twist rate?

[trapperdan]

I know that several here have tried or have gone the shortened 20 vartarg route or similar shortened cartridge. Well, I’m going to give it a try. Have the drawing made up for a 20 vt shortened .200 ( inspired by NorCalMike) and am taking it to PTG to have a reamer made and will be sending it to whidden for dies. I am building this for shooting Hornady 24 gr NTX bullets. Getting ready to order a barrel. My question is what would be the best barrel twist rate for this small cartridge and small bullet. Thanks.
Dan

 

[RGRobinett]

Measure some bullets - follow instructions - and use this calculator.
< http://www.jbmballistics.com/cgi-bin/jbmdrag-5.1.cgi >
If the link does not work, go to JBM Ballistics, open the 'calculator' tab, scroll down to BULLET DRAG/TWIST, and have at it - you will not be disappointed. RG

P.S. Where called for,dimensions must be converted to calibers!

 

[trapperdan]

Thanks for the link RG. I tried several times and kept getting three input errors “Less than allowed minimum value” so apparently it does not recognize that small of a bullet. I am hoping that someone shoots these small calibers that can shed some light on it.
Thanks again. Dan

 

[22vartarg]

What is your project velocity?
I shoot the 20 VarTarg and run a 1:12 for my 32 to 39 grain bullets and a 1:11 in another one that I shoot up to 40 grains.
I can get the calculator to work, but don’t have velocity for 24 grain bullet.

 

[trapperdan]

22 vartarg.
Since this is still in the planning stage, I can only estimate the velocity. My 20 viper (20 vt .107 short) with the 24 gr ntx shoots in the 4000-4030 range with 18.8 gr of powder. The 20 vt “ short” ( 20 vt .200 short ) will have about 2 grains less powder so I am estimating 3800-3900 velocity. That’s the best I can do for now. Thanks for the help.
Dan

 

[22vartarg]

I say 1:12 should be fine..... maybe 1:11 to be overly safe.

 

[Tommie]

Just do 1-11”
No down-sides and all the bases are covered.

 

[Sniper338]

I run a 12t for 32gr boolits

 

[trapperdan]

Thanks for the help guys. Another question to complicate things more. I had a guy offer to sell me a Wilson 20 cal barrel at an attractive price but it is a 10 twist. How would that handle the 24 and 32 grain bullets? Would it over twist them?
RG
I am still working on the twist calculator and about have it figured out. Thanks.
Dan

 

[Evan]

10 twist at 4000fps is 288krpm. Most bullets won't start popping until you hit 300krpm. I'd say that a 10 twist is safe.

I'd love to see pictures of a loaded round and the rifle, along with results when you get it together. I've always had a soft spot for the 20 Squirrel, but I haven't found the time or money to build one yet.

 

[RGRobinett]

Thanks for the help guys. Another question to complicate things more. I had a guy offer to sell me a Wilson 20 cal barrel at an attractive price but it is a 10 twist. How would that handle the 24 and 32 grain bullets? Would it over twist them?
RG
I am still working on the twist calculator and about have it figured out. Thanks.
Dan

Dan, I looked 'em up, and since the NTX are a 'composite copper' core, with a 'tip', they are likely LONG relative to their mass: we need to know the over-all-length of the bullet. We can then 'plug & play' with the specific gravity, to match the weight, and obtain a relatively precise twist rate.

Just for example, for Sg1.5, let's take a .224cal bullet (just made up length), 0.80" long, with a 0.070" BT length: with a aluminum core, @18.0gr, we'd want a 1:6.9" twist; copper, 24gr (cast, or, rolled, thus, likely somewhat more dense than the NTX core), 1:10.8"; lead - 52gr - 1:11.7" twist.

Thus, specific gravity:length plays a significant roll in determining the overturning moment. Again, we must know the OAL of the bullet, or, it's your best guess against the next guys - why gamble on this important attribute of a custom build?!?

That said, if we take a 20cal, hypothetical OAL 0.70", featuring a 9*x0.060" BT, & a solid copper core, or, monolithic construction, thus, about 39gr, a 1:10" twist should be almost perfect (Sg1.5) . . . but, if we adjust the specific gravity to attain 24gr, we need 1:8.8" twist - a 10" twist wouldn't be a total wreck, but it would be undesirable. So, to come up with a reliable twist, we must know the OAL! Oh, all of this modeling was done at 4K fps. RG

Oooops, I forgot - if the pendulum swings to the shorter side, that's better - perhaps much so! Yet another, "I forgot" . . . it's almost impossible to, 'over-spin' a quality bullet - that is, a bullet with zero, or, very near zero (as in custom made) center-of-gravity off-set!! The only penalties for excessive spin:
1) may exacerbate bullet failure - usually high velocity and thin jacket combination.
2) much more subtle, will exacerbate vertical component wind-drift - gyroscopic precession. A prairie dog probably won't notice . . .RG

 

[trapperdan]

Thanks for the help RG. I really appreciate it. I will get more information to you later tonight. And you’re right about getting it right the first time. If it doesn’t shoot good I will always wonder if it was the wrong twist, wrong load or just my shooting ability.
Dan

 

[trapperdan]

10 twist at 4000fps is 288krpm. Most bullets won't start popping until you hit 300krpm. I'd say that a 10 twist is safe.

