When Did SMK 175 Change Ogive?

[otnot]

This morning I opened a new box of smk 175 and went to seat them with my seating die and when I checked the ogive they were off. So I scratched my head and reset to the right depth and went and shot them. My groups were terrible. I had some of my old loads and everything shot fine.
I went home and checked the old stock and base to ogive was .580 the new box was .605. As I was doing this the mailman dropped off another box so I measured it and it was an average of.600. Now I have to find a good seating depth and probably powder load because the bullet is now set so far into the case. And I have 1,000 of them. My old load was .020 from jam with a OAL of 2.808. To be .20 from jam with these the OAL is 2.770. Sorry for the book and the whine.

 

[drop_point]

I hadn't seen that but I have several thousand new and unopened of the same lot. I'll have to check them. I know they did change the 168s sometime around 2012 and the ogive was CLOSER to the meplat.

 

[otnot]

It definitely jumped up the pressure and really flattened the primers

 

[Ned Ludd]

What you are describing in terms of bullet BTO variance (~.020" to .025") falls within what can reasonably be expected between two different Lots of the same bullet. In fact, although a BTO range of .020" to .025" within a single Lot of bullets wouldn't be considered excellent, it would not be unheard of either. The increase in pressure in the situation you describe is likely similar the the effect you might have observed had you seated the original (shorter BTO) Lot# of bullets at about .020" to .025" deeper into the case. In my hands, such a change in seating depth isn't typically sufficient to cause pressure/velocity to increase astronomically. Rather, it might be worth a tenth grain or two of powder, maybe even three tenths, but I wouldn't expect it to be much more than that.

With the new (longer) Lot of bullets, you can most likely do some limited charge weight testing to reproduce your previous average velocity, which might take a tenth grain or two less powder as a guess. Once you have identified the charge weight that reproduces your previous average velocity, I would do a new seating depth test as if you were starting from scratch with a new bullet. In other words, cover a sufficient range to identify the seating depth optimum for this new Lot of bullets, regardless of where the previous Lot# tuned in. Given your previous seating depth, I'm guessing from about .010" to .030" off the lands might be sufficient. Because this new Lot# of bullets is clearly different, it is possible that the entire ogive radius might be slightly different than the previous Lot#. Increasing bullet BTO between Lot#s can certainly raise the pressure a bit if the length increase is significant, but it can also mean a markedly different seating depth optimum of the ogive radius of the new Lot# of bullets is different. In other words, Lot-to-Lot bullet average BTO can increase solely because the bearing surface (or boat tail length) of one Lot# is a bit longer, but there can also be changes in the shape of the ogive. It's usually possible to address any change in the pressure/velocity aspect of a load using a new Lot of bullets by simply adjusting charge weight to reproduce the same velocity as with the previous Lot. However, I find it's always best to do a full-blown seating depth test because you never know how close the new Lot will tune in as compared to the old. Sometimes it will very close or exactly the same as before, sometimes not.

 

[Straightshooter1]

This morning I opened a new box of smk 175 and went to seat them with my seating die and when I checked the ogive they were off. So I scratched my head and reset to the right depth and went and shot them. My groups were terrible. I had some of my old loads and everything shot fine.
I went home and checked the old stock and base to ogive was .580 the new box was .605. As I was doing this the mailman dropped off another box so I measured it and it was an average of.600. Now I have to find a good seating depth and probably powder load because the bullet is now set so far into the case. And I have 1,000 of them. My old load was .020 from jam with a OAL of 2.808. To be .20 from jam with these the OAL is 2.770. Sorry for the book and the whine.

Yup. This is why I always sort my bullets into batches with labels so I can seat them to a consistent depth. Of one box of 500 175 SMK's I currently have, they're around .590 and the previous box of 500 was around .580 (with some variances of course). In the past, I've seen a variance between lots of 175 SMK's at about .030 on the BTO's and I've found that differences of .020 - .030 or so does make a significant difference. . . especially like in your case when seated close to the lands or into the lands and getting a much longer BTO.

 

[RegionRat]

I had to adjust the Wilson seater die on new boxes when they don't have matching batch stickers.

My gripe, is that most of the big box mail order places no longer make an effort to fill your order with respect to batches.

I started my own thread on my frustration, so you are not alone. The worst part, is that even after screening for external dimensions, you can still get burned by tribological changes due to the jacket material. The only way to "see" that is to watch your seating forces and hope that you can get that under control with sizing the neck up or down to try and get things to stay the "same".

I don't like to let seating forces get really low on magazine fed rounds, so I try and keep them high enough and keep that under control. Things like batch to batch variations get frustrating when you don't get to keep the batches large.

 

[Old Navy]

tribological? I had to Google that one. LOL

 

[RegionRat]

tribological? I had to Google that one. LOL

friction and wear...
I used to tell folks it meant going to New Guinea to study the natives....

