OzFclass

Lee,
Some facts to consider:

Lets take the following worst case data and see what computes...

Temp 0C (good Tassie summer day)
Air Pressure 1016 hPa (almost below sea level)
RH = 50%
2800fps (32" 284 Shehane barrel should be good for 2900fps)
180gr Berger Hybrids (longer bullet than the VLD)
Speed of sound (mach 1) @ above atmospherics = 1087fps

Results for 9 twist 284 barrel...

Sg = 1.37 (Litz recommends 1.4)
Velocity at 900 metres = 1547fps
Velocity at 1000 metres = 1427fps (1356fps if launched at 2700fps)

The bullets are stable at supersonic speeds (above mach 1 (1087fps)) so lets make an allowance and require the bullets to stay above mach 1.2 (mach 1.2 = 1300fps for the extreme atmospherics given above) to be on the safe side.

Even if we launch these Hybrids or VLDs under these conditions at 2700fps we still achieve mach 1.2 velocities at 1000 metres. Considering that most ranges max out at 1000 yards, a 9 twist barrel used to launch bullets at a puny 2700fps will still stabalise these bullets. Heavy bullets punish the 284 case and I would not go above the 180gr bullets for optimum target use.

My advice is to use the least amount of twist rate in the barrel to achieve a Sg = 1.4. The answer is simply that the higher the twist rate and the great the Sg the greater is the aerodynamic jump. Vertical deflection (up and down) increases with increased stability. If you want to minimise the vertical deflection caused in cross winds then keep Sg at the required 1.4 and not any greater.

One final thing about the lighter bullets, I am using the Hornady 162gr Amax and new Match bullets and it seems while the BC might be a bit less than the Berger 168s they group better. Something to try when you build your gun.

Ian

Hi ya Ian,
luckily I'm half way through my new copy of Applied Ballistics so I managed to understand what you are saying. Two months ago it would of been a different story. Going by what is above and looking at a few tables of figures 1:8.5 certainly is the way to go.
If the Berger projectiles didn't do what I wanted the other option was to try the Amax's. Do Hornady cover most of the range when it comes to bullet weights last time I had a look they only had 162g Amax's and Bthp Match rounds for the 284.
Thanks for the info.

Lee,
The Berger 180gr VLDs @ 2900fps (cant get the hybrids yet) are my favorite load and the best long distance 7mm projectile I have used to date!

I dont think you can go wrong getting a 1:8.5 twist but I have just bought a new 32" 5R in my old fave 1:9 twist. My motto is "less is best" providing it gives Sg = 1.4 as Litz recommends. Factor in energy wasted increasing the bullet spin rpm and increasing aerodynamic jump, even if the difference is only 0.5" in the twist rate. I'll take the minimum twist rate that will do the job every time, until someone can show me a better way.

I'm just suggesting trying the Hornady 162gr bullets alongside the 168gr Bergers and you be the judge of which projectile has the tighter groups. I have found the 162gr bullets excellent out to 600 metres and others are shooting best scores at 1000 yards with them (check 6mmbr.com). I jump the 162 grainers 20 thou and that seems to work pretty well. I currently hold the SA FO 600 metre record using the 162gr Amax bullets. Mind you the records in SA have only started officially this year.

The Bryan Litz book (2nd Ed) is worth buying even if you have his first book. I found the aerodynamic jump info excellent and its now sits on a spreadsheet just to check what vertical deflection is possible in cross winds.

Keep us informed of your rifles progress.

Ian

Sorry I should of been clearer,
it is the 2nd edition I'm currently reading. Good way to loose a few hours at a time.
It will be good to spend the time up at my small range testing loads and projectiles when its all done.
Will post updates on how it goes as it comes.
Cheers

As an instigator of the proposal to get FTR going, I will explain my reasoning:-
I see "F" class standard as akin to Australian Rules Footy. I've watched, played, and umpired Aussie Rules. It's a great game and I love it , along with millions of Australians. However, if you want to play on the international stage, the game is Rugby Union or Soccer. Similarly, "F" Standard is a great shooting discipline, and I enjoy it. Again, if I want to compete in the World Championships, I must shoot FTR or "F" open.

Australia is competitive internationally in RU and Soccer. Without a strong domestic competition, this would not be the case. The fact of the matter is that Australia needs a domestic FTR competition, if we are to be competitive internationally. The Australian RU selectors don't select a group of RL or AFL players to contest the world cup. Neither would I expect the NRAA to select a group of open or standard "F" class shooters to compete in an international FTR world championships.

Greg Warrian.

See Greg Warrian's announcement on ICFRA FTR on the NSW F Class website.

Link: www.nswfclass.com.au