I'd love to see pictures of a loaded round and the rifle, along with results when you get it together. I've always had a soft spot for the 20 Squirrel, but I haven't found the time or money to build one yet.

Hi Evan. Here are the pictures. I have included 3 others for comparison. Left to right, 204, 20 vartarg, 20 viper an the one that I am working on now, the 20 vt "short".

The rifle is a cz 527 that is my switch barrel for all 4

Fun projects. Dan

 

[Evan]

Hi Evan. Here are the pictures. I have included 3 others for comparison. Left to right, 204, 20 vartarg, 20 viper an the one that I am working on now, the 20 vt "short".

Those look great! What powder are you targeting with the 20 short? Seems like you may not get enough 4198 in there. Pistol powders?

 

[trapperdan]

RG

To the best of my ability, the following measurements are what I came up with.
24 grain ntx
Overall length with plastic tip 0.638
Length without tip (if it maters) 0.403
Base diameter of boattail 0.1715
Boattail length 0.066
G1 BC. 0.195
Sectional density 0.103

Hope this helps. Thanks again.
Dan

 

[Oso]

Curious mind - I like projects. What is impetus for the 20VTG Short using 20gr class of bullets? What is expectation of performance of it compared to something like a .17 Vartarg using 25gr bullets?

FYI - I have a 20VTG with 1:11 for bullets in the 30's gr.

 

[trapperdan]

Your right Evan. I think h 4198 is out but it is my go to for 20 VT. The powders I plan on trying are cfe blk, A2200 and RL 7. I have tried LIL Gun in other small cartridges but it gets a little scary for me but it would probably work fine. My 20 viper which is only .093 longer than the “short” shoots a little over 4000 w 18.8 G cfe blk and the 24G ntx
I'm not familiar with the 17 vartarg so can’t compare. I do shoot a 17 spatan which is also a 20 vt .200”short” necked down to 17 caliber. My favorite squirrel round. It shoots the 15.5 ntx as fast as 4141 but accuracy was terrible but shoots in the 3’s &4’s at around 3800.
Dan

 

[Evan]

Your right Evan. I think h 4198 is out but it is my go to for 20 VT. The powders I plan on trying are cfe blk, A2200 and RL 7. I have tried LIL Gun in other small cartridges but it gets a little scary for me but it would probably work fine. My 20 viper which is only .093 longer than the “short” shoots a little over 4000 w 18.8 G cfe blk and the 24G ntx
I'm not familiar with the 17 vartarg so can’t compare. I do shoot a 17 spatan which is also a 20 vt .200”short” necked down to 17 caliber. My favorite squirrel round. It shoots the 15.5 ntx as fast as 4141 but accuracy was terrible but shoots in the 3’s &4’s at around 3800.
Dan

I've come to despise AA2200. One of the least stable powders I've ever used. Can get a lot of it in there, and it produces excellent speed, but it's scary how fast the pressure builds as ambient temperature increases. Worked up safe loads at 70 degrees and at 90, bolt lift was stiff and brass was trashed. Even on seriously conservative loads going nice and slow, it moved so much with temperature that I couldn't stay in a node from morning to mid-day. Ramshot X-terminator was better, but it's another step slower - moving in the wrong direction for you.

 

[RGRobinett]

RG

To the best of my ability, the following measurements are what I came up with.
24 grain ntx
Overall length with plastic tip 0.638
Length without tip (if it maters) 0.403
Base diameter of boattail 0.1715
Boattail length 0.066
G1 BC. 0.195
Sectional density 0.103

Hope this helps. Thanks again.
Dan

Yup - 'I was wrong again'! This bullet - much shorter than I imagined - should be an almost perfect match for a 1"11.1" twist (Std Conditions/Sea Level, and 3-4K fps, Sg 1.5) ! Good shootin! RG

 

[trapperdan]

Evan: Thanks for the heads up on the AA2200 powder. I will use extreme caution if I do use it. Cfe blk works so good in 3 of my other calibers that I am hoping this will be my go to powder for this one.

RG
Thanks for helping on this. I do appreciate it. That’s one less thing I can blame if this doesn’t work out. PTG gave me a lead time of 8-12 weeks for the reamer. Going to check with Pac-nor tomorrow on an 11 twist varmint barrel and hopefully will have it shooting before the squirrels come out in Feb.

Thanks for the help and suggestions from all that responded.11 twist it is.
Dan