Case Master Gauging Tool and my old 6MM

[reinert]

Some few months ago, and after a lot of indecision on a concentricity gauge purchase, I happened onto a "new old stock" RCBS Case Master Gauging Tool. I got it on ebay for considerably less than a current new one, including the Sinclair tool I was thinking about. I've been using it ever since I got it for my reloads, and it's quite interesting/revealing to check case neck/bullet runout on the finished rounds. Quite the tool for checking any part of a cartridge for concentricity; cool, fun, and now an essential part of my equipment. No looking back on that purchase, for certain.

Lately I've been working on a new load for my old 6MM Rem (tang safety M77 Ruger), using VARGET propellant and my tried and true Hornady 87gn. S.P.s., and just neck sizing the cases. I'm fortunate to have a good supply of some new, unfired cases for the rifle, and quite a bit of old factory brass I saved from back in the days before I got into reloading. I found an old box of Rem. brass that had a date from back in '82 when I reloaded them last. I cleaned them up, neck sized them, trimmed them, checked the primer pockets for uniformity, used my flash-hole uniformer on them, and loaded them up with VARGET and the 87 grainers. Then I checked the run-out...

Geeze, some of those rounds where the bullet shank meets the ogive were better than twelve thousandths out of round. Some were less than four thousandths out; wow, a whole "New World" of reloading info with the run-out gauge! Then, I remembered a while back on a TFL thread, and it may have been Uncle Nick who mentioned a trick I'd never heard of before, which was about tweaking a badly out-of-round cartridge in a simple jig (drilled hole) to straighten out the bullet along with the case neck. So, I decided to try it:

I had a left over piece of oak from a project that measured 1 and 1/2" by 3/4" by 2' long (you don't need that much length for sure) and drilled a 1/4" hole through it. Then I used a small round file to enlarge the hole just enough so the case neck and bullet fit snugly in the hole right up to where the neck meets the shoulder. I took each cartridge and ran them through my new concentricity gauge, and when the gauge needle "ran" out the farthest, I marked the case neck mouth where it meets the bullet and made a mark there with a Sharpie. I used that Sharpie mark for my reference to "tweak" against.

With my new oak jig chucked up in my bench vice, I put the cartridge in the hole so when I tweaked the case neck/bullet, I'd bring it back into "plum." It's just like straightening out a bent Easton XX75 arrow shaft (2216s were what I used) back in the old bow-hunting days. I brought back each of those 6MM cartridges (12 of them) all within less than four thousandths of an inch.


O.K., I thought, but will they shoot after that tweaking business? So, I went out this morning before the wind started blowing and took 5 shots over my daypack. My first shot, with a cold clean one, was out of my center hold about an inch a half. My next four shots were an inch and a quarter low and right of my first shot and grouped right at, farthest hole to farthest hole, @ exactly a half an inch.

Next, I went out to the 200yd. gong, spray painted it black with a one and a half inch (approx.) white dot in middle. Again, over my daypack, I took one shot holding at the right edge of the white dot, and nailed the left edge of the dot, holding dead on elevation. I quit there, as the wind started to pick up. But I am starting to be a believer in the tweaking business. My testing will definitely continue.


Whoever it was who mentioned that tweaking trick on a round, and again, I think it was Uncle Nick, thanks a bunch! This is the kind of info stuff TFL is GREAT for!

BTW, my old Ruger (bought it new in '80 as I recall) has well over 900 rounds through it by my records, and is still a good shooter. I know not what kind of barrel life that old rifle has left, but it's an old dog that can still hunt.

FWIW, here's the 6MM Rem. load data I used in the loads this morning:

Rem cases; neck sized, and trimmed @ 2.225"
VARGET propellant; 39 grns.
Bullet; 87 grn. Hornady S.P.
CCI 200 LR primers
C.O.A.L.; 2.895"

[F. Guffey]

I have the RCBS TOOL, the dial indicator will be the first thing to go. The indicator on my tool has been boxed up for a number of years; among other things I am not hurting for dial indicators.

F, Guffey

[Unclenick]

Yes, I did write about that method. You can get an official tool for doing it from NECO, but I've always found holes in some wood or scrap aluminum plate worked fine.

So, now you are ready for the other half of the experiment. When your cartridges have been tweaked to lowest runout, set them in the gauge and mark them when it reads the high spot. A line on the head from the perimeter of the primer pocket to the edge of the rim is good. This line does not have to match the high spot. It just has to be at the same relative location from one to the next. So you could make a 6:00 line instead of a 12:00, etcetera, as long as you are consistent.

At the range, orient the lines to point to the same side of the chamber every time before closing the bolt. If the turning bolt moves them, that's fine as long as it's consistent. The object of the exercise to get all the high spots at about the same place in the chamber before firing. This should cut the effect of the canted bullets on accuracy in half.

[reinert]

Thanks for the replies. Uncle Nick, I’ll surely try the next step you suggest; seems kind of like indexing a black powder cartridge round. It really was a pleasant surprise at the range with those “tweaked” 6mm rounds and the accuracy potential they showed. Thanks again!