OzFclass

John T wrote:Buffalo Boys.

what are your chamber necks for any 7mm?

.002" to .003" a side is what I like.
Keith H.

To this point, there has been nothing to suggest that low clearance, such as 0.001, 0.002 or 0.003, improves accuracy. In fact, low clearance could be detrimental (Pete Smith).

I have thought that the larger the clearance, the greater the blow-back, with consequential change in chamber pressure. This would vary with the irregular outside surface of each unturned case. But anecdotal evidence suggests that any such effect must be so small that it does not impact F Open accuracy.

Thus ender the lesson.

john,
to a large degree blowback due to insufficient obturation can be connected to the amount of partial anneal applied to the neck, and also the upper body of the case.
black powder cases need more partial anneal to obturate well, as pressures are lower.
a good even partial anneal also achieves, as well as correct neck grip, even release of the bullet as it starts its journey.
0.003" per side sounds a lot, but compared to factory ammo in factory chambers it is nothing.
the saami site shows min ammunition and max chamber which when they come together are interesting.
bruce.

Bruce,

I follow the norm and express neck clearance as the diametric total, such as, chamber 0.321 less neck 0.320 equals clearance 0.001.

Regards,
John.

that is why I said "per side."
in case there might be confusion.
0.003" per side = 0.006 as per your definition.
some factory chambers have been over max.
one of note was a ruger 338 win mag where a fired case was, in front of the belt, almost equal in diameter to the belt!
imagine how well that headspaced!
bruce.

I don’t turn the creed necks