I have heard "over 1.0" and I have heard "0.5 is enough for a cast slug".

Having seen the sins of overspin on the 6.5 Swede, at what minimum stability level index # does the perfect cast lead bullet spin at?

To clarify which stability index # I am talking about, go here:

http://www.eskimo.com/~jbm/ballistics/drag/drag.html

What is that minimum stability index # again??

Oldfeller

[ 11-09-2003, 20:31: Message edited by: Oldfeller ]

OF, anything around 1.3 thru 1.5 is OK below 1100 fps, and no more than 2.0 anywhere up thru 2400 fps. For example, the 32-30 (32 Winchester Special) has a 16 twist, and the RCBS 170 boolit does exactly this specification. A natural shooter, that winchester lever is. If you are shooting pistols, a little more twist won't hurt just in case the boolit is destroyed somewhat up front, but a 16 twist in a 45acp is absolutely nuts. My next barrel, if there ever will be one, will be that barrel having a 24 twist, made by somebody offbeat for that gun. ... felix

Kelly I don't think you can hang your hat on a specific number. Alot depends on how far you want the slug to fly, as 1000 yd 45/70 shooters like Forrest for instance find the 16 twist makes a rounder hole in the cardboard vs the 18 often used per 45/70. Higher revs at exit means a longer range I guess you could conclude before the bullet starts it's destabilization thing [forget the term]. To contrast that, my 20" 45/70 shoots the long Postell find at 100 yds-- but 300 show'd some tipping [I think?]. Interesting thang is the Wingryo program sez this slug is on the edge of stability with a 20 twist too. Methinks that program is set on 1.5 You can download a Wingryo copy on the net free too.

I definitely believe a slug can be over stabilized. This goes to max accuracy attainable with that combination. BR guys use 14's and even some 15's shooten the 6 PPC or the 6 BR, finding the best accuracy there. 12's will often group much better in a 308 vs a 10 using the 165's or so. The last Palmer type rifle I saw in 308 wore a 13 twist shooten 155 gr match slugs.

Aladin, keep in mind, though, a flat boolit versus a pointed boolit at the same length, the flat boolit will require more twist. ... felix

Specifically, I am talking about the .177 Viper pellet we are getting ready to cut (in ever increasing numbers, it seems). It only has to fly about 150-200 yards while remaining stable (which turns out to not be an issue with any of the versions listed below, BTW).

The rub is we are now told by the airgun lists some of the Chinese airguns have only a 16 twist rate on their rifled barrels.

The Viper pellet was designed for a 10-14 twist world (premium brand adult air guns), with a very minimal stability number at 14 twist as we wanted as high a BC as we could get, keeping the long ogival nose. I own a Gamo Sporter at 14 twist, so I was very mindful of those 14 twist numbers as we did all the original calculations.

Now it looks like I may have to either shorten the design some or have folks complaining about their Chinese airguns "going keyhole" on them. Or I can tell the 16 twist folks that this pellet is too long for their slow twist guns and for them to simply not buy it (a little late in the game for that now).

Trouble with a 16 twist rate is it is just plain tough to plan around a 16 twist. Here is the existing design at 16 twist. As you can see, the numbers are very poor (they suck).

Spin calculations courtesy of http://www.eskimo.com/~jbm/ballistics/drag/drag.html

Drag/Twist Output, original design at 16 twist

Input Data
Caliber : 0.177 inches
Rotating Band Diameter: 0.177 inches
Total Length: 0.460 inches
Nose Length: 0.220 inches
Nose Type Secant
Meplat Diameter: 0.000 inches
Nose Radius: 15.000 cal
Boattail Length: 0.060 inches
Base Diameter: 0.140 inches
Weight: 35 grains
Twist: 16.0 inches
Specific Gravity: 11.14
Drag Function G1
Temperature: 59.0 �F
Barometric Pressure: 29.92 in Hg
Relative Humidity: 0.0 %
Altitude: 0.0 feet
Air Density: 100 % of Sea Level

