CBI 260 Match: AI Mag Length OAL questions / issues

[capreppy]

Rifle barrel is a CBI 260 Rem Match (longer freebore). I realize now I probably didn't need the longer freebore as I am running mag length.

Using the Hornady OAL tool. Hornady 140 Amax: lands at 2.3390 (ogive using the .25 comparator as I apparently don't have a 26 one, but getting consistent readings). Sierra 142 MK: lands at 2.3530.

My OAL for the 140 Amax at .02 off the lands works out to be 2.8980. Mag Length is 2.860 and that is pushing it. If I load the Amax at 2.850 (0.01 short of mag length), I am about 0.065 off the lands. From my reading, the Amax likes to be 0.02 off the lands.

My OAL for the 142 SMK at .02 off the lands works out to be 2.8770. If I load the SMK at 2.850 (0.01 short of mag length), I am about 0.05 off the lands. From my reading, the SMK likes to be 0.02 off the lands.

I did run a set of ladders from 41.9 to 43.1 (.3 increments) at mag length for both and I was at more than 1 MOA accuracy and that is a far cry from what I had expected.

I would like to try the following as long as it does not sound completely off base. I will rerun my ladders at 0.02 off the lands for both the 140 Amax and the 142 SMK. These will be single fed and I will hopefully come up with a fairly accurate load. Once that charge weight is determined, run another set of ladders varying the OAL to see if there is an OAL that can be mag fed that is still as accurate.

Thoughts?

 

[LRJammer]

I just finished building a rifle with that same brand of barrel and also use AI mags. I ended up going with the standard chamber in lieu of the longer freebore and have been very happy with my choice. The Sierras like to be jammed and since that is my primary bullet choice, that was the primary driver in my choice. I'm sure you can get it to shoot with some jump if you have to. If you arent satisfied with the SMK or Amax, give Bergers a try. They have a reputation for preferring some jump for best accuracy.

 

[capreppy]

I just finished building a rifle with that same brand of barrel and also use AI mags. I ended up going with the standard chamber in lieu of the longer freebore and have been very happy with my choice. The Sierras like to be jammed and since that is my primary bullet choice, that was the primary driver in my choice. I'm sure you can get it to shoot with some jump if you have to. If you arent satisfied with the SMK or Amax, give Bergers a try. They have a reputation for preferring some jump for best accuracy.

I'd like to find a load and COAL that will work in a mag. Might burn a few bullets trying to find one, but hopefully not too many. I've got a metric crap ton of the 140 Amax and 142 SMK so definitely want to get these to work. In a couple of years when I have to replace the barrel, I'll definitely be avoiding the longer freebore and go with the standard 260 Rem.

 

[FCS]

Absolutely no reason you shouldn't be able to get well below 0.5 MOA accuracy jumping 140 A-max or 142 SMK 0.065", no truth to either bullet needing to be 0.020" from lands to shoot well. If you really need the longer OAL, mod a magazine or get an accurate 300 WSM mag and adjust feed lips.

What powder, primer, brass are you using?

 

[capreppy]

RP brass, H4350, and CCI BR-2.

Going to redo my ladders and go up to 45 and seat at 2.855.

 

[cp3head]

I am in the same boat; however, I have not had my barrel chambered yet. I am I interested in your results with the 142 gr smk as this is what I am planning to shoot out of my rifle/magazines. Please post your results.

 

[FCS]

Running a standard AICS magazine, 260 Tac Match reamer with 0.088" of freebore is about ideal.

If you are running a 591 w/notched feed ramp, 70/FN SPR, R700 w/notched feed ramp, Stiller TAC with AW or similar action cut then longer freebore that allows for getting bullet out of case some is a good thing. However, you'll need to modify AICS magazine or buy Accurate 300 WSM mags and adjust lips.

If running standard AICS magazine, pretty hard to beat 6.5 Creedmoor & 140 VLDs as it gives up nothing to 260 when the 260 is limited to 2.860" OAL.

 

[LRJammer]

I'd like to find a load and COAL that will work in a mag. Might burn a few bullets trying to find one, but hopefully not too many. I've got a metric crap ton of the 140 Amax and 142 SMK so definitely want to get these to work. In a couple of years when I have to replace the barrel, I'll definitely be avoiding the longer freebore and go with the standard 260 Rem.

Reworking your ladder test with the deepest seating depth your magazine will allow should give you the max charge for that combination. If you arent happy with the accuracy of your results, you could always have the chamber recut after removing the difference between the two throats removed from the chamber end of the barrel. It shouldnt be enough that you wouldnt still have plenty of thread to work with. Given the cost and lack of availability of reloading components these days, it could actually save you money in the long run.

 

[capreppy]

I am going to take these new ladders to the range tomorrow. Hoping for a little better results. If these don't run well, I will try an Alpha Type 2 mag. That should get me out to where I need to.

As long as I can get close to half MOA, I'll call it done. I've got enough components to burn out this barrel. I won't be competing so half MOA at distance is fine for me. I'll rebarrel in a few years when I might be in a position to compete.

 

[capreppy]

Well reran my ladders at 2.855 versus 2.850 and groups tightened up a lot. I think I am going to order an Alpha Type 2 mag and see if loading them even longer helps. I don't have my notes in front of me, but I don't need that much more room to get a .02 jump. This is the cheapest alternative and if it works, I'll just switch out the mags I do have for a set of Alphas.