Another twist rate question...help

[rmist]

If you want to shoot mid range F TR with a .223 and heavy bullets (70-90gr) . Would you have a 6.5 twist or 7 twist ??
Thanks again
Rmist

 

[Laurie]

Seven.

 

[Tozguy]

You can take Laurie's answer to the bank.

 

[rmist]

Thank You. Would a 28 or 29 inches be alright? I'm looking at a Krieger 4 groove #18 lb contour.
Suggestions ?
Thanks again

 

[r bose]

With 70-82g bullets you can use an 8 twist.

90's require at least a 7 TW. My first barrel was a 6.5 TW but I just bought a 7 TW replacement knowing I will be giving up about 5% of the 90's BC. (see bergers twist stability calculator)

 

[Laurie]

With 70-82g bullets you can use an 8 twist.

90's require at least a 7 TW. My first barrel was a 6.5 TW but I just bought a 7 TW replacement knowing I will be giving up about 5% of the 90's BC. (see bergers twist stability calculator)

I'm not at all convinced by this. First, the Miller formula is known to under-state the Sg for HPBT match or plastic tip bullets. In the case of very long for weight / calibre bullets where the top of the lead core is well below the bullet tip, it seriously underestimates the Sg value. As a practical example of this, the 0.308" 155gn Lapua Scenar was long the favourite handloaded match bullet in the UK in 'Target Rifle' over Viht N140 fired from Palma type rifles and loaded from 2,925 to 3,000 fps at most. It was, and still is, happily used in 1-13 inch twist barrels, and in the early days when 1-14s were still common, it worked at 1,000 yards in that twist too. Now, this bullet is very long for its class thanks to a huge void up front, and if the Miller rule applies, it's barely stabilised at all under standard conditions in the 14 twist, 1.01 at 2,950, but falls below 1.0 under colder conditions - and we Sir, do shoot all year round in the UK and even if it's not Montana or Wyoming, freezing or just above freezing is not unknown at Bisley which is also not much above sea level. So, at 32F, Sg falls to 0.95 and if it's high pressure anticyclonic conditions, you'd see a slightly lower value still.

Even in the 13 inch twist, which Miller says is sub-optimal, it's 1.09 Sg in winter, only 1.17 in standard conditions which will if true produce substantial BC degradation. Since we're talking 2,950 fps (and often much less because the tightwads who shoot TR here don't reckon a 308 barrel done if it's not the wrong side of 6,000 rounds), these bullets are invariably supersonic at 1,000 in all conditions and perform happily in both twist rates.

From my own experience of the 0.224" 90gn VLD at 2,910 fps in a 7 inch twist, I found that ballistic program range cards continually underestimated the required elevation additions (using a scope whose adjuster value accuracy had been verified) and that shot by shot windage settings compared favourably with 308/155 loads used at that time, that is getting on for 3,100 fps. In the US v Scotland FTR team match held at Blair Atholl in June 2011, the 223 was in the top half of the scores posted in the 1,100 yard stage and in an informal 1,200 yard practice (actually 1,224 yds) held a couple of days earlier, the cartridge held a less than 1-MOA elevation, actually nearer to half-MOA, for 9 out of 10 shots with one a half-MOA low in the group. (This same rifle and ammo shot 1,000 yard 4.8" 5-round groups in a 1,000 yard BR match shortly after being built.)

 

[rmist]

Laurie... Thank You for a very informative reply !! More research to do before going forward. I'm still going to go forward with this one. I don't have any 1000 yard ranges that shoot to much F TR. There are a couple that shoot 600. This is the what I'll shoot primarily.

 

[amlevin]

You can take Laurie's answer to the bank.

I actually took it to Benchmark and they built me a new .223 barrel in "7 twist" along with only three grooves. It shoots fantastically with 73 gr to 90 gr bullets.

 

[Tozguy]

Now we are hummin !

 

[Laurie]

You can take Laurie's answer to the bank.

I actually took it to Benchmark and they built me a new .223 barrel in "7 twist" along with only three grooves. It shoots fantastically with 73 gr to 90 gr bullets.

That's interesting and good to know! We starting seeing Benchmark barrels here in the UK maybe 18 months ago and their users have already put up some really good results. I'm sure that 223 could (and hopefully will in due course) benefit from user and gunsmith experience building up on issues like the optimum number of grooves and so on. After all, we are where we are with knowing what works for the 308 Win after 60 years of shooting the cartridge!

 

[rmist]

You can take Laurie's answer to the bank.