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

Calculated Table
Tangent Radius / Radius: 0.120
Sectional Density: 0.137

Velocity Mach CD Form BC Twist Stability RPM
(ft/sec) Number Factor (G1) (inches)
500 0.448 0.115 0.555 0.247 11.11 0.724 22500
700 0.627 0.115 0.567 0.241 10.70 0.671 31500
900 0.806 0.118 0.457 0.300 9.89 0.573 40500
950 0.851 0.169 0.594 0.230 9.54 0.533 42750
1000 0.896 0.250 0.734 0.186 9.07 0.482 45000
1050 0.940 0.345 0.866 0.158 8.32 0.406 47250
1100 0.985 0.611 1.340 0.102 7.44 0.324 49500
1150 1.030 0.907 1.762 0.078 10.19 0.609 51750
1200 1.075 0.847 1.476 0.093 10.41 0.635 54000

==========================

Now here are the numbers at 16 twist when trying to completely "fix it" at 16 twist by shortening the body and the nose and the boat tail to the maximum that still yeilds something that looks like a Viper pellet (proper proportions still, looks very much like the .22 Viper pellet sized down to a much shorter, lighter .177 diameter).

Drag/Twist Output at 16 twist with shortened everything
-----------------------------------------------------------

Input Data
Caliber : 0.177 inches
Rotating Band Diameter: 0.177 inches
Total Length: 0.370 inches
Nose Length: 0.180 inches
Nose Type Secant
Meplat Diameter: 0.000 inches
Nose Radius: 15.000 cal
Boattail Length: 0.040 inches
Base Diameter: 0.150 inches
Weight: 25 grains
Twist: 16.0 inches
Specific Gravity: 11.14
Drag Function G1
Temperature: 59.0 �F
Barometric Pressure: 29.92 in Hg
Relative Humidity: 0.0 %
Altitude: 0.0 feet
Air Density: 100 % of Sea Level

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

Calculated Table
Tangent Radius / Radius: 0.086
Sectional Density: 0.114

Velocity Mach CD Form BC Twist Stability RPM
(ft/sec) Number Factor (G1) (inches)
500 0.448 0.139 0.675 0.169 14.49 1.231 22500
700 0.627 0.141 0.692 0.165 13.91 1.133 31500
900 0.806 0.148 0.572 0.199 12.82 0.963 40500
950 0.851 0.258 0.903 0.126 12.38 0.899 42750
1000 0.896 0.354 1.038 0.110 11.80 0.816 45000
1050 0.940 0.460 1.155 0.099 10.92 0.698 47250
1100 0.985 0.721 1.581 0.072 9.80 0.563 49500
1150 1.030 1.044 2.028 0.056 13.61 1.086 51750
1200 1.075 0.989 1.723 0.066 13.88 1.128 54000

This still has a low stability rough spot at the sonic transition zone 1,050-1,100 fps when fired through a 16 twist barrel. Sad thing is, you can get there now that the pellet is so light.

Downsides to this change are that the pellet will speed up as it is much lighter in weight now. You may be able to get up into the sonic zones again with some of the 1,250 supermagnum guns (remember the sonic disruption effects on accuracy and the fact this shortened pellet still DOESN'T like the sonic zone). Plus the lighter much shorter pellet loses a good bit of its desirable BC and long range energy retention characteristics.

(Remember, we intended the original very heavy weight to store energy at the 100% sub-sonic speed ranges so you didn't ever have to deal with the sonic disruption zone in the first place)

With 30-35 grains weight, the original pellet would run 800-950 fps out of a 1,250 adult supermagnum airgun where it still had fairly good stability numbers (even at the slowest 14 twist originally planned for). It would have run about 500-600 fps out of my old Gamo, just about the book perfect "match accuracy" speeds. Stability numbers on the original design at 14 twist would have looked like this:

Original design:

Drag/Twist Output at 14 twist with pellet never exceeding 1,000 fps

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

Input Data
Caliber : 0.177 inches
Rotating Band Diameter: 0.177 inches
Total Length: 0.460 inches
Nose Length: 0.220 inches
Nose Type Secant
Meplat Diameter: 0.000 inches
Nose Radius: 15.000 cal
Boattail Length: 0.060 inches
Base Diameter: 0.140 inches
Weight: 35 grains
Twist: 14.0 inches
Specific Gravity: 11.14
Drag Function G1
Temperature: 59.0 �F
Barometric Pressure: 29.92 in Hg
Relative Humidity: 0.0 %
Altitude: 0.0 feet
Air Density: 100 % of Sea Level

