Variation in ogive to bullet base

[mtang45]

Anyone want to comment as to the importance, or not, of consistency of this dimension? The reason I ask is I was checking 100 Sierra 175gr MK's and found two distinctive populations out of the 100. The first measures .658" +- .002 and was approximately 60% of the total. The second group was .642 +- .005. Plus 4 that were somewhere between the two groups. Personally I was surprised that there was .016 average difference and an extreme spread of .023 out of the same box but I'm not sure if this dimension really means anything. I weighed the groups and surprisingly the short groups average weight was slightly higher than the long. 175.0gr vs 174.9gr

Anyone want to let me know if I just collected useless data or if the differences really mean something?

 

[Graham]

I reset my seating die almost every new box of 500 DTACS.

 

[mtang45]

I reset my seating die almost every new box of 500 DTACS.

Graham, so you adjust seating depth based on ogive to base dimesion? Is this to maintain shank seating depth primarily and then ogive to lands would vary?

 

[Graham]

No, I adjust it because the ogive isn't in the same place. The only way to measure where it is, is by using the base.

 

[mtang45]

No, I adjust it because the ogive isn't in the same place. The only way to measure where it is, is by using the base.

Please correct me if I'm seeing this wrong, but it seems that by changing seating depth based on ogive to bullet base then you are fixing the position of the bullet base and allowing the ogive to lands vary. Maybe I'm not visualizing it correctly.

 

[88blazer]

I just got done going through all 300 of my Hornady 285 HPBT Match for my 338RUM. I had a spread of .055"(wasnt too happy). OTM's and SMK's are way better. The close ones I sort, the way off ogives are loaded for fouling shots. I think is what graham is saying is you want to adjust as necessary to maintain ogive to lands for consistency. Check your loaded round, ogive to case head and adjust seating die as needed. The ogive to base does vary from bullet to bullet batches/lots. Thats what I do. Took care of my flyers.

 

[redbullitt]

I'm with Graham. Usually I have to do the same thing bullet lot to bullet lot. That spread you are seeing is pretty common from lot to lot. If you are seating close to that lands then IMO it does matter that you take care of that variation. If you are not running very hot loads and have plenty of jump to cover the variation then it may not matter much.

Thats also why you have to sort pulled smks by ogive. Otherwise you might jam .050 into the lands and not even know it until you pull the bang switch lol.

 

[Graham]

Please correct me if I'm seeing this wrong, but it seems that by changing seating depth based on ogive to bullet base then you are fixing the position of the bullet base and allowing the ogive to lands vary. Maybe I'm not visualizing it correctly.

It's not a method. I'm just saying that the bullet lots are different. The base to ogive of the bullet is the only way I can tell a variation on a bullet.

 

[918v]

Anyone want to comment as to the importance, or not, of consistency of this dimension? The reason I ask is I was checking 100 Sierra 175gr MK's and found two distinctive populations out of the 100. The first measures .658" +- .002 and was approximately 60% of the total. The second group was .642 +- .005. Plus 4 that were somewhere between the two groups. Personally I was surprised that there was .016 average difference and an extreme spread of .023 out of the same box but I'm not sure if this dimension really means anything. I weighed the groups and surprisingly the short groups average weight was slightly higher than the long. 175.0gr vs 174.9gr

Anyone want to let me know if I just collected useless data or if the differences really mean something?

If you load into the lands, then your data is crucial.

If you load for accuracy, then your data is important.

If you load for making noise, then your data is useless.

 

[mtang45]

If you load into the lands, then your data is crucial.

If you load for accuracy, then your data is important.

If you load for making noise, then your data is useless.

Thank you 918v, I would fall in category 2

But that being said, what makes this dimension crucial for loading into the lands but only important at any other seating depth?

 

[918v]

Three scenarios:

A) Your OAL is 2.800" and you're jumping .050" to the lands.
B) Your OAL is 2.890" and you are touching the lands.
C) Your OAL is 3.006" and you are jammed .016" in the lands.

Scenario A is most forgiving in terms of spiking pressure.
Scenario B is least forgiving.

Combine scenarios B and C on a hot day and you will see pressure issues. BTW, I have some 175SMKs that are even worse. Some are 1.224" long, some are 1.240", while others are 1.250"+. Their ogjives are all over the place too. That's why I don't jam into the rifling. That's a practice most appropriate in short throated single shots using Johnson Precision Bullets. If you measure a box of his bullets you'll see a max variance of .002". And that's for length and ogjive. Those are the only bullets I'm comfortable with jamming.

In other words, if you jam, even if you load the SMK to the same exact OAL, some of them are gonna be way deeper in the rifling than you may suspect.

 

[mdmp5]

You don't need to measure the variance between bullets. To seat them all the same length based on the ogive, just seat them a little long, measure, make adjustments on the seating die, then seat again.

 

[918v]

and then you will still have some bullets seated deeper than others.

 

[mtang45]

OK all thanks for the explanations. However sometimes I'm just plain dense; anyone have a bat?

If I keep my seating die at the same setting, regardless of bullet base to ogive dimension, doesn't the seating stem sitting on the ogive, seat every bullet to the same distance between ogive and cartridge base? The the only thing that varies is the depth of the seat in the case and that a load that is setup to be touching the lands will always touch the lands? If this is the condition then variation would be defined as the friction difference in the case neck because of seating depth, plus maybe internal case capacity.

Again I'm just trying to understand the relationships. Thanks

 

[918v]

No, because the seater stem does not contact the bullet at the ogjive. It contacts it north of the ogjive closer to the tip. So any variance will transpose itself to the relationship between the ogjive and the rifling.

 

[mtang45]

No, because the seater stem does not contact the bullet at the ogjive. It contacts it north of the ogjive closer to the tip. So any variance will transpose itself to the relationship between the ogjive and the rifling.

Ahh I think I got it; the light bulb came on. So the cartridge length (ogive to case base) could be very consistent if the seater contacted the ogive at the exact same location as the lands.

 

[918v]

Yes.

 

[KYpatriot]

As long as your OAL is such that the round fits in the mag and feeds well who cares what it is. Seat with respect to the ogive on the bullet, keep the distance to the lands constant, and let the tip of the bullet end up wherever it ends up. Check it with a comparator. It doesn't matter where the seater contacts the bullet, you will measure with the comparator and then adjust the seater until the comparator reads correct.

 

[Enough Said]

As long as your OAL is such that the round fits in the mag and feeds well who cares what it is. Seat with respect to the ogive on the bullet, keep the distance to the lands constant, and let the tip of the bullet end up wherever it ends up. Check it with a comparator. It doesn't matter where the seater contacts the bullet, you will measure with the comparator and then adjust the seater until the comparator reads correct.

and then you will still have some bullets seated deeper than others.

^^^ THIS ^^^

Reason: Your measure does not take into account the length of each bullet, Base of Bullet to Ogive.