize.

[9.3X62AL]

Take a look at "Greenhill Formula" for some info on optimizing twist rate. There is a cool little online calculator for this function that can be Googled up.

[popper]

Look at the thread under lead alloys, adding copper to alloy. No Cu won't 'melt' in your pot. 1800 is NOT the limit. Stability is a function of fps, twist and length of the projectile. Read Bama's thread and one from Larry Gibson. Actually, for 30 cal 1800-2000 something at 100 yds for hunting is fine.

[BHuij]

...Stability is a function of fps, twist and length of the projectile. Read Bama's thread and one from Larry Gibson. Actually, for 30 cal 1800-2000 something at 100 yds for hunting is fine.

Even if your numbers are correct, and I can get 2000 FPS out of cast bullets before I lose stability, that's still shy of the 2600ish velocity of factory ammo. Surely this isn't because jacketed bullets are longer than cast? If that's the case, wouldn't I just want to find a super long mold so I can push it harder before losing stability?

[JBinMN]

I am posting because I am interested in the replies here, to see about gathering some more knowledge, and I have a question.

Not important to your goal(s) here, I am curious in how ya figure you spend $15 to shoot 50 rounds? $0.30 each cartridge. You don't have to answer, as it is kind of a rhetorical question based on a mental exercise for me this morning, to figure out what I did, just for fun.
;)

I figured:

Powder ( 23.1 gr,/IMR4227 @ approx. $30.00/lb) = approx. $0.10 each
Lead boolit @ 185gr. with lead approx. $1/lb = approx. $0.03 each
Primers at approx. $4.00/100 using 50 = approx. $0.04 each

Total "before" brass cases, which is multi use with one time cost = $0.17 each x 50 loaded cartridges = $8.50

"Time" I do not figure in, as I consider it a "hobby" & folks have different quality & priced tools & varied amounts of experience. Too hard to add time in properly, to get a good figure,IMO. That is what I tell myself anyway. LOL
;)

I do not add in brass as it is used over & over until it is un-serviceable. Even then it is around $1 each so I can't figure out where ya got the $15.00 per 50.

----------------------

Like I said....I "am" interested though, in seeing what sort of replies ya get as far as feedback on your goal(s).
:)

G'Luck!
:)

[BHuij]

Here's my math:

Lead: Free (WWs)
Powder Coat: Less than $0.01/round, so effectively negligible.
Powder: for ease of math, 25 grains of IMR 4227. 1lb = 7000 grains = $30. 7000/25=280 rounds per $30, or just shy of $0.11/round.
Primer: CCI 250 Large Rifle Magnum. 1000 primers = $30, so $0.03/round.
Brass: I bought 60 rounds of Prvi Partizan factory ammo so I could run them through my Mosin and see if this rifle was accurate enough for me to keep it. Shipped, those 60 rounds were about $50. Assuming I can get 10 uses out of each casing (not counting the first shot), that's 600 shots for $50, or about $0.08/round.
Time: Not factored in since I enjoy this hobby and don't consider it "work" ;)

All told, I'm paying $0.11 in powder, $0.03 in powder, and $0.08 in brass costs over time every trigger pull, or about $0.22/round. 50 rounds per range outing = $11. So I guess $15 was a bit of an overestimate :)

$0.22/round is pretty stellar for shooting a rifle like this, but compared to $0.05/round for shooting 9mm, it feels expensive haha. Certainly I'd love to find a caliber I could shoot for barely over the cost of a primer. Just seems like if I'm set on casting boolits for said rifle caliber, I'm really not going to be able to get the velocity that the caliber is designed for. But the cheapest .224 bullets I can find are still like $0.08/each... a LOT more than the $0.00/each I can do with cast ;)

[JBinMN]

Thanks for the answer, even though ya did not need to do it!
:)

:drinks:

[GhostHawk]

Question, do you realize that for just target shooting you do not have to push the speed?
3 to 5 grains of Red Dot can be just as accurate as 15 - 20 grains of IMR 4227 or other powder, at roughly one quarter the cost.

Lower your speed some, but lower your costs significantly. And if you really wanted to go low cost, you might consider something like the Lee .314 90 grain truncated cone mold. Size to fit your bore + 1 or 2 thousandths.

Shoot over 4.5 to 6 grains of Red Dot.

You use up a lot less lead. BPI still has promo/Red Dot for about 24 or 25$ a pound with hazmat if you buy in bulk. That brings your powder cost down to 2 cents ea, 3 - 4 cents for primer. No gas check. 6 cents ea or 6 dollars per hundred. Not quite as cheap as .22lr, but a lot more pop.

