6mmbr stiff bolt

[Woodlander]

I use 30+ gr of N150 with 105 Scenar and have recently been getting heavy bolt lift, sometimes having to knock the bolt up with the palm of my hand. I use Forster fl die, set to what I thought it had always been set to, so adjusted to size a little more, now all’s good. I think, as has been said, bump those shoulders back a few more thou.

 

[Laurie]

surely that would be stiff on close as well?

Not necessarily. If the case has just barely an interference fit in the chamber, the bolt's camming action is so efficient you barely notice any extra effort and it's a lot easier to push the bolt handle down than lift it in extraction.

Fire such a round though and the internal pressure makes it a very tight bolt-face to chamber shoulder fit, enough to need noticeable effort to lift the bolt-handle.

I learned all about this phenomenon back in the days when I neck-sized everything. Using light loads you can start with a new case and get away with the practice forever. Load up to factory pressure levels, say 57-59,000 psi in most modern cartridges and you get noticeable resistance to bolt opening on the second firing. If neck-sized again for a third firing there's noticeable resistance to closing the bolt and it's now really hard to open it.

 

[scotch_egg]

@stephentri why did you go with a tight neck?

The 6mmbr and Lapua brass is a well known performer. Are you shooting BR competition?

 

[stephentri]

Not shooting competition, went with what the Smith had when rebarreling rifle

 

[scotch_egg]

Not shooting competition, went with what the Smith had when rebarreling rifle

That’s a pain having to go with what was on the shelf. I had a trio of standard neck 6mmbr rifles built by @Ronin all three shot the .1’s with the old 105gr A-max and 30gr Varget. My dad still has his and had it fluted after the build and it still shot the same. Quite impressive really after a re-thread and crown.

I think a light weight 6mmbr is on the cards for my next rifle. Just need to find the right action for the small case.

 

[Edinburgh Rifles]

Not necessarily. If the case has just barely an interference fit in the chamber, the bolt's camming action is so efficient you barely notice any extra effort and it's a lot easier to push the bolt handle down than lift it in extraction.

Fire such a round though and the internal pressure makes it a very tight bolt-face to chamber shoulder fit, enough to need noticeable effort to lift the bolt-handle.

I learned all about this phenomenon back in the days when I neck-sized everything. Using light loads you can start with a new case and get away with the practice forever. Load up to factory pressure levels, say 57-59,000 psi in most modern cartridges and you get noticeable resistance to bolt opening on the second firing. If neck-sized again for a third firing there's noticeable resistance to closing the bolt and it's now really hard to open it.

Do you not get a case head swipe on extraction with such a snug fit?

 

[Laurie]

Do you not get a case head swipe on extraction with such a snug fit?

Yes usually, anyway, after a full pressure firing. Nothing to do with guns and ammo though seems to be without its exception (or three).

Sometimes of course, head swipes can be a result of simple massive pressure / rearwards thrust on the bolt-face (and I don't mean brass swipe off a raised extractor mark). I've got a Lapua 6.5X47 case I picked up at Diggle a couple of years ago on a firing point that sees regular McQueens and Tactical matches. At first sight, slightly dulled after a couple of weeks in the grass, I thought it was in good condition. Primer looked perfect (but don't they always in these small primer precision cartridges), but there was just a hint of a shiny ring covering the inner half of the circumference around the primer pocket. When I got it home and buffed it up a little you could see it plainly in some lights and from some angles. Then out with the LEDs-illuminated magnifying glass (one of my best purchases ever, can't manage without it these days), and I found an ejector mark further out and the brass swiping, and eventually a second one that is so faint you can hardly see it. Serially overloaded and potentially weakened. Anything other than one of these tough small primer 0.473" head-diameter Lapua cases and it just might have failed with a primer leak or blown primer. (I suspect, likely most uncharitably, that if the shooter hadn't lost it in the grass it would have had another loading or two.)