7s&8s
02-24-2008, 10:32 PM
I went down this path as a result of a couple European Mausers (7mm’s and 8mm’s) whose bores run larger than what usually drops out of the molds.
A mold purchased on eBay (advertised in “very good” condition) arrived with fairly-well rusted cavities, and was able to re-negotiate price to $15 delivered. So, with not much too loose, I cast a couple bullets, chucked them in the old man’s lathe, drilled and tapped them so they could be used as laps.
http://i254.photobucket.com/albums/hh102/thol1_photo/0802_mold_lapping_04a.jpg
Then, I chucked the tap in the rechargeable drill, added a lap (drilled and tapped bullet), a smear of lapping compound and drop of oil… turn slowly in the closed mold. After a minute or two, things seemed to loosen up a bit, so cleaned up the mold and lap and recharged with lapping compound. Repeated this process a couple of times per grit.
First with 220 grit; then 320 and finished with 600 grit; using a new bullet for each grit. I used denatured alcohol for clean up between grit changes and final clean up.
A couple of lessons learned:
Go slowly and check frequently. After the second charge of 220 grit, clean-up and cast a couple to check progress. My first subject ended up .003 larger vs. the .002 desired.
Shave the base of the bullets used for the 220 and 320 grits so that the lapping does not enlarge the mold diameter at the base for gas-check seating (unless gas-checks fit loosely). The first pass resulted in oversize base with gas checks that have to be forced on.
When taking the mold back to the casting bench, I find that there is a pretty fair “break-in” period, that I suspect may be residual oil from the lapping process. The next candidate will receive the boiling water bath to clean the oil out.
This process achieved the .002-.003” increase in diameter as desired, and seem to be making some good progress in bringing group size down a bit on the first candidate RCBS 7mm-168 gr that now drops at .288-9. As a result of the first test, the second (RCBS 8mm -170 gr) has been lapped and cast .325-6, but not yet loaded...
http://i254.photobucket.com/albums/hh102/thol1_photo/0802_mold_lapping_01a.jpg
A mold purchased on eBay (advertised in “very good” condition) arrived with fairly-well rusted cavities, and was able to re-negotiate price to $15 delivered. So, with not much too loose, I cast a couple bullets, chucked them in the old man’s lathe, drilled and tapped them so they could be used as laps.
http://i254.photobucket.com/albums/hh102/thol1_photo/0802_mold_lapping_04a.jpg
Then, I chucked the tap in the rechargeable drill, added a lap (drilled and tapped bullet), a smear of lapping compound and drop of oil… turn slowly in the closed mold. After a minute or two, things seemed to loosen up a bit, so cleaned up the mold and lap and recharged with lapping compound. Repeated this process a couple of times per grit.
First with 220 grit; then 320 and finished with 600 grit; using a new bullet for each grit. I used denatured alcohol for clean up between grit changes and final clean up.
A couple of lessons learned:
Go slowly and check frequently. After the second charge of 220 grit, clean-up and cast a couple to check progress. My first subject ended up .003 larger vs. the .002 desired.
Shave the base of the bullets used for the 220 and 320 grits so that the lapping does not enlarge the mold diameter at the base for gas-check seating (unless gas-checks fit loosely). The first pass resulted in oversize base with gas checks that have to be forced on.
When taking the mold back to the casting bench, I find that there is a pretty fair “break-in” period, that I suspect may be residual oil from the lapping process. The next candidate will receive the boiling water bath to clean the oil out.
This process achieved the .002-.003” increase in diameter as desired, and seem to be making some good progress in bringing group size down a bit on the first candidate RCBS 7mm-168 gr that now drops at .288-9. As a result of the first test, the second (RCBS 8mm -170 gr) has been lapped and cast .325-6, but not yet loaded...
http://i254.photobucket.com/albums/hh102/thol1_photo/0802_mold_lapping_01a.jpg