.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.

 

[xphunter]

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.

You are welcome.
160 grain AccuBond or the 160 grain LRX.
You will want a 8T for the 160 LRX

 

[JMRhino]

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.

View attachment 29629

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.

Yes Sir. Good point. I have quite a few Nosler 150 and 160gns that may fit into that paradigm. This forum never lets me down.

Thank you.

 

[Aussie Wanderer]

Don’t underestimate the 150bt in the 280ai. Driven at 3,000fps it is a much underrated combination. That is a bullet that hunters seem to overlook and I’m not sure why, it is lethal.

 

[JMRhino]

No doubt. I have dispatched many whitetails with that bullet only going 2650-2800 out of a 308 Win with boring efficiency. No drama ever…

 

[xphunter]

Don’t underestimate the 150bt in the 280ai. Driven at 3,000fps it is a much underrated combination. That is a bullet that hunters seem to overlook and I’m not sure why, it is lethal.

I have killed javelina, deer and cow elk with the 150 NBT (30 cal) at 3000 fps.
Lazzeroni cartridge: 7.82 Patriot.

 

[Aberhan]

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?

View attachment 29626

More free bore can be added at a later date if you thought it was a necessity. However I do understand the logistics of doing it during the initial build. I have done that myself on. 257 Roberts, never regretted it.

 

[RL338]

I wouldn’t sweat it over a 8 twist in a 7mm barrel , it will allow you to run the longer solids if you choose.

 

[JD338]

My custom 700 is a 280 AI with a 9t barrel. I load 160 gr ABs long and they easily fit the 3.600 length magazine. More than enough for anything I would hunt in NA.
You need to let your gunsmith know on a specific bullet so he can give you the proper throat.
I’ve settled on the 150 gr ABLR and it’s been a solid performer on WT deer while more than capable for bigger game.
Going with a 8t is a good choice.

JD338

 

[JMRhino]

My custom 700 is a 280 AI with a 9t barrel. I load 160 gr ABs long and they easily fit the 3.600 length magazine. More than enough for anything I would hunt in NA.
You need to let your gunsmith know on a specific bullet so he can give you the proper throat.
I’ve settled on the 150 gr ABLR and it’s been a solid performer on WT deer while more than capable for bigger game.
Going with a 8t is a good choice.

JD338

That’s amazing. Thanks for the reply.

 

[SJB358]

I tend to set my chamber up for the longest/sleekest nose bullet I’m going to use to fit at magazine length. You’re probably using a Tikka will be the 3.4” length you’re constrained to unless you’re getting an altered action.

Chance are a 180 ELD or Berger at 3.6 will still allow you to get within useful distance to the lands with blunter nosed bullets like the Accubonds and PTs.

 

[Aussie Wanderer]

This might help with some of your thinking. I chose the Hornady stuff for simplicity just because it is in their calculator and typically shoots okay. I ran the 150eldx at 3,000fps, the 162eldx at 2,900fps and the 180eldm at 2,700fps. Figured they were all around the mark, allowed for a 200m zero which is what I suspect most hunters do. There really isn't a lot in it.

One of the things I didn't graph it was largely irrelevant, is that all projectiles met the 1 second flight time rule somewhere around 800 yards. For hunting I wouldn't want to go beyond that, but for me, I am not generally going beyond 400 yards.

 

[JMRhino]

Very helpful information. All I need to do is start sourcing the action and barrel...

Thank you all.

 

[Blkram]

With the bullets and weights mentioned, not sure you would need more freebore...but if you were to play with the heavier Bergers, you might want it. But I think that you will be very happy with the performance of the 150-165 gr bullets with a normal chamber in your barrel, both in velocity, accuracy, and on-game performance. It just plain works! = priceless!

If I didn't already have the 7mm-08 (140 gr AB), 280 Rem (150 gr AB or BT), 7mm PRC (175 gr TA) and 7MM STW (160 gr AB), and was so happy with their performance, I would also look at the 280 AI with the 155 gr TA bullet. And who knows, I may play with one at some point in the future, as I just love the 7mm' performance!