Inconsistency with Sierra 168 matchking projectiles

[oldfudd]

I realize I may be stepping on some sacred cows, but here goes. I have found that when checking the ogive to base length of new 168 SMKs (30 caliber) projectiles, that this measurement may vary by as much as 0.030". In addition, generally this variance falls pretty much in even 0.010" increments. I'm using a Sinclair bullet comparator that is used on a granite base with a dial indicator. The bullets I checked came out of the same 500 piece sealed box.

Is this normal? Since I seat using a Forster micrometer seating die (uses the ogive) this can create COAL problems in the ammunition that I load in a magazine. In all the 308 I load I'm trying to keep the base of the projectile at a consistent seated depth in the trimmed brass.

Comments, good, bad, or just plain ugly, please. Thanks.

 

[Graham]

Inconsistency with Sierra 168 matchking projectiles

Thirty thou is a lot, but SMKs jump just fine. I wouldn't worry about it. Seat them to 2.8 and be done with it.

 

[oldfudd]

Sorry, I just saw a similar thread by Brux on 5/15. Didn't mean to repeat.

 

[M40_A1]

Base-to-ogive variation will not change your COAL.

 

[armorpl8chikn]

I never measured them. My rifle does not care in the least. They still shoot just as good as ever.

 

[oldfudd]

M40 -- what I'm concerned about is the varying amount of "free" space changing between the powder and the base of the seated projectile, not the COAL. By my thinking, this variation will likely change the pressure build-up when the cartridge is fired, and as a result change the projectile's
velocity at the muzzle. Maybe I'm making too much of an issue over this?

Base-to-ogive variation will not change your COAL.

 

[Assault116]

M40 -- what I'm concerned about is the varying amount of "free" space changing between the powder and the base of the seated projectile, not the COAL. By my thinking, this variation will likely change the pressure build-up when the cartridge is fired, and as a result change the projectile's
velocity at the muzzle. Maybe I'm making too much of an issue over this?

The only way you'll know for sure (with your load and rifle) is to test it. Sort them by length in groups, run say 10 of them at the short end of the tolerance, and 10 more at the high end. Run them over a chronograph and see. Check the group size, the speed, ES and SD and compare the two groups.

I'd be interested just from a purely learning standpoint in what you find.

 

[M40_A1]

M40 -- what I'm concerned about is the varying amount of "free" space changing between the powder and the base of the seated projectile, not the COAL. By my thinking, this variation will likely change the pressure build-up when the cartridge is fired, and as a result change the projectile's
velocity at the muzzle. Maybe I'm making too much of an issue over this?

And this would be my concern as well. It is a different concern than what you originally stated.

If case capacity means anything, then seating depth at the base of the bullet should likewise be a concern. You could seat the bases all to the same depth by adjusting your micrometer accordingly, but then your jump will vary. If you already have a long freebore, then a variation in jump would probably not make any difference. On the other hand, if you are not shooting past 500 yards, then variation caused by the bases being at different depths would not likely make a noticeable difference either. The better route to take would depend on your freebore and the distances you are shooting.

 

[Sig Marine]

M40 -- what I'm concerned about is the varying amount of "free" space changing between the powder and the base of the seated projectile, not the COAL. By my thinking, this variation will likely change the pressure build-up when the cartridge is fired, and as a result change the projectile's
velocity at the muzzle. Maybe I'm making too much of an issue over this?

It can make a difference but you won't know how much until you chrono the loads and test for accuracy. I had a similar issue with some 175s, about .050 difference, and called Sierra about it. While they were somewhat sympathetic, they said it can happen and that I would probably have to re-work the load. I've since sold the rifle but I quickly learned that since Sierra's lots can have these large variations, it's best to buy in large quantities. Also, I think there are better projectiles made by other manufactures that are more consistent but they come at a higher price.

 

[Sig Marine]

I just looked over my notes and saw that the differences in base to ogive did make a difference in my COAL. While my seating die doesn't register off of the ogive, the shape of the bullet changed from the ogive to the meplat causing it to seat differently. My concerns were as yours regarding bullet base and powder volume as well as now having a shorter bearing surface. While maybe not what you want to hear, the simple answer to the problem, as given my Sierra, is to re-work the load.

 

[leadbullet]

life of a reloader is a lot simpler when you stop measuring thing that dont matter.

 

[M40_A1]

And threads are a lot shorter when those that don't care don't post.

I just looked over my notes and saw that the differences in base to ogive did make a difference in my COAL. While my seating die doesn't register off of the ogive, the shape of the bullet changed from the ogive to the meplat causing it to seat differently.

Again, separate issues. Variation in ogive contours and meplats cause inconsistent COALs, not base-to-ogive variation.

 

[Sig Marine]

And threads are a lot shorter when those that don't care don't post.



Again, separate issues. Variation in ogive contours and meplats cause inconsistent COALs, not base-to-ogive variation.

Agreed. I measure case head to ogive for seating purposes vs COAL.

 

[oldfudd]

Thanks guys. I appreciate not being flamed. As suggested above I'll make up groups of 10 (grouping by differences in ogive to base), seat to the same COAL, chronograph and let you know what I find. Thanks again.