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

Calculated Table
Tangent Radius / Radius: 0.120
Sectional Density: 0.160

Velocity Mach CD Form BC Twist Stability RPM
(ft/sec) Number Factor (G1) (inches)
500 0.448 0.115 0.555 0.288 11.11 0.945 25714
700 0.627 0.115 0.567 0.282 10.70 0.876 36000
900 0.806 0.118 0.457 0.350 9.89 0.748 46286
950 0.851 0.169 0.594 0.269 9.54 0.697 48857
1000 0.896 0.250 0.734 0.218 9.07 0.629 51429

So, what do I do? If I react by shortening the design up for the 16 twist folks I lighten things way up but only get a .563 stability number in the sonic zone for all the trouble.

Or do I stick with the original design, tell the Chinese airgun folks that it is only good for 14 twist or faster guns, and give them a massive slow pellet at match accuracy speeds with a .700+ stability number at the worst condition they can possibly reach (950-1000 fps at the muzzle)? BTW, the original design became more and more stabilized as the fps speed dropped, with the index numbers increasing past 1.000 as you dropped below 500 fps. That was its true design cleverness, it got better and better the longer the distance it traveled.

This means telling people to measure the twist in their barrels and if it is a 16 twist offer to send their check back to them? Not the best thing to be doing at this stage of things, not to me anyway.

This 16 twist situation sucks, damn if you do, damn if you don't.

================================================
================================================

A compromise is possible, a half shortened .177 Viper design that keeps most of the weight that is needed to keep things nice and slow. Like all compromises it gives some up on both ends. The advantage is no $21.50 checks need to be returned to anybody.

Nose length is ...................... .190"
Bore rider band to bore rider band is .160"
Boat tail length is ................. .040"
Over all length is .................. .390"

It looks about like this at 16 twist:

Velocity Mach CD Form BC Twist Stability RPM
(ft/sec) Number Factor (G1) (inches)
500 0.448 0.139 0.673 0.203 13.72 1.102 22500
700 0.627 0.140 0.690 0.198 13.19 1.019 31500
900 0.806 0.143 0.556 0.246 12.19 0.871 40500
950 0.851 0.237 0.832 0.164 11.79 0.814 42750
1000 0.896 0.329 0.964 0.142 11.25 0.741 45000
1050 0.940 0.431 1.082 0.126 10.42 0.636 47250
1100 0.985 0.689 1.510 0.091 9.37 0.515 49500
1150 1.030 1.005 1.953 0.070 12.85 0.968 51750

It gives a 16 twist Chinese airgun person slightly better than the old 14 twist premium airgun stability numbers, which were considered "good enough" back then. At this stage, let's call them "marginal" but as good as we are going to get at 16 twist period, without totally screwing up the design.

===============================================

and it looks like this at 14 twist now:

Drag/Twist Output

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

Input Data
Caliber : 0.177 inches
Rotating Band Diameter: 0.177 inches
Total Length: 0.390 inches
Nose Length: 0.190 inches
Nose Type Secant
Meplat Diameter: 0.000 inches
Nose Radius: 15.000 cal
Boattail Length: 0.040 inches
Base Diameter: 0.150 inches
Weight: 30 grains
Twist: 14.0 inches
Specific Gravity: 11.14
Drag Function G1
Temperature: 59.0 �F
Barometric Pressure: 29.92 in Hg
Relative Humidity: 0.0 %
Altitude: 0.0 feet
Air Density: 100 % of Sea Level

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

Calculated Table
Tangent Radius / Radius: 0.093
Sectional Density: 0.137

Velocity Mach CD Form BC Twist Stability RPM
(ft/sec) Number Factor (G1) (inches)
500 0.448 0.139 0.673 0.203 13.72 1.440 25714
700 0.627 0.140 0.690 0.198 13.19 1.331 36000
900 0.806 0.143 0.556 0.246 12.19 1.138 46286
950 0.851 0.237 0.832 0.164 11.79 1.063 48857
1000 0.896 0.329 0.964 0.142 11.25 0.968 51429
1050 0.940 0.431 1.082 0.126 10.42 0.831 54000
1100 0.985 0.689 1.510 0.091 9.37 0.672 56571
1150 1.030 1.005 1.953 0.070 12.85 1.264 59143
1200 1.075 0.950 1.656 0.083 13.09 1.311 61714

This compromise will allow everyone (16 twisters included) to have a little better than what we orginally planned for the 14 twist premium airgun crowd. The pellet should still always stay in the sub-sonic zone and it will keep most of its original BC and long range energy retention characteristics.

