175 Sierra Matchkings variable ogive lengths

[TackDriv3r]

I am trying out the 175 SMK again after doing a load development last week, they shot well. Made a batch today, after seating a bullet and i checked ogive to check proper settings, it was spot on. Seated a 2nd bullet and checked ogive again, it was off by .008" and I did not readjust the seating die and resumed a 3rd and it was spot on again, 4th one was off .004" and so on. I measured the ogive of each bullet from the box and they measured .641", .644" and .649" out of 25 bullets. There are 3 different lengths and is it normal?

I was thinking to sort each bullet to its own respectable length or is it a waste of my time?

I just got a 500 round batch of the 168's and would it be worth it to sort them by ogive length?

 

[barefooter56]

You may want to first check and make sure that the tip of the bullets is not contacting the base of the of cavity of the bullet seating stem. That can cause variances you are seeing.

 

[Ned Ludd]

You state that you "measured the ogive of each bullet from the box". I assume you mean you measured the bullet base-to-ogive? If so, just be aware that bullet BTO measurements have little effect on on cartridge base-to-ogive (CBTO) measurements. The seating die stem pushes the bullet into the case neck from a contact point well out on the nose. You then measure the seating depth of the bullet (CBTO) using a caliper insert tool that has a contact point just above the bearing surface. It is the distance between these two points that is critical for uniform seating depth (see cartoon below). The length of the bullet below the ogive (i.e. BTO) has little impact on how far the seater stem pushes it into the case neck, or how long a loaded round measures after seating (CBTO).

Per your questions, bullet dimensional variance is normal. How much variance you might routinely see can depend on a number of things including the brand, bullet type, Lot #, etc. You can certainly sort bullets using BTO. However, that is unlikely to be the sole cause of your seating depth variance as outlined above. To improve seating depth consistency, a tool such as Bob Green's Comparator tool that sorts bullets on the basis of their length in the critical region (i.e. the region between seater die stem and caliper insert tool contact points) would be more useful. However, before spending any more money, I'd suggest first taking a close look at your die setup and/or your measuring technique, because I can generally seat bullets to within .001" or less, even if they have a fair amount of BTO length variance, without ever changing the die micrometer setting. Having seating depth variance of .008" is a problem you really want to fix before moving on to anything else.

 

[TackDriv3r]

I did some checking, took the die apart to check bullet seat in the seating stem and it rests on the ogive, not the tip. Next thing I did was sort each bullet according to BTO mesurement and loaded 20 rounds, perfect and spot on at 2.260" Using the .641" batch, all seated the same.

 

[Dusty Stevens]

Gotta sort em

 

[Willie]

I did some checking, took the die apart to check bullet seat in the seating stem and it rests on the ogive, not the tip. Next thing I did was sort each bullet according to BTO mesurement and loaded 20 rounds, perfect and spot on at 2.260" Using the .641" batch, all seated the same.

That points out that the length of your Sierra bullet, to the radius that the seater touches, is consistent with your BTO sort. If all bullets had this form, consistently, life would be good and BTO would be the only sort needed.

 

[Twoboxer]

Glad you got them seated where you want them. Having a micrometer seating die, I found it easier to seat each bullet a bit long, measure CBTO, adjust the micrometer as required to hit your target, and do the final seating. You still have to measure every bullet . . . but it seems easier to me. Just a thought.