Relationship between bullet weight, twist rate and velocity?

[Vandis2]

I'm starting out and am interested in some theory of load development if someone has already figured this out.

Is their a noteworthy relationship between velocity and twist rate given a fixed bullet weight (i.e., 95gr MK)?
Given my .243 barrel with its twist rate of 9.125" and a fixed seating depth relative to the lands would various velocities effect, if any, this bullets' potential to stabilize for accuracy?
Also what role do slower or faster burning powders play in this?

Curious

 

[Vandis2]

Will do!

 

[boltgunluvr]

please note...the longer a bullet is, one must consider the bullet's bearing surface relative to the barrel twist. And since you mentioned a 1 in 9.125 twist, I'll assume you are shooting a Rem? If so, and if you're having problems finding the sweet spot, try seating to mag length (2.8-2.820) using 40.5ish gr of H4350. Let me know. I use the 95 SMK jumping .065 and it is oh sooooo sweet.

 

[Lapua40X]

Will this help in any way?

http://kwk.us/twist.html

 

[357Mag]

Vandis -

Howdy !

You can make use of the digi " Powley Computer(s) " , that can be found on the net. Use of Homers computer(s) will help make these relationships MORE clear. Especially, as regards " expansion ratio ".

IF you want to go old-school, order a copy of " Guns & Ammo 1974 Annual ". That edition includes many pages and graphics authored by Mr. Powley. After the "BIBLE", it was the one publication that most profoundly influenced my thinking........ no joke.


With regards,
357Mag

 

[Vandis2]

please note...the longer a bullet is, one must consider the bullet's bearing surface relative to the barrel twist. And since you mentioned a 1 in 9.125 twist, I'll assume you are shooting a Rem? If so, and if you're having problems finding the sweet spot, try seating to mag length (2.8-2.820) using 40.5ish gr of H4350. Let me know. I use the 95 SMK jumping .065 and it is oh sooooo sweet.

Thanks Boltgunluvr

Yes its a new .243 700SPS 26" varmint barrel, which I assume you are shooting as well?
I have replaced the stock with a freefloating H-S Precision thumbhole and added a 1lb Jewell trigger. I am now out of money so am buying my reloading equipment and supplies piece by piece.

Thanks for the load info.
I am using this time to study up before buying primers and powder. H4350 seems to be the most popular for over 80gr. Your formula will be very helpful.

 

[JeffG]

My (relatively inexperienced) belief was that any given bullet has an optimum rotation-per-second that allows it to use it's ballistic coeffiecient most effieciently and accurately;

i.e. your 95g MK in a 9.125" twist barrel achieves best accuracy at (?? 3100 fps??) which equals 4076.7 rotations-per-second. You could put the same bullet into a slower twist barrel but you would have to push it faster to achieve the same rotations-per second...and I always thought that similar rotations per second (more than similar velocity, in different twist rates) would equal similar accuracy.


Trying to quantify accuracy ballistics is a little like trying to quantify God... in the end it's too big a mystery for me to understand.

 

[boltgunluvr]

Vandis2.......Yep I have same model. Also, I have played around with H4831 and H1000, but H4350 and IMR4350 were clear winners. Then I went after my seating depth and found the magic. Have same model in 223 and 308. All with B&C stocks, Timney triggers, EGW bases, Burris rings and Weaver GS Tactical (MidwayUSA exclusive) 3-10x40mm optics.

 

[lpreddick]

based on what i'v read and my experience , in general, the heavier the bullet, the longer it is and therefore the longer it's bearing surface and the faster the twist to achieve stability. also, in general, the heavier bullets shoot best with slower burning powder. THERE ARE ALWAYS EXCEPTIONS. longer bearing surfaces mean more resistance (drag) and the pressure curve needs to be more "gentle", i.e. not as sharp a spike. faster powders produce a sudden, sharp spike which is ok with a light,short bearing surface bullet that has little drag and leaves quickly. velocities tend to slow as bullet weight increases as velocity is a function of pressure and the pressure measured at propellant sublimation is due to the sublimation itself AND barrel resistance to bullet movement. powder charges can be increased to achieve more velocity. pressure will also increase and at some point get too high and pressure signs appear. powder volumn will reach max also. more velocity can now be achieved by going to a faster powder allowing you to add more. accuracy and velocity don't always go hand in hand. i have suspected that there is a relationship between bullet weight and barrel twist that can control accuracy. we know a fast twist will "shoot" light and heavy bullets, but the heavier ones tend to produce the more accurate groups. the lighter bullets seem to be most accurate in barrels with slower twist. you ask a very complex question and i obviously don't completely understand the process.

 

[BoydAllen]

If you want to get a better feel for this subject, I suggest that you spend some time on this site.
http://kwk.us/twist.html

 

[Vandis2]

Thanks all!
My knowledge base is increasing. I have an engineering background so this is fascinating stuff.
What a science but I am definitely do not want to get a calculator out just yet!

So far from what I have dug up online, there are excellent mathematical formulas out there for analyzing all aspects of ballistic theory and practice but the big picture is still not so clear.
Somebody has to be addressing the big picture.

On the practical side, there seem to be many, many knowledgeable competitive shooters out there sharing an abundance of info and who have an excellent sense for what works and doesn't work. The proof is in their accomplishments.

At this point there are too many variables in riflery for me to make sense of it all. Why, for example, as 'lpreddick' explains above, do heavier, longer bullets tend to shoot better with slower powders? You make a lot of sense. The relationship between weight, shape and bearing surface relative to barrel pressure, velocity and spin rate to produce ultimate stability yielding consistency and accuracy.

I will either become knowledgeable or dig a deep hole that only gets darker and harder to see as 'JeffG' suggests above... LOL

'357Mag', maybe the "Guns & Ammo 1974 Annual" you mention is kind of what I am looking for. I will check the library in the nearest big city.

Meanwhile I will post links to readings that are most promising...

 

[lpreddick]

i kind of joke when describing precision handloading and shooting to a novice: this hobby/pastime is affected by 2000 variables and we have "some" control of about 12.

 

[Luke_NL]

Hehehehe. I've used a similar phrase, except I stated that we feign control over 10 % of the variables. The return question, whether or not it was frustrating to have results influenced by the 90 % outside of control was answered by my girlfriend before I had a chance to:

"No - it's his excuse for buying more and better toys".

I might pop the question at some point

 

[Vandis2]

Vandis2.......Yep I have same model. Also, I have played around with H4831 and H1000, but H4350 and IMR4350 were clear winners. Then I went after my seating depth and found the magic. Have same model in 223 and 308. All with B&C stocks, Timney triggers, EGW bases, Burris rings and Weaver GS Tactical (MidwayUSA exclusive) 3-10x40mm optics.

I looked long and hard at replacing that junk x-mark trigger.
I had the pull adj. screw almost screwed all the way out and it still had 2-3lbs of pull!
Almost went with the Timney like you.
But after reading the history of the Jewell, i.e. design development, I jumped and spent the cash.
Now I'm back to saving money again...

It came factory with 1lbs of pull and can adjust down to a few ozs!
As I shoot off my truck for game or on a benchrest I would not want more than 1lb.
And maybe sometime I might want to shoot in a competition.
Put it in myself and you do not need a brass hammer!

Its functioned real well with all the dust here in S. Texas, but we will see.

 

[357Mag]

Vandis -

Howdy !

Send me a PM with your E-mail address, and I'll send you YOUR copy the " Powley Papers ".


With regards,
357Mag