Head space.....000 or more?

B

Bnhpr

Guest
I'm trying to understand the best way to go with my headspace, and would like some opinions....

I'm building another HBR rifle (30x47), and was advised on another post to cut a tight chamber.

http://www.benchrest.com/forums/showthread.php?t=57355

Here's an experience I had and I'd like some thoughs. I recently chambered a 700 in 257 roberts for a friend of mine. The barrel was a Shilen rem factory varmint contour. I cut the chamber a little tight, the bolt handle went down on factory win brass a little firm, but not real firm.

The rifle shot .500 groups with vlds on the lands. I was really disappointed. I messed with the load, seating, brass prep....nothing helped. I removed the barrel and re-indicated it, and reamed .003" deeper..so the bolt closed easily, like a factory rifle.

With the same ammo, the rifle shot hole over hole, no problem. Why?

So here's my theory, and I'd like some feedback. The winchester brass has some neck runout, typically. A slightly larger chamber allows the jammed vld's to self align more into the throat, with no or less side load on them. This is why it shot better....???? who knows...

Ben
 
Ben

That sounds plausible.

But, in your first shooting, did you use brass right out of the box?? You do know that brand new brass doesn't really have the same "shape" as a fired and sized case. It generally has quite a bit more body taper toward the neck.

With the new brass in the chamber that was fitting tight on the headspace, it could have been cocking the case in the chamber an excessive amount as the shoulder hit the chamber.

Besides, new brass will not fit a seating die as well either. We do rely on the close fit of the seating die and case to achieve the loaded round trueness that is required for extreme accuracy.

Did you check any of the initial loaded rounds to see just how straight they actually were. Brand new brass is not very staright with its self right out of the box. Even premium brands such as Lapua and Norma will exibit some runnout.

Heck, there are a lot of things we can't explain. Just be thankfull the Rifle is performing up to expectations now.........jackie
 
Ben,
First of all we need to establish a universal definition of 'tight chamber'. Yours was tight by being shorter than SAAMI specs. My definition of 'tight' would be a 'minimum chamber' in diameter. In other words, not oversized in diameter from not having the barrel indicated in properly. In my mind length is not a factor so long as the bolt won't close on a 'no-go' gauge.
 
I think it is important to use a steel go gauge when chambering for "factory" cases. In which a minimum chamber would be one that at the bottom of the bolt throw you can just "feel" the go gauge. That creates a repeatable standard.
 
Jackie, you might be onto something. I normally run everything through a re-sizer before loading, even hunting ammo. For this rifle, I picked up a rcbs fl resizer set. I resized and prepped 20 brass, weighed each with powder, and went to seat the bullets.....then...holy crap!. the bullets fell into the case!...I was pist.

The expander ball was too big, in the brand new die set! So, from then on, I just used the seating die for my qualification of the rifle. I need to get another expander next time I remember and am in town. Anyway, this was a rare case where I was forced to use new brass not resized.

As far as the chamber is concerned, I did check runout when i put the barrel back in the lathe, and after indicating it in on the throat, when i moved the needle back into the neck and body, the needle hardly twitched, so I felt the chamber was quite good. It was definitely related to headspace.
 
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