This is the design compromise I propose to the group as the final design. A new CAD drawing will be forwarded to LEE before the pellet molds are cut (and after the discussion dies down).

Your comments are desired, so please do give your comments below. I will be posting a link to this discussion on the air gun boards, so expect some visitors. Use some of the other gyro calculators to see what sort of number they give you (they are all slightly different).

Oldfeller

=========================================

P.S. Vintage old Lothar/Walther 18 twist barrel guys, you buy this thing at your own risk -- your numbers are quite poor with that very slow old twist rate no matter what we do to the .177 Viper design to improve it.

[ 11-10-2003, 01:11: Message edited by: Oldfeller ]

"It only has to fly about 150-200 yards while remaining stable (which turns out to not be an issue with any design, BTW)"

Ya lost me. Is yds really feet?

The ( ) statement really looses me. I've shot my share at LR and have seen the holes understabilized slugs make. Please explainer??

Aladin, it starts out of the airgun's muzzle at 900 fps or so at the worst stability factor it will ever see. As it travels down range and slows down the stability factor increases to well over 1.0 as the speed drops (see the previously attached tables).

I know most folks are used to shooting the kiddie airguns they had when they were young over feet distances instead of yard distances, but this one is intended to shoot yards (like 100 or more) using adult supermagnum airguns and good scopes.

Right now the limiting factor to longer distance airgun shooting is the high ballistic air drag that is inherent with the diablo skirted pellets. They lose over half their speed in 35 yards and have an exaggerated rainbow trajectory past 50 yards because of this effect.

Oldfeller

[ 11-09-2003, 22:09: Message edited by: Oldfeller ]

I have found a source for 1-10 or even 1-9 twist .177 barrels. They are 25" long, 7/16 diameter (perfect size for Talons breech end, by the way, with a bit of ingenuity to thicken up the barrel diameter for the stock size bushings or turn new bushings to fit, which is what I have already done). I have ordered mine and it will be here Monday I think. They are Redman barrel liners from Brownells. The part number from Brownells is 770-000-001. Retail is only $35.17 plus shipping. This is a brand new item they have never listed before.
Go here for the complete scoop:
http://www.brownells.com/aspx/NS/store/ProductDetail.aspx?p=10974&title=.17+%26+.22+BARREL+LINER&s=41096#41096

It is not that hard to drill out a barrel and install this barrel liner using AccraGlass for the folks with the Chinese guns, thus keeping the pellet weight up to what was designed in the first place. Myself, I don't want the pellet lightened, it would just defeat the purpose this project was initiated for in the first place. Thanks for reading.
Comments or additions?

Fellas,

This pellet was not set up for the chinese guns even though that's what I have and plan to shoot it plenty. The extra weight is why I wanted it. At $21.50 it's not a lot to invest.

Mike

The Shotgun News I received yesterday has an article on Chinese Air rifles and pistols. In general it didn't give them a very good rating. It did say the quality is going up in some cases.

Here is the drawing for the minor re-tune of the .177 Viper pellet so it better accomodates 14-16 twist rate barrels.

http://images.ofoto.com/photos493/7/55/93/2/92/0/92029355705_0_ALB.jpg

Thanks to the Talon guys for pointing out the twist rate issues with the slower barrels. We could not figure out how to accomodate the 18 twist Lothar / Walther barrels, but were able to accomodate the 14-16 twist rate barrels.

18 twist is just too too slow for the size and weight of this pellet.

For those of you who wanted to hold off sending your $21.50 check for fearing "twist rate problems" you need to get up with Mike as soon as you read this to get your check in to Mike.

Mike is planning to close the order for the .177 Viper this week. He is already the best part of 2 weeks behind his initial planned schedule and has enough orders for the production run and I don't think he is going to wait any more.

Oldfeller

[ 11-12-2003, 20:31: Message edited by: Oldfeller ]