And it will punch paper at 100 or ring steel just as well. Might not do as well at 200, I have not tried that.

Brass, personally I don't figure in. I count that same as tools, cost of doing what I love to do.
Cared for, annealed when it needs it, trim when it needs it. It lasts a LONG time.

YMMV, just something to consider.

[BHuij]

Good insight. I am primarily wanting to just punch paper in terms of high-quantity practicing. Maybe shooting super light, super cheap target loads out of the Mosin is a good way to do it. I suspect I'll do most of my practicing with ironsights at 100 yards (don't have a scope, don't have plans to get one for this rifle). It would be nice to be able to get out to 200 yards though. Do you think a cheap load of Red dot would get me out there?

[Silverboolit]

In my .308, 10-13 grains of Promo/Reddot shoots very well @ 100 and not too bad at 150. Have'nt taken further, but would expect similar results.

[runfiverun]

faster twists are used to stabilize the longer bullets at lower velocity's.

[303Guy]

My idea for adding copper was to improve the terminal performance. Toughening as opposed to hardening. And the copper gets dissolved into the alloy.

I was aiming to use copper with a lower antimony and higher tin content (because the tin is available and the antimony not so much). That and the brittleness factor.

When I was playing around with a ductile alloy, I found that boolit swaging in the throat would form tail finning. Harder alloy boolits didn't do that.

[plainsman456]

No faster than you are pushing those boolits the brass should last a long time.

As for a fun shoot Red dot of unique will get them going pretty good.

But some regular primers.
But the powder coating i can't help with cost because i have not been bitten with that bug yet.:bigsmyl2:

[Larry Gibson]

Stability and the adverse affect RPM will have at a certain level on a bullet in flight are completely unrelated to each other. You can easily have a bullet that is inaccurate under the RPM Threshold because the bullet is unstable. Conversely you can have a stable bullet over the RPM Threshold that is inaccurate because of the adverse affect of the RPM.

The RPM Threshold is not a "limit". The RPM Threshold will generally occur between 120,000 and 140,000 RPM with ternary cast bullets. That is exactly where it occurred for the OP. The RPM Threshold can be moved up and it can be moved down. Adding Cu to the alloy, using a better designed cast bullet and using a slower burning powder are some ways to move the RPM Threshold up. Using to soft an alloy, to fast a burning powder and poorly cast bullets are ways to lower the RPM Threshold.

"A harder and smoother projectile (i.e. j-word vs cast lead) can spin faster before destabilizing and losing accuracy (hence we can't push boolits as fast as we can push bullets)"

The concept is correct except the faster you push such in your MN the inaccuracy is not from destabilization but from your cast bullets exceeding the RPM Threshold. If we control the RPM and keep it below the RPM Threshold we can shoot cast bullets just as fast as jacketed bullets......at least to 3000 fps. However, you're not going to do that with your MN and it's issue barrel. If you want to shoot "cheap" with your MN then a Lee TL314-90-SWC over 3.2 gr Bullseye will be just about as "cheap" as it gets.

I shoot a lot of cast 314299s through my M91/30 sniper and my Finn M39 (which I use in CBA Military rifle matches). I cast them well out of #2 alloy and load them over 28.5 gr milsurp IMR4895 and use a 1 gr Dacron filler. These run 1825 - 1850 fps and are very accurate. This load also holds up quite well to 500+ yards. Not as cheap as the previous load by any means but certainly cheaper than factory or even handloads with commercial bullets.

A read of my posts in this thread should give you a better understanding;

http://castboolits.gunloads.com/showthread.php?208186-RPM-Threshold-barrel-twist-velocity-chart

[BHuij]

A perfect example of the kind of clarification I was looking for, thank you! I always assumed that being below or above the "proper" RPM threshold caused destabilization which then compromised accuracy. From what you're saying, it sounds more like going above the RPM threshold may not actually destabilize the projectile, but compromises accuracy through some other mechanism.

If you're having success out to 500 yards with a projectile going about 1800 FPS, maybe I'm overthinking the need for high velocity. Since I'm using these only for target shooting, I don't need anything specific in terms of terminal ballistics. I just assumed that going 1800 FPS instead of 2600 FPS was going to preclude the possibility of shooting accurately out to longer ranges. If I can accurately achieve even 300 or 400 yards at 1800 FPS with enough energy to punch through a sheet of paper at the end, then I'm worrying about nothing.

[44MAG#1]

I bet this is going to turn ugly.

[Bama]

Hopefully I've posted this in the right subforum.

