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It's probably not a lube groove but a "dirt-catcher," designed to give fouling left in the rifling grooves from previous shots someplace to collect without being mashed into the first groove-diameter band. This is a common feature of bore-rider designs going as far back as the black powder era but not really needed with smokeless. Here's a typical example dating from the early 20th century, Lyman/Ideal 311332 -
https://www.milsurps.com/images/impo...0/311332-1.jpg
What's odd about the 6.5 bullet shown in this thread is the apparent step in the forward band - probably from nose diameter to base diameter. Putting a dirt-catcher in front of such a conical step is questionable design. Note that Lyman's 150-grain 266673, below, has the step but not the forward groove.
https://www.milsurps.com/images/impo...0/266673-1.jpg
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New Info
Finally slugged my rifle barrel and measured the bullets.
First off, I purchased the bullets shown and they are supposed to be Lyman #266673. They measure .2625 just forward of the gas check, and quickly drop to .255 just behind the forward "dirt groove." When I slugged my bore I got .2635 . I should have purchased my bullets from Beartooth. They offer a 140gn bullet that is sized to .265.
Needless to say, it looks like the bullets I purchased are undersized.
About the Trail Boss. To develop the load I used the method suggested by IMR.....
Find where the base of the bullet will be on an empty casing, mark the case and fill with trail Boss to that line. Weigh the charge. This is a max load. Beginning load is 70% of the max load.
I was using slightly more then the beginning charge in my loads. I'm not ready to throw in the towel with the Trail Boss, as I have used it with excellent accuracy with jacketed 6.5 and with cb for my Mosin Nagant. Seems more likely that the lead bullets I was using were simply sized too small to stabilize in my rifle.:banghead: Does that make sense?
Shoot often, Shoot Safe, Shoot 10's!
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I have been developing loads for my milsurps with cast bullets using TrailBoss(TB) for the Cast Bullet Associations postal matches. My M-N M38 as done well with Lee 150g FP meant for my Winchester 94, sized to 308. I use 8 grains for 50 yard plincking. I also have tried the Lee mold in 160 grain meant for the SKS sized to 311 with the same 8 grain load. I find TB to be suited for light plincking loads I would not use it for heavy hunting loads with cast. I also load 30-06 for my 1903 and Savage, 308 for my Enfield 2A and 8x56R Hungarian for my Styer. Another reloader uses TB to allow him to enjoy his 38-55. He has had rotator cuff surgery. My next step will be to slug my barrels and step up to 100 yard shooting. The CBA is a wealth of knowledge about cast bullet shooting.
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Take just a bullet and inert it in the muzzle to see how much play there is before it hits the dirt catcher band. I have a 358 Winchester I barreled up and normal 358 cast bullets are designed to bore ride on .350 top of lands. The top of my lands are .352 and show yaw thusly I am having a bullet mold made up with .353 bore ride which should alleviate the condition. I figured this out when I tried a bullet in muzzle and held it to one side and the yaw was clearly visible to the naked eye. Thusly when the pressure curve it, the bullet flopped to the side and stayer that way out the muzzle.
FWIW the Marine Corps rifle team used to shoot 168 Sierras at 1000 yards and won matches with their M14s with every round yawing as it went through target. They were stable to 900 yards and by the time it got to 1000 it had dropped below mach 1 but accuracy wasn't hurt so they kept shooting them.
