Tom Acheson
posted this
09 November 2007
Was afraid to jump into this but here goes”¦
I have a print of a 30 BR reamer that was used by one of our local CBA BR shooters to do a few local CBA BR rifles. The neck section is .327� long and is 0.331� dia. It then tapers at 45º down to 0.310� dia. The lead is .173� long and from that point it tapers again at 45-minutes (1.5º included angle) into the rifling.
This shooter also did my XP-100 in 30 PPC for me. Here he gave me a note describing the chamber as”¦..330â€? neck, .310â€? x 45 minutes (1.5º included angle). For this gun I use an Eagan MX3-30 AR mould. If I did the math correctly, using Don's drawing of the bullet, the bullet has a taper on it of 0.527º.
And lastly, I had a reamer made for a silhouette pistol. The chambering is 30 Silhouette (a 223 case necked-up to 30 cal.). Between me and JGS we came up with the following:
It has a 0.330â€? dia. neck section that is 0.300â€? long. It then tapers down at 45º to 0.310â€? dia. The “leadâ€? is 0.090â€? long where a 45-minute (1.5º included angle) starts and goes into the rifling.
All three of these examples are intended to use “bumpedâ€? bullets. Bore riders with straight bodies take a lot more oooomph in the bump die press than the tapered bullet does. It's interesting that the Eagan bullet with a 0.5º taper needs just a bit more “bumpâ€? (about ¼º!) to get the bullet shape to match the chamber.
I was coached to tap a sized and “bumped” bullet into the chamber until the proverbial “light goes awayâ€? to measure the starting seating depth. Then the bullet is removed and in an “idealâ€? condition we should see witness marks all around the leading edge of the front driving band, confirming that the bullet made the “sealâ€?. Dan needs to help me out there”¦.the end game here is to have a cone-in-a-cone bullet positioning with a loaded round that is slightly longer than the previously measured OAL (and upon bolt closing the bullet is pushed back slightly into the case) after the bolt is closed. Supposedly the cone-in-a-cone gets us as close to optimum concentricity between the bullet and the bore as we can get? Of course with the neck sizes seen above this is a project for neck-turned brass.
My somewhat confusing 2-cents on the 30 BR reamer subject...and I'm always open to correction of mis-understood “theories"!
Tom