VLD Bullets - How much should MV increase?

[Guest]

Cybra, Cauterucio makes them. They make 156 and 176 VLD's. THe BC of the 156 is 690 and the 176 is 750.

The 156 gets 3500 for a 30" 1:9 twist 7mm RUM. Shoot VERY accurate at 3450.

I'm trying to figure what an STW would do with the same twist a 4" less of barrel.

 

[Cybra]

Ok, lets see...

You know, the funny thing with this is that different powders will really play a major role here. For example, by nitration content, a lot of VV's powders are very clean burning, and ignite efficiently, often completely converting from solid to gas in half the time/pressure ratio that different powders of similar speeds will. Another route to explanation; if you use a very slow burner, such as...H870, Retumbo, 5010...you would notice a LOT more velocity drop for those four inches, especailly with a bullet of that weight, and also because the shank length is quite short <minimum bullet-bore friction>. The bottom line is it is hard to approximate right to the last fps because of all the variables.

Here's what I found; assuming you were using H870 <I don't know what YOU were using, but H870 was a powder I found would work, and would match up with the pressure/velocity situ. rather nicely> with a 30" tube and 156gr projectile, I snarffed up some figures that got me to 3450fps. Chop 4" off, the speed drops to about 3320fps. BUT!--remember, the type of powder used will likely have the biggest effect--a faster burning powder will show less of a decrease, presumably. My earlier guess was based on RE25 for powder. I'd say you should be above 3300, anyhow. I used 3450 as you indicated that to be the accurate load, and for me, if it isn't accurate, it isn't worth a d*mn!

Dave

 

[Cybra]

So far I get a highest "experimental" velocity with H870. It's about 3340fps, up to 65000 psi and COL at about 3.70". I'm also assuming you're trying to get the bullet up to the lands. If not, ad on another 20-30 fps.

 

[Guest]

I'm shooting a 140 Barnes XCL Boat Tail in my 1:9 twist 7mm STW with a 26" tube.

Trying to figure out what king of velocity I might expect with a 156 VLD.

Thanks to all!

 

[Cybra]

John

First off, help me out; who the hell makes a 156gr VLD in 7mm? JLK? I honestly don't know, but doing some interpolating with a 150gr Sierra MK boosted to weigh 156grs, I would think you'd find yourself around...3300fps <give or take 40fps> I programed for a fit to the lands, and an over-all length of 3.68". This is with about 65000 psi. Of course, with a 1:9" twist, you may not get there within that pressure limit. A 1:9 is really better suited to 160gr+ bullets--hell, I think a 1:9 will stabilize a 180gr VLD, if I'm not mistaken? If accuracy and max loading are your goals, perhaps you'd have better luck with the 168s by Berger? I realize recoil goes up, though. I'll be interested to know who makes the 156gr VLD--I might have to get some of them! <I shoot a 7-08>.

Anyhow, I would think 3300 to be tops, maybe; twist might get ya, first, although I know folks who load to about 70000 psi <brass don't last too long, though!

> You may find your best accuracy about 150fps below max, from my limited experience with that weight range, twist, and bore size. Best of luck.

Dave