I'm trying to get my head wrapped around the relationship between twist rates, projectile diameter/composition/mass, and accuracy. I've done a bunch of reading and I think I have the basics down, but I came here to get clarification and correction if I've got something wrong.

Here's what I think I've learned:

-A smaller diameter projectile must spin faster to stabilize. Spinning too fast or too slow will cause inaccuracy
-A heavier projectile must spin faster to stabilize.
-A harder and smoother projectile (i.e. j-word vs cast lead) can spin faster before destabilizing and losing accuracy (hence we can't push boolits as fast as we can push bullets)

As an example, I have a Mosin Nagant I recently started reloading for. I'm shooting Lee 185 grain round nose GCs out of it. The 1-10 barrel twist means I hit maximum accuracy with this projectile as I approach 1800 FPS. Some informal load workup testing has confirmed this. I'm not actually chronoing, but it's easy to graph a line between the minimum load velocity and maximum load velocity in my literature, and sure enough, my most accurate load comes at about 1730 FPS.

Supposedly the accuracy falloff from pushing this boolit faster comes from the fact that WW alloy lead destabilizes as it spins faster than whatever RPM is achieved at 1800FPS. However, if I decrease the mass of the projectile, i.e. by going to a 155gr mold, I have to spin even slower to properly stabilize, which translates to an even lighter load becoming necessary (maybe somewhere in the neighborhood of 1500 FPS or so?).

Of course, factory ammo comes out at well over 2000 FPS to my knowledge. It certainly kicks like a mule compared to the mouse fart 23.1 grain load of IMR 4227 I'm getting best accuracy out of with my homebrew boolits.

Basically, due to the 1-10 twist of the rifle, if I want to shoot cast rather than buying j-word ammo, I'm basically capped at 1800 FPS unless I want to sacrifice considerable accuracy. Theoretically I could go to a heavier projectile (200 grains +?) if I wanted to be able to shoot faster, but I don't see an extra 15 or 20 grains of mass getting me even up to 2000FPS before my accuracy craps out. Is this all correct?

The reason I ask is because I'm a fairly bad shot with a rifle, and even casting and reloading for my MN, I'm still spending close to $15 to shoot 50 rounds every time I go to the range. I'm looking to get a rifle that I can reload for dirt cheap, where I can go and shoot hundreds of rounds for practice while keeping costs way down. 22LR is an obvious choice for economy (minus reloadability), but I'd like something I can get out a little further so I can practice shooting at 200 yards + for cheap.

I've looked into several small-caliber centerfire rifles, like .17 Hornet, .22 Hornet, etc. Something where the lead and powder cost per round become almost negligible and the price to shoot comes down to barely more than a primer. Unfortunately, it looks like most of these calibers are made to be well over 3000 FPS, often into the 4000+ FPS territory. If I can't push my MN past 1800 FPS, how fast can I reasonably expect to push a lead .17 projectile before it destabilizes? I've even heard stories of smaller projectiles spinning so fast that they completely disintegrate a few yards out of the muzzle.

I'd like to find something that costs pennies per round to shoot, which indicates small caliber. But also something where I'm not neutering the round's speed or accuracy by shooting lead instead of buying factory projectiles. Kinda seems like if that's what I want, I need a long barrel with a very slow twist, which probably puts me into "super expensive custom rifle" territory, defeating the purpose of trying to shoot cheap.

Any and all feedback is welcome. Thanks in advance!

Look at a 6x45 6mm in a 223 or 556 case. With the popularity of the AR you can usually pick up all you would ever need. Case forming is a no brainier, just lube case and run it through a full length forming die, add primer and you are ready to add powder and bullets--o yes you have to champher inside of case and check overall length. Unless using Red Dot, most of charges are from 20 to 25 grains. Recoil in non existant and it is accurate even out to 350 yds. It also does well with cast boolits and jacketed. It doesn't break the bank to shoot a couple hundred a week AND Grand Kids love them.

[BHuij]

This is an interesting caliber to look at. I'm also looking in to 222 Remington. Both seem very economical to shoot.

[EDG]

You might check the articles on cast bullet alloy strength and pressure at the Los Angeles Silhouette Club site.
If your pressure is too high for a specific alloy it may strip in the rifling.
For rifling twist issues you need to understand that bullet length is the most important characteristic. There are several bullet design calculators on line. Play with them to understand the affect of bullet length. PM me if you cannot find them.

[Outpost75]

Some related links:

http://kwk.us/twist.html

http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

http://bulletin.accurateshooter.com/2015/09/ballistics-tip-understanding-bullet-stability-twist-rate-and-mv/

http://bisonballistics.com/articles/bullet-stability