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Determine oal


rooster
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I bought some 9mm FMJ seconds to save a few bucks. My question is how do I determine my oal if the length of the bullets are all different. Let’s say my target oal is 1.120. My bullets measure .581 to .589 in length. I set my die to 1.120 with a randomly picked bullet. I start loading and my oal is all over the place. From 1.114 to 1.126. I’m worried about pressure at the shorter length. So I took the shortest bullet and set up my die to the 1.120 oal. This gives me .210 of bullet seated in the case. My seating die works off the ogive so even at the longer oal’s I’m still seated at .210 in the case . Is doing it this way correct?

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Most likely, there is variation in the ogive of the bullets, and because you are seating off the ogive, that is what is causing your OAL variation. 

Be leery of seconds. We (PD) only sell bullets as seconds when the issue is purely cosmetic, and will have no effect on loading or shooting.  I wouldn't purchase "seconds" if they were outside of normal dimension or weight tolerances. 

 

All that being said,  +/- .006" isn't really "all over the place", and shouldn't cause pressure concerns. 

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2 hours ago, Ssanders224 said:

All that being said,  +/- .006" isn't really "all over the place", and shouldn't cause pressure concerns. 

 

 

This.

 

I'd have to be loading on the ragged edge (over max/unpublished data) and crushing the bullets into the case with a hammer before I even started to worry about pressure. More importantly, at the longest lengths (1.126") your rounds should still plunk and spin freely.

Edited by 4n2t0
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2 hours ago, Hi-Power Jack said:

There are two different seating stems - flat point and round point.

 

And, two different types of bullets - right, flat and round.

 

Possible that you are mixing up the seater and bullet types ?

I am using a Redding micrometer seat die, there is only one stem, and it looks like it seats off the ogive. So the portion of bullet that is in the case looks to me like it would always be the same regardless of the length of the bullet. I seated the smallest bullet which was .581 and marked the bullet where the case was, then took a bullet that was .587 and did the same. The portion that was in the case measured the same .210 on both bullets. So I think I have fixed my problem. The oal would always be different, but the part that is inside the case would be the same.

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38 minutes ago, Hi-Power Jack said:

 

Guess you can't expect the OAL to always be the same, then.

 

Is this affecting velocity or accuracy ?

No, doesn’t effect accuracy maybe velocity by being shorter or longer.  You will only get the same oal if your bullets are the same length. But seating off the ogive you should always have the same length of bullet in the case. If you die seats off the bullet tip, then it would be just the opposite. Your oal would be the same but the amount of bullet in the case would change. I was concerned in raising pressure by being 1.115 instead of 1.120.

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3 hours ago, rooster said:

doesn’t effect accuracy.   I was concerned in raising pressure by being 1.115 instead of 1.120.

 

I wouldn't worry about the pressure, but you could confirm by measuring velocity.

 

And, if it doesn't affect accuracy, I wouldn't worry about it.    :) 

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27 minutes ago, Hi-Power Jack said:

 

I wouldn't worry about the pressure, but you could confirm by measuring velocity.

 

And, if it doesn't affect accuracy, I wouldn't worry about it.    :) 

10/4. I guess from previous post the ogive could also be inaccurate, but I measured some last night and the ogive seemed to be pretty consistent. So I took the shortest bullet, and set press with that bullet seated to 1.120, now I shouldn’t get any oal shorter that that. Which became true after measuring about 20 rounds. These are seconds, but they are from a well known company. Thanks for your help.

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  • 3 months later...

 
All that being said,  +/- .006" isn't really "all over the place", and shouldn't cause pressure concerns. 


I’m glad to read that. I just started reloading after a few years and was concerned I was getting +/- .006 ranges in OAL. I figured it was due to the fact that I’m using mixed head stamp range brass.

I also looked at my chrono data going back to 2007 and found larger velocity deviations which I also attributed to the brass.



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The effects of shortening and pressure are very much powder dependent.. For higher velocity loads with 124 gr. JHPs I use Silhouette quite a lot. I can hold OACL tolerances to +/- .002" in many cases, but I seat and crimp in 2 separate operations.

 

Don't take anything for granted and trust your chronograpgh. I typically see variations in velocity when I increase or decrease OACL by .010". These are typically 1200 FPS + loads. On the other end, with light loads you may not see much change depending on the powder your using. But ask your chrono just to be sure.😉

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