 

[otnot]

What you are describing in terms of bullet BTO variance (~.020" to .025") falls within what can reasonably be expected between two different Lots of the same bullet. In fact, although a BTO range of .020" to .025" within a single Lot of bullets wouldn't be considered excellent, it would not be unheard of either. The increase in pressure in the situation you describe is likely similar the the effect you might have observed had you seated the original (shorter BTO) Lot# of bullets at about .020" to .025" deeper into the case. In my hands, such a change in seating depth isn't typically sufficient to cause pressure/velocity to increase astronomically. Rather, it might be worth a tenth grain or two of powder, maybe even three tenths, but I wouldn't expect it to be much more than that.

With the new (longer) Lot of bullets, you can most likely do some limited charge weight testing to reproduce your previous average velocity, which might take a tenth grain or two less powder as a guess. Once you have identified the charge weight that reproduces your previous average velocity, I would do a new seating depth test as if you were starting from scratch with a new bullet. In other words, cover a sufficient range to identify the seating depth optimum for this new Lot of bullets, regardless of where the previous Lot# tuned in. Given your previous seating depth, I'm guessing from about .010" to .030" off the lands might be sufficient. Because this new Lot# of bullets is clearly different, it is possible that the entire ogive radius might be slightly different than the previous Lot#. Increasing bullet BTO between Lot#s can certainly raise the pressure a bit if the length increase is significant, but it can also mean a markedly different seating depth optimum of the ogive radius of the new Lot# of bullets is different. In other words, Lot-to-Lot bullet average BTO can increase solely because the bearing surface (or boat tail length) of one Lot# is a bit longer, but there can also be changes in the shape of the ogive. It's usually possible to address any change in the pressure/velocity aspect of a load using a new Lot of bullets by simply adjusting charge weight to reproduce the same velocity as with the previous Lot. However, I find it's always best to do a full-blown seating depth test because you never know how close the new Lot will tune in as compared to the old. Sometimes it will very close or exactly the same as before, sometimes not.

You answered all the questions and options I’ve been tossing around. Thank you! I had no idea that there was that much variation from lot to lot. My plan is to back out from jam the same amount as the old bullets .029 and drop my charge down.5 grains and go up from their. And thanks again.

 

[otnot]

Yup. This is why I always sort my bullets into batches with labels so I can seat them to a consistent depth. Of one box of 500 175 SMK's I currently have, they're around .590 and the previous box of 500 was around .580 (with some variances of course). In the past, I've seen a variance between lots of 175 SMK's at about .030 on the BTO's and I've found that differences of .020 - .030 or so does make a significant difference. . . especially like in your case when seated close to the lands or into the lands and getting a much longer BOT.

You would think in this day and age you could manufacture to much closer tolerances. Without measuring I can see that the ogive is much more forward. I guess if this was an easy hobby there would be a lot more excellent marksman.

 

[Ned Ludd]

You answered all the questions and options I’ve been tossing around. Thank you! I had no idea that there was that much variation from lot to lot. My plan is to back out from jam the same amount as the old bullets .029 and drop my charge down.5 grains and go up from their. And thanks again.

Unfortunately, there can be significant variation between Lot#s for any of the components we use in the reloading process. That doesn't mean there has to be, I've had plenty of Lot#s of bullets for example that tuned in exactly where the previous Lot# did. However, not always, and measuring/comparison won't usually tell you whether the load will need to change, and if so, exactly how. So the tendency is to do a full load workup every time a component has changed, just to be sure. This is one of the primary reasons shooters that compete in various disciplines like to buy as large a quantity of a given component from a single Lot# as they can afford, so as not to have to go through the process any more often than necessary.

 

[otnot]

Unfortunately, there can be significant variation between Lot#s for any of the components we use in the reloading process. That doesn't mean there has to be, I've had plenty of Lot#s of bullets for example that tuned in exactly where the previous Lot# did. However, not always, and measuring/comparison won't usually tell you whether the load will need to change, and if so, exactly how. So the tendency is to do a full load workup every time a component has changed, just to be sure. This is one of the primary reasons shooters that compete in various disciplines like to buy as large a quantity of a given component from a single Lot# as they can afford, so as not to have to go through the process any more often than necessary.

And that is what’s so frustrating right now with the shortages especially up here in Alaska because no one will ship up here so we’re at the mercy of just a few vendors.

 

[RegionRat]

You would think in this day and age you could manufacture to much closer tolerances. Without measuring I can see that the ogive is much more forward. I guess if this was an easy hobby there would be a lot more excellent marksman.

Batch to batch variations and their effects on tuning is a huge challenge.

It is one of those chicken or the egg debates... if you don't shoot well enough to even notice, then you never notice...

If you shoot well enough to notice and learn to keep things in tune, you have won half the battle.... and then if you can to learn to drive the gun well and dope wind... you become a champion.

Yes it is frustrating, but don't give up without a fight!!!