260AI Barrel ONLY shoots when Bullets are Jammed

[CaptainMal]

754 documented rounds through my 26" Palma hunting 260 AI. Been frustrating because it only shoots appx. 1" groups or larger. You name the bullet, primer, powder etc. and I tried it multiple times. Never tried jamming a bullet as I wanted it for hunting. Well three weeks ago I used some A-Max 140's and jammed them .020 in front of 47.3 grs of Rel. 22. Shot .3's.

Last week it was the same thing. This week I tested three different hunting bullets at .020 jam and then touch and out .015. All the jammed shot less than .4's while all the touch and jumps were an inch or larger.

Jamming is NOT an option for a hunting rifle. What if I had the threads cut off this barrel, shortening it to 25" and had a new chamber cut and throated? Is the jammed shooting a product of the barrel (Krieger 5r), the throat or both?

Just not sure it would be worth it so plan on rebarreling but...

What do you think?

 

[necchi]

What's wrong with 1" for a Hunting Rifle?

 

[broncman]

What effect does more kneck tension have on the rds you jump?

 

[Forum Boss]

Three thoughts...

1. Lot of 260 brass has donut problems, if it has been reformed from other cases. When jumping you may be driving your bullets into a donut which can definitely harm accuracy.

2. Is the chamber really fat (wouldn't think so because the .260 AI isn't a factory chambering). I ask because I've found some guns that wouldn't shoot jumped because the cartridge just sat in the bottom of the fat SAAMI chamber so the bullet axis was below the centerline of the barrel.

3. Scope your chamber and throat. I've seen the issue you've described when the lands did not extend uniformly into the leade area. When a bullet jumped to the lands it would encounter one land first and not align properly.

 

[alf]

What effect does more neck tension have on the rds you jump?

I'd pursue this thought......

I wouldn't have a problem running .020" in with adequate tension.

 

[CaptainMal]

Getting into your thoughts.

Chamber was made with a Pacific reamer. I use Lapua, blue box, 260 brass. Fireforming was a .030 jam that shot sub MOA. I still fireform to make another 100 cases and fireforming still outshoots all jumped rounds.

Tightest bushing for the Forster dies I have is a .292. Used to use a .293 but would have to order a .291 to go tighter. Neck tension is only about .0025 right now.

My guess is the chamber is centered but that's nothing I can measure. Two friends also have chambers from the same reamer and each of them experienced troubles. One had it re-chambered with a different Pacific reamer. Not sure his results were much if any changed.

That throat issue is on my mind. Might call my 'smith and ask him if he can lengthen the throat another .020". Built this on a long action Remington and there's plenty of room both in the action and in the neck. My longest bullet OAL to leade was a 139 Scenar at 2.354 OAL to touch. The 140 SST are but 2.307" and could easily come out .030" and STILL have bearing surface below the bottom of the neck.

Appreciate the ideas.