What's the deal with Hornaday HAP and XTP

[nwhpfan]

Why does HAP and XTP bullets reach such high pressure at low charge weights with low velocity compared to bullets like the Golden Saber or Gold Dot HP? Here is some data from Accurate and data from Hodgdon shows the same theme.

Ramshot ZIP

124 HDY XTP 3.3 888 3.7 967 34,893 1.060
124 MIL FMJ 3.8 879 4.3 985 34,750 1.100
124 REM GS 4.1 970 4.6 1,055 34,880 1.145
124 SPEER GDHP 3.8 888 4.2 979 34,218 1.105

Accurate #2

124 HDY XTP 3.3 877 3.6 938 34,518 1.060
124 REM GS 4.3 989 4.8 1,054 34,859 1.145
124 SPEER GDHP 3.9 917 4.3 989 34,347 1.105
124 WIN FMJ 3.6 880 4.2 992 34,675 1.100

[Hambo]

Call or email Hodgdon or Western and ask them.

[nwhpfan]

Call or email Hodgdon or Western and ask them.

Gee thanks. Like, why have a forum then

[rjohnson4405]

My guess is they slightly oversize for accuracy in a wide range of barrels.

[Tokarev]

The Golden Saber has less surface area than probably anything else in its weight class. The Gold Dot has a concave base that gives the loaded round a bit more powder capacity/air space. Both of those should produce lower pressures.

[Hambo]

Gee thanks. Like, why have a forum then

If you want guesses, a forum is great. If you want to hear from the people who pressure tested the data, not so much. I've called powder, bullet, and reloading companies and always found them helpful.

[WDR]

My guess is they slightly oversize for accuracy in a wide range of barrels.

I'm not sure if it's still the case, but the 9mm versions were .356" vs the standard .355" for a jacketed 9mm bullet. Possibly because they are also often used in .38 Super. I don't believe the .40 or .45 bullets are oversize, but I could be wrong. I've still got about 700 of the now discontinued 121gr 9mm HAP bullets kicking around, and they do measure .356"

Other reasons for higher pressure can include bearing surface length, jacket composition, bullet hardness... the list goes on...

[Lester Polfus]

Hornady's 9th Edition Reloading Manual doesn't even list reloading data for 9x19mm with the 121 and 125 grain HAP bullets. They are both .356" diameter bullets designed for the .38 Super.

[olstyn]

I think at least some of the answer is right there in the data - look at the cartridge overall lengths - the XTP/HAP bullet's profile is such that it has to be loaded shorter than a lot of other bullets, which of course leads to higher chamber pressures. In the event that you have a gun which has a generous enough chamber, you may be able to load the XTP/HAP longer than the data specifies, in which case you'll have lower pressures for any given powder charge.