124 grn FMJ vs 124 grn XTP... What makes em different?


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TrickyD
February 13, 2012, 12:18 AM
While loading 9mm, I see some load books list 124 grn FMJ and 124 grn Hornady XTPs separately. I am curious what the difference is. I would consider the the XTP to be a FMJ and if its the same weight, whats the difference? Do I have to use load info that names the XTP specifically?

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Lerk
February 13, 2012, 12:23 AM
The XTP's (along with the Gold Dot's) have a large expansion. My Lee Manual (Page 96 if you're curious) has a specific warning about them as they need to be seated deeper then most standard bullets and that will affect pressures.

I actually had looked this exact thing up today for 10mm as I wasn't finding very consistent load data with the XTP's for certain powders.

jcwit
February 13, 2012, 12:26 AM
I am guessing the bearing surface is different between the two bullets.

I would start at the lowest starting load for the two bullets and work up to an accurate load from there and stop. This should keep you well away from a max load. You have no need for full max loads, do you?

Plus what Lerk says.

rcmodel
February 13, 2012, 12:38 AM
Hornady XTP bullets use a Secant Ogive shape which is different then most others.

It tends to make the full dia bearing surface longer, which make bore friction higher.

See this about that:
http://www.thehighroad.org/showthread.php?t=643158

rc

joneb
February 13, 2012, 12:40 AM
has a specific warning about them as they need to be seated deeper then most standard bullets and that will affect pressures.
That's a biggy, seat a bullet deeper in the case and the pressure increases. This is the main reason you do not mix load data with different bullets.
The driving band length, bullet diameter, jacket composition and thickness are other factors.

918v
February 13, 2012, 02:22 PM
Hornady XTP bullets use a Secant Ogive shape which is different then most others.


The reason you have to treat them differently is Hornady's truncated cone nose profile will cause the bullet to touch the rifling sooner than a FMJ when both are loaded to the same OAL. In most 9mm chambers, you can load a FMJ to the max recommenced OAL of 1.169". An XTP at that OAL will be jammed into the rifling and the slide won't close. XTPs have to be seated closer to 1.100" in order to chamber. 9mms are very sensitive to seating depth because they run at high pressures and have a low internal capacity. When you see the same data for both the XTP and the FMJ, it defers to the XTP which has to be seated deeper.

gamestalker
February 13, 2012, 06:44 PM
My approach, and that of most seasoned reloaders I know, is to seat bullets for AL cartridges to the longest OAL possible that will accomodate chamber fit, magazine fit, and cycles properly. And to be completely honest, I couldn't tell you what OAL I seat to without measuring them, simply because I use the barrel, not a number to determine seating depth. And as to whether XTP's or Gold Dots require a deeper depth to enable chamber or magazine fit, again, this would not mean anything to me, even if they do seat deeper. The only XTP that may present some specific OAL consideration is the 147 gr. XTP. But it still doesn't change anything as to how I determine the proper depth using the barrel as my guide.

Work up procedure isn't any different or require any extra attention to detail. Find the correct OAL for your firearm, use a low end starting charge, and then work the powder charge up, stopping when you've reached what your going for, or pressures have indicated your at maximum.

918v
February 16, 2012, 01:24 PM
And as to whether XTP's or Gold Dots require a deeper depth to enable chamber or magazine fit, again, this would not mean anything to me, even if they do seat deeper.

It should mean this: geometric differences between different bullet types force your hand when determining OAL, even if you use your barrel as a guide.

rcmodel
February 16, 2012, 01:45 PM
+1

Reloading manuals that specify a generic 124 grain FMJ are talking about the original 9mm FMJ-RN military bullet.

XTP JHPs are not shaped like that, and are longer for weight because of the large HP cavity taking up space inside them.

rc

1SOW
February 17, 2012, 02:08 AM
Work up procedure isn't any different or require any extra attention to detail. Find the correct OAL for your firearm, use a low end starting charge, and then work the powder charge up, stopping when you've reached what your going for, or pressures have indicated your at maximum.

I don't disagree with what you've said. I use a variation of this same procedure too.

The problem is the idea that the MAX usable oal is the best load oal, is not always correct.
One simple example is if you have multiple pistols shooting the same loads. Some chambers are shorter than others. Conical-nose HP tend to be the worst offenders here.
Then there is accuracy and consistency to consider. The longest oal that will fit, is not necessarily the most accurate that can be shot. I do acknowledge that many loads do tend to shoot accurately loaded long.
Not all powders or load charges follow this same rule.
Do you run all loads to max speed in all pistols? Why not?
The same is true of OAL.

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