Can I use 125gr load data for 124gr bullets?

[Airweight38]

Battery is croaking on my laptop, so I'll be brief.

Bought a Lee 124-gr .356 9mm mold. Cast some bullets.

Looking at load data in my Lee book, I see Unique loads for 125 gr bullets but not 124. Logic tells me that I can use load data for lighter bullets within reason, but not the other way around. So I'm good there. Question is:

1) Why are the powders SO DIFFERENT from 124gr bullets to 125. Accur #2 is the only powder that appears in both places.

and 2) Why are starting OALs so different? 124gr the starting Min OAL ranges from 1.095 to 1.142. In 125 it's 1.100 - 1.169.

Given this disparity, is it even safe to substitute 125gr load data? These seem so different!

I'm really not interested in buying a new powder for these loads.

Help.

Thanks,

Jason

[lifeon2]

perfectly safe and to be honest at a 1 grain difference makes no difference at all

[qajaq59]

Just go with the starting load for the 125 and work your way up. One grain isn't going to make any difference.

[Bob Krack]

Could be that the shape/size of the projectile requires more space to be consumed in the boiler room meaning smaller combustion chamber and perhaps much more pressure.

The weight difference by itself makes no practical difference.

Bob

[Oldtimer]

Also make sure you are looking at data for cast loads and not jacketed. The data for the one grain difference as others have said makes no difference, but if you compare data for 124 cast, vs 125 jacketed, you will see a good bit of difference. Just my .02 cents. Bob

[mike in co]

what bob and oldtimer said...
plus

1.169" is MAX OAL for a 9mm luger/9mm nato/9x19mm/9x21 in a 9mm frame( 9mm in a 45 frame can go longer)

use listed starting lengths, with the listed min load and work up.

once you get close to what you want, you can tune the load by changing the length and/or the powder load.


mike in co

[mooman76]

You can use heavier data for a lighter bullet but not the other way around. This is from the book but like others have said 1 gr. isn't going to make a difference.

[jonk]

The variation in your bullet weight is going to be more than that. I'd bet that you get a 1-2 gr variation at least, maybe 3-5. It's taken into account by the load data.

You can generally substitute data as printed for a heavier bullet as you suggest, but within reason, and up to a point. Too heavy and you might have a squib load with the small powder charge. For instance, if you were shooting a 100gr bullet (not in a 9mm but in general) and used the data for a 250 grainer, you might get incomplete ignition or even a secondary explosion effect.

But within reason, sure, no problem.

[Leftoverdj]

You can use heavier data for a lighter bullet but not the other way around. This is from the book but like others have said 1 gr. isn't going to make a difference.

Coulda fooled me. Bullets have more than one grain variance, sometimes even factory jacketed bullets.

I don't worry about anything less than a 5% variance for starting loads. Anywhere between a 120 and 130, I'll happily use 125 grain starting data and work up.

There is such a thing as picking too many nits.

[AZ-Stew]

Lots of possible variables.

All things equal (both bullets jacketed or both cast) and with the same bearing length, there is no practical difference in load data for a 1-grain difference in bullet weigh.

If the data is for a cast bullet and I'm shooting jacketed, I'd look for specific data for the jacketed bullet, since it will have more resistance in the bore, resulting in higher pressures.

Inversely, if the data is for jacketed and the bullet being loaded is cast, the jacketed load data should be safe with the cast bullet. Again, the last two examples assume bullets with the same bearing length and protrusion into the case and with weights within a few percentage points of each other.

In my experience, quality jacketed bullets usually vary no more than a few of tenths of a grain within a box/lot. Good cast bullets don't vary much more. In practical use, a variation of 1-2 percent in weight will have little effect with regard to load safety or accuracy, bench rest competition excepted. Again, all other things being equal.

The OAL data you've seen will be greatly affected by the bullet shape. Look at the two bullets in question. Is one more "pointy" than the other? If so, this is likely the reason for the difference.

Regards,

Stew