I actually took it to Benchmark and they built me a new .223 barrel in "7 twist" along with only three grooves. It shoots fantastically with 73 gr to 90 gr bullets.

What are your chamber dimensions? How long is the barrel?
Thanks again

 

[amlevin]

You can take Laurie's answer to the bank.

I actually took it to Benchmark and they built me a new .223 barrel in "7 twist" along with only three grooves. It shoots fantastically with 73 gr to 90 gr bullets.

What are your chamber dimensions? How long is the barrel?
Thanks again

Benchmark chambered it with a 5.56mm NATO reamer that they use on Bolt action rifles. Only know that it has less taper in the case body area (down to minimum I believe). Fired cases have little or no "bulge" just above the case head. Had it throated for a couple of dummy cartridges I provided using 77 gr Nosler Custom Competition bullets seated with the pressure rings about .020 above the shoulder/neck junction. This allows me to fill the case with powder with the bullet not compressing it. So far this has been an awesome setup with 73 gr, 75 gr, 77 gr, bullets and shows great promise with 90 VLD's (if I could find any to replace what I shot).

Forgot to add, the barrel is 26" long and I opted for the MTU profile. I like nice heavy barrels 8)

 

[barefooter56]

rmist,
Go to our (Berger) website and use the "TWIST RATE CALCULATOR' to confirm or find the best twist rate for your application.
Just plug in your parameters and hit go. Laurie has excellent points and suggestions also on this. Plug them in too and see what comes up just for fun!

 

[manitou210]

With 70-82g bullets you can use an 8 twist.

90's require at least a 7 TW. My first barrel was a 6.5 TW but I just bought a 7 TW replacement knowing I will be giving up about 5% of the 90's BC. (see bergers twist stability calculator)

I'm not at all convinced by this. First, the Miller formula is known to under-state the Sg for HPBT match or plastic tip bullets. In the case of very long for weight / calibre bullets where the top of the lead core is well below the bullet tip, it seriously underestimates the Sg value. As a practical example of this, the 0.308" 155gn Lapua Scenar was long the favourite handloaded match bullet in the UK in 'Target Rifle' over Viht N140 fired from Palma type rifles and loaded from 2,925 to 3,000 fps at most. It was, and still is, happily used in 1-13 inch twist barrels, and in the early days when 1-14s were still common, it worked at 1,000 yards in that twist too. Now, this bullet is very long for its class thanks to a huge void up front, and if the Miller rule applies, it's barely stabilised at all under standard conditions in the 14 twist, 1.01 at 2,950, but falls below 1.0 under colder conditions - and we Sir, do shoot all year round in the UK and even if it's not Montana or Wyoming, freezing or just above freezing is not unknown at Bisley which is also not much above sea level. So, at 32F, Sg falls to 0.95 and if it's high pressure anticyclonic conditions, you'd see a slightly lower value still.

Even in the 13 inch twist, which Miller says is sub-optimal, it's 1.09 Sg in winter, only 1.17 in standard conditions which will if true produce substantial BC degradation. Since we're talking 2,950 fps (and often much less because the tightwads who shoot TR here don't reckon a 308 barrel done if it's not the wrong side of 6,000 rounds), these bullets are invariably supersonic at 1,000 in all conditions and perform happily in both twist rates.

From my own experience of the 0.224" 90gn VLD at 2,910 fps in a 7 inch twist, I found that ballistic program range cards continually underestimated the required elevation additions (using a scope whose adjuster value accuracy had been verified) and that shot by shot windage settings compared favourably with 308/155 loads used at that time, that is getting on for 3,100 fps. In the US v Scotland FTR team match held at Blair Atholl in June 2011, the 223 was in the top half of the scores posted in the 1,100 yard stage and in an informal 1,200 yard practice (actually 1,224 yds) held a couple of days earlier, the cartridge held a less than 1-MOA elevation, actually nearer to half-MOA, for 9 out of 10 shots with one a half-MOA low in the group. (This same rifle and ammo shot 1,000 yard 4.8" 5-round groups in a 1,000 yard BR match shortly after being built.)


Holy smokes Laurie 2900 + fps your cases must be good for one firing ?
manitou

 

[rmist]

Glad your 90's are working out for you !! Sounds like you had a great day with the 223 !! After reading your post I picked up some H4895. What primer are you using? I'll be shooting the 80.5 gr FB's out of 30" tube. What would you suggest a good starting point for powder weight? I have Fed 205's and CCI 450's. When/why would you go to the magnum primer? I only use them in my 6BR right now.
Thanks again