Both the Army Ordnance drawing in our Tech Issues section and the later one in the book of Army Ordnance drawings from Nicolaus Associates show the distance from the hood to the gage ball to be 0.684" + 0.020".
This probably matters only to those with a 0.450" gage ball.
It could be. Fortunately it is an error that will enhance function rather than detract. I do note it says Late Ordnance Specifications reading that as a change from early specs. It would be interesting to know the measurement with the ball and after finish reaming with a Manson reamer that gives .060" of free bore or leade.
Quote:
I do note it says Late Ordnance Specifications reading that as a change
from early specs.
I've only got 3 different GI (not NM) barrel blueprints, and that "0.684 + 0.020" dimension is on the two most recent ones. Of these two, the earliest drawing has a 1961 basic date. The most recent of these two has the same basic date of 1961, but its last update/change was in 1994.
It is possible, I suppose, that there is an Ordnance drawing more recent than 1994 containing that "0.694 + 0.020" found in Kuhnhausen's book.
The third and earliest drawing I have is dated 1942 and it doesn't have a gage ball shown. Perhaps the gage ball method was in use back then, but its details are on some other drawing.
As far as what the distance to the gage ball would be after a certain amount of finish reaming, it sounds like you've got a reamer but no gage ball. I've got two of the gage balls but no reamer. PM me if you'd like to either buy or borrow one of them to find out.
That's a kind offer. I find it interesting that the mystery at least to me, is this dimension is from the hood extension and does not take into account how far the end of the hood is from the breech face. Even though the head space is correctly identified. So if the hood where as short as was acceptable with the greatest amount of space between it and the breech face and the .450" ball dimension where the minimum amount the lands would be really close to the shoulder. For me the finish reamer is the answer as everything is predicated from the breech face.
FWIW, the distance from the center of the 0.450" OD gage ball back to to the stop shoulder in a mid-spec Ordnance barrel works out to be:
All this doesn't help much for barrels manufactured to SAAMI specifications, however, because the gage ball system is not used by SAAMI. Thought I'd worked out some dimensions for a gage ball in a SAAMI chamber, but if I did I can't seem to find it. Probably kind of moot anyway - don't think too many folks have a 0.450" gage ball.