Why is there such a difference in charges for JHP vs. FP-XTP?

[Kappy]

I just made up some loads for 125 gr. .357 magnum rounds. I got a shipment of Vectan BA9 powder.

Looking at the load data, for 125 gr. bullets, I have:
125 gr JHP
min 8.0 gr
max 10.0 gr

and

125 gr FP-XTP
min 7.4 gr
max 9.3 gr

I'm using Xtreme 125 gr copper plated bullets, which I've always treated as basically being a JHP.

I loaded them at 9.4 gr of powder, and now I'm worried that I'm exceeding the max for the FP-XTP.

What I don't get is... why would two bullets which weigh the same be treated so differently? Am I going to blow up my gun?

Is it better to test this in a shorter barrel so that there is less pressure? Do I have to bust up 300 .357 magnums?

[JagerDog]

Shorter barrel won't change chamber pressure.

I don't see a "so different" in the above data (~7% reduction).

You're dealing with a new powder with limited data. That's the price you pay for "pioneering".

[Spafxer]

Is the C.O.L. the same for both loads?

[opos]

I just made up some loads for 125 gr. .357 magnum rounds. I got a shipment of Vectan BA9 powder.

Looking at the load data, for 125 gr. bullets, I have:
125 gr JHP
min 8.0 gr
max 10.0 gr

and

125 gr FP-XTP
min 7.4 gr
max 9.3 gr

I'm using Xtreme 125 gr copper plated bullets, which I've always treated as basically being a JHP.

I loaded them at 9.4 gr of powder, and now I'm worried that I'm exceeding the max for the FP-XTP.

What I don't get is... why would two bullets which weigh the same be treated so differently? Am I going to blow up my gun?

Is it better to test this in a shorter barrel so that there is less pressure? Do I have to bust up 300 .357 magnums?

I been through the same dog and pony show...Hodgden's charts often show the Hornady bullets and ignore others...I shoot a lot of Speer and went to the speer Manual to find reasonable load data..I keep several books on hand and also I buy the little "one caliber one book" booklets on e bay for the caliber I'm using...it gives good cross references between published data from the manuals and it is cheap and simple to use...I also look at MD Smith's reloading pages...I've not had problems with any of it as long as I begin with a reasonable starting point and keep my eyes out for anything strange I will say I ran into a strange situation a while back..I really like the 4013 Speer 38 caliber 125g sjhp bullet...in old books it was listed with W231 powder as a "do not reduce"load (only one load shown)...it disapeared and I con'tacted Speer and found they do not recommend it for a 38 special with W231 any more as there have been cases of leaving the jacket in the barrel... so it pays to keep on your toes and double check things. I always had a bit of a difference when trying to load Hornady bullets using powder data and a "generic" bullet..but slow and steady always works fine for me.

[Kappy]

Is the C.O.L. the same for both loads?

Yeah... I think they're both 1.57".

I'm not at home with a manual right now, but that's what I'm seeing when looking at manual's.

I think I run mine slightly longer (1.6"), so maybe that will help.

[Kappy]

I been through the same dog and pony show...Hodgden's charts often show the Hornady bullets and ignore others...I shoot a lot of Speer and went to the speer Manual to find reasonable load data..I keep several books on hand and also I buy the little "one caliber one book" booklets on e bay for the caliber I'm using...it gives good cross references between published data from the manuals and it is cheap and simple to use...I also look at MD Smith's reloading pages...I've not had problems with any of it as long as I begin with a reasonable starting point and keep my eyes out for anything strange I will say I ran into a strange situation a while back..I really like the 4013 Speer 38 caliber 125g sjhp bullet...in old books it was listed with W231 powder as a "do not reduce"load (only one load shown)...it disapeared and I con'tacted Speer and found they do not recommend it for a 38 special with W231 any more as there have been cases of leaving the jacket in the barrel... so it pays to keep on your toes and double check things. I always had a bit of a difference when trying to load Hornady bullets using powder data and a "generic" bullet..but slow and steady always works fine for me.

Odd that you mentioned that. W231 (or HP38) was my .38 spl powder of choice. I guess I'll be switching to something else.

I'm just worried because the midrange for this powder is a hair above the max for another bullet.

[Kappy]

Shorter barrel won't change chamber pressure.

I don't see a "so different" in the above data (~7% reduction).

You're dealing with a new powder with limited data. That's the price you pay for "pioneering".

I'll let you know how it works out. I may have to type it left handed.

[Rotting]

I just made up some loads for 125 gr. .357 magnum rounds. I got a shipment of Vectan BA9 powder.

Looking at the load data, for 125 gr. bullets, I have:
125 gr JHP
min 8.0 gr
max 10.0 gr

and

125 gr FP-XTP
min 7.4 gr
max 9.3 gr

I'm using Xtreme 125 gr copper plated bullets, which I've always treated as basically being a JHP.

I loaded them at 9.4 gr of powder, and now I'm worried that I'm exceeding the max for the FP-XTP.

What I don't get is... why would two bullets which weigh the same be treated so differently? Am I going to blow up my gun?

Is it better to test this in a shorter barrel so that there is less pressure? Do I have to bust up 300 .357 magnums?

A lot of technical babbling could be done on this, but here's really what it boils down to -- are the bullets themselves identical in diameter and height? The answer is almost certainly no, though the differences may seem negligible. This height or width difference can have an impact on the pressure conditions in the chamber/barrel at ignition.

Secondly, asked in a semi-technical fashion-- "How different are the obturation qualities of the two different projectiles?" That is to say, under pressure, how do these two projectiles "expand" causing it to, at least somewhat, fill in empty space in the chamber and barrel? This is a more significant consideration with lead bullets, but it happens with all bullets in all chambers/barrels.

These two qualities alone can explain the (rather slight) differences you are seeing in powder charge requirements for seemingly identical bullets.

So, let's take it in order.

Bullet A may be .005" taller (and/or wider) than bullet B. If it is taller, to achieve the same COAL, it must be seated deeper into the brass. You can see that this will have an effect on pressure conditions, and, it follows, on powder charge.

Now, let's look at obturation. Let's say that Bullet A "expands" to "fill the gaps" 5% more than bullet B does. Well, if the gasses that are propelling the bullet down the barrel can't "escape" around bullet A as readily as it does with bullet B, you can see that there is a different pressure equation to be had here.

Again, there are several other factors that contribute, these two alone account for a giant portion of why you see disparities such as your case here.

['ol shooter]

I like that avatar, used to watch Rollie and the A's a lot as a younger fellow. Just because two projectiles are the same caliber and weight doesn't mean they will fly alike, the ballistic coefficient can be very different between them.

[Kappy]

Update: The rounds worked just fine. Better than I hoped for, frankly. Full house loads which are seriously accurate. 5-shots under 2" at 10yds. For me, that's a winner.