There have been some discussions here in the recent past about how Hammer Bullets are capable of generating higher velocity than conventional bullets of the similar weight. Particularly the Absolute Hammers because of the very low engraving pressure by design. There have been a few folks on here that have said that it is not possible to generate higher vel with one bullet of the same weight as another without utilizing higher pressure. Charlie from the Gordon's Reloading Tool has been doing some pressure testing with his 7mm test rifle in order to calculate the start pressures for calculating loads for the Hammer Bullets. We sent him all of our Hammer Hunter and Absolute Hammers to test. His initial test was with the Hammer Hunter Bullets slightly jammed into the lands so that he could compare initial pressures compared to lead core bullets. His findings were that they were basically identical to the cup and core when jammed. This is not normal for pure copper bullets. They usually are higher. He then tested with a jump of 50 thou. With Charlies permission I am just going to copy the emails that I received from him regarding his findings.
I finally got around to running the tests to determine the basic IP settings for the Hammer AH and HH bullets in 7mm that you provided. So sorry it took so long. I have a reason, just too old to get up the energy to do so at times. Additionally, my helper got sick two months ago and has died just this week. That is what happens when your help pool and organizer are all in their mid 70s.
As you know the initial tests were with bullets loaded to what I term soft jam (basically just plunk able in the chamber). I did this in order to determine the charges for each bullet weight that represents pressure nearly to cartridge Pmax, or MAP in SAAMI speak. This week I loaded up the remainder of the HH bullets and set the COAL -.050" shorted.
For the 177 HH bullets using the same charges as the Plunk rounds and running the PTII device, it becomes apparent that with .050" additional bullet jump available the velocity will drop approximately 20 to 30 fps, and the peak pressures remain nearly the same, within the limitations of the measuring device. In order to balance the simulation the no jump bullets require a IP (Initial Pressure / Start Pressure) setting of about 2900 psi, while the .050" jump bullets require a reduction in IP down to about 1500 psi. So it appears the jump actually allows the bullets to engrave easier into the rifling. The 155 HH and 120 HH responded in a similar manner. No jump to .050" jump produced nearly the same velocity and peak pressures, but the jumping bullets all required an IP reduction to balance the simulation.
Next I tested the 172 , 155, and 120 AH bullets. There was a bit of surprise when loading these. Of course the 172 AH required a bit more propellant than the 177 HH to get similar velocities, however, the COAL required to find Plunk and Plunk -.050" were exactly the same as the 177 HH bullets.
This surprised me as the ogive ahead of the first driving band is cut away on the AH bullets. I had added an additional .66 grs. of propellant as compared to the 177 HH in order to balance the simulation to the 5 grs. less weight. The results on both the chrono (MSV3) and PTII were reversed from the 177 HH results. The rounds that were shorter by -.050" actual gained a bit of velocity and produced a bit more pressure than the Plunk rounds. On the order of 30 fps and 2200 psi. These are well within the variation between shots normally, and the differences may level out with a greater number of shots. The big thing that appeared was that the IP required to balance the simulation was drastically reduced from the HH bullets even for the Plunk rounds. It appears with my 284 Win and long freebore throat (completely different from standard 284 Win) the HH and AH bullets touch the lands nearly at the same place on the first driving band, however the cutaway of the AH allows the required IP to be much lower than that required by the HH. Velocities were nearly the same for both the HH and AH, but the relative pressure produced was approximately 3000 to 4000 psi less. This change could be seen in the primers as well.
With the 155 AH the results were similar except that the velocities for the HH and AH bullets were all within 10 fps no matter the type or jump. However, the AH produced approximately 4000 psi less pressure overall, and required a reduction in IP of 1400 psi between the Plunk and -.050" rounds.
The 120 AH followed suit. I am still studying the data, but I think it is safe to say that the AH type allows one to either obtain the same velocity for weight at a lower pressure, or obtain more velocity at the same pressure as the HH type. My limited testing does not indicate if one type is more inherently accurate that the other. I had no grouping of greater than 1 MOA, and most were within .75 MOA and a couple were below .5 MOA. The way my barrel is chambered I never see greater than 1 MOA with every bullet I have tried except for Noslers'.
If I were deciding between the AH and HH types, I think I would be choosing the AH and save the strain on the barrel.
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Response from Ed.
Simplified, 1500 PSI for HH and 1400 PSI for AH? That should raise some eye brows in the GRT and QL community!!!
Ed
Response from Charlie.
Yes, it appears the design actually does relieve the start pressure requirement that most monolithic designs required.
Of course, it was impossible to get a true reading since the bullet weights were not exactly the same between the AH and HH designs.
Never the less, it is apparent that the AH requires a smaller IP as compared to the HH type in 7mm, and I would think all other diameters.
The few bullets that were just above the nominal diameter by .0005" or so did produce higher measured pressures, but only in the order of 3 – 4000 psi.
Even lead core bullets generate a pressure differential of +/- 2000 psi most of the time.
I ran my targets through the GRT Group Analyzer and the worst group was 1.1 MOA, and the best .4 MOA. Most of the larger groups were shooter problems.
At 76 years of age, my ability to hold point of aim is compromised by floaters in my eyes, and my Parkinson like genetic twitch.
Both GRT and QL programs lump the IP as a single value depending of several factors, but in truth IP is different for every bullet / cartridge / propellant, with primer being a factor
In the smaller case volume types. Both programs assume the bullet is touching the lands, only GRT has compensation for bullet jump, but that option was developed using
Straight walled cartridges and cartridges with very shallow shoulders like 300 AAC.
I chose my 284 Win since it has 35 degree shoulders, and bullet jump becomes a very small portion of the IP as the propellant packs at the shoulder from primer impulse
and prevents much initial pressure from the ignition cycle from reaching the bullet base. Significant propellant burn has to take place before the bullet moves, unlike the 300 AAC
who's bullet will advance to the lands upon primer impulse and before a significant propellant burn takes place. Variable neck tension in these straight wall cartridges makes
it hard to achieve small SD/ES numbers, but if the reloader can set the bullets very near to the lands the SD/ES numbers in these straight walled cartridges will usually improve.
Charlie
Project Relations
Gordons Reloading Tool
Berlin, Germany
[email protected]
https://www.grtools.de/
https://discord.com/invite/3FEYWG4
