.280AI New Rifle Build - Barrel Twist & Free Bore Question

[JMRhino]

I want to build a "big brother" to my current Tikka Lite in 7-08. I believe that I have decided on a Tikka action with a TBD brand barrel chambered in .280AI. I would like to have a 1/8 twist rate and longer free-bore to have the capability to use heavier bullets with higher BCs; however, I still want to be able to hunt with more traditional weights and projectiles in 7mm for whitetails. Is there a penalty for having too much free-bore for traditional projectile weights in 7mm (think 120gn -165gn) with traditional seating depths and COALs? I will mostly be hunting with Barnes TTSX 120s, 140s or NOS 140 ABs but want the capability to go heavier if ever needed.

Ultimately, I know it's not super critical either way because I know 165s out of a 280AI will handle everything I want to do at reasonable hunting ranges, but I thought since I am building from the ground up, might as well give myself options for future tinkering...

Final question (for now) - I have seen .280AI brass advertised as "280 Ackley Improved" and "280 Ackley Improved w/ 40-degree shoulder." Don't all 280AIs have a 40-degree shoulder? Why the distinction?

 

[xphunter]

One and the same case: 40 Degree shoulder
Are you potentially wanting to go with heavier bullets for hunting?
If so, what would the max weight be?
Specific bullets in mind?

 

[Aussie Wanderer]

150 to 165 grains seems to be the sweet spot in the 280ai. Unless you are really going to stretch out there those bullet weights are about a perfect match to the powder capacity you have to play with. Absolutely one of my all time favourite cartridges but just don’t ask more of it than it can provide. It is super well balanced.

 

[JMRhino]

One and the same case: 40 Degree shoulder
Are you potentially wanting to go with heavier bullets for hunting?
If so, what would the max weight be?
Specific bullets in mind?

Your questions have prompted me to look more into the "hunting" heavy weights for .284 and there really aren't many great options above 165ish except for the Partition. As Aussie Wanderer has pointed out the sweet spot may live in that middle weight range; therefore, I probably shouldn't be too concerned about needing a larger free-bore. I assumed there would be more options, but most true heavies are probably intended for banging steel.

My main intention for wanting that rifle/cartridge is for a good all-purpose Elk gun and can still not be complete overkill for whitetails at moderate distances.

Many Thanks.

 

[JMRhino]

150 to 165 grains seems to be the sweet spot in the 280ai. Unless you are really going to stretch out there those bullet weights are about a perfect match to the powder capacity you have to play with. Absolutely one of my all time favourite cartridges but just don’t ask more of it than it can provide. It is super well balanced.

Yeah, it seems almost perfect with respect to balance between ballistics, recoil and rifle dimensions. In Goldilocks terms this porridge is just right.

Thx.

 

[NYDAN]

Is there a penalty for having too much free-bore for traditional projectile weights in 7mm (think 120gn -165gn) with traditional seating depths and COALs?

I found that some the very high B.C. bullets have a very long boat tail. By the time you get the full diameter of the shank into the neck of the cartridge, the C.O.A.L. isn't necessarily that long. Look at the 6.5 mm bullets in the photo below.



Compare the Nosler 140 BT to the 142 Sierra HPBT M and the Nosler 142 ABLR. You can see how much deeper the long boattail bullets have to seat into the neck of the cartridge. For the same bullet jump, the optimum free bore (to minimize the amount of full diameter shank below the neck) the 142 HPBT M and the Nosler 142 ABLR will require less free bore than the 140 BT.