Inconsistant COAL reloading .300BLK

[7.62x39]

I normally have no problem maintaining +/- .002 COAL with other cartridges, but for some reason .300BLK is giving me fits.

125gr matchkings call for 2.210 COAL. I can produce 3 or 4 good rounds and then it will produce a short one (minus as much as .008 to .010) and then turn around and spit out another round that's right on the money. I tried to press exactly the same every round.

I'm using brand new RCBS dies and new brass. Any ideas?

 

[Fixxah]

Is the brass all the same lot?

 

[Steve Marshall]

If this really bothers you, change seating stems, reseat after measuring or sort bullet length, base to ogive.

 

[NavelOfficer]

If every bullet were exactly perfect and uniform, you'd likely find better consistency. Often slight variations in bullet profile cause the top punch to contact the bullet in a different spot and create variations in COL.

 

[EddieCoyle]

If you're measuring tip-to-casehead, try checking with a bullet comparator instead.

 

[Hvymax]

If every bullet were exactly perfect and uniform, you'd likely find better consistency. Often slight variations in bullet profile cause the top punch to contact the bullet in a different spot and create variations in COL.

Exactly. Most dies seat from the meplat of the bullet so you don't damage the tips. If doing one at a time you can kind of hand seat each then crimp. Or make a die that pushes from the tip.

 

[7.62x39]

Yup, Inconsistant bullet lengths is the culprit.
I have been using the Sierra matchkings (in several different weights) almost exclusively for my .308 and .223 reloads. I have never seen the variations in those calibers as I'm seeing with these particular bullets. In just a dozen bullets I measured I'm seeing OAL of anywhere from 1.111 to 1.131. Although most were @1.124

These particular ones (#2121) Are designed specifically for .300BLK (co-designed with AAC) and are very pointy, much more so than any other .308 SMKs I have. I assume this is to help with feeding this crossbreed caliber. I wonder if this has something to do with the wide variation of lengths that I'm seeing?

 

[Steve Marshall]

The last two posters are confusing ogive and meplat. Seating stems for rifles tend to use the ogive of a bullet to seat. If the ogive varies, so too will the OAL.

 

[EddieCoyle]

The last two posters are confusing ogive and meplat. Seating stems for rifles tend to use the ogive of a bullet to seat. If the ogive varies, so to will the OAL.

Yup. The metplat is the tip and the ogive is the curved side.

This is why you should ignore the casehead-to-tip measurement in favor of the length you get with a bullet comparator.

 

[Hvymax]

The last two posters are confusing ogive and meplat. Seating stems for rifles tend to use the ogive of a bullet to seat. If the ogive varies, so to will the OAL.

Doh!!! Stupid technical terms!!!