16ga.com ~ View topic - Can someone please explain the relationship

Joined: 21 Dec 2006 Posts: 40 Location: Memphis, TN

among load weight, velocity, and pressure? I mean, I know that a heavier load with a higher velocity is generally going to have a higher pressure. But it seems to me that the crimp, type of powder, type of primer, type of hull, type of wad can all effect the velocity and pressure - and not necessarily in a 1;1 ratio.

One example of a seemingly anomalous situation is steel shot - the loads are lighter and the velocity is higher, but the pressures are MUCH higher.

So, how does this work? Thanks.

There is usually no real way to tell short of testing in a pressure gun. Every time we change one componant, we alter the peak pressure of a load. The commonest change is adding or subtracting a fraction of a grain of the powder being used. this will usually raise or lower pressures somewhat predictably--but not always. Every once in a while, lowering the amount will raise the pressures slightly. Who knows why. It just happens. However, it usually is never enough to be dangerous. Adding too much of course always is and so is not using enough sometimes. Just stay within the range of suggested powder drops with the componants called for in the manual youare using and you will be safe.

Different powders with different burning rates will raise or lower pressures. Adding shot will always raise pressures if all else is the same. A hotter primer can too. A deeper crimp sometimes will, and sometimes not.

Changing a wad type or shell type should be carefully done. Shells with smaller internal capacities will raise pressure. So will using a wad with a more efficient gas seal, or a stiffer one, or one with a tighter fit. Even plastic density wad to wad will alter pressures slightly.

Until a reloader is well advanced along in his experience and can read loading manuals well enough to recognise predictable trends from componants, just stick to the basic recipes and be safe. the manuals are written with a biuilt in safety or fudge factor. You will not get in trouble if you follow the recipes and directions exactly.

If you have componants but can't find data for them, call the powder company of the brand you intend to use in the load. talk to one of the ballistics lab techs before you try to put a load together. These folks sometims know how safe the load you are contemplating will be.

A good example of this was how I learned switching an R16 one ounce wad to replace a Winchester AA16 wad was a safe move if all other componants were the same and used in a safe recipe to begin with. The guys at Alliant knew this from long experience. R16 wads predated the Winchester wad by a decade and were commonly used in Winchester compression formed hulls before 1984. I had seen the evidence based on comparing old and new data using the same powder, Unique, in a WW16AA compression formed shell with one ounce of shot, and a Winchester 209 primer. I got the okay, and have been swapping an R16 for the now long gone Winchester wad ever since.

Free lance reloading is going out on a limb. Be absolutely certain you know what you are doing before doing it and use a very strong gun in excellent shape if you do. That is my best advice.

Recoil is entirely a function of shot weight, shot speed (fps), and the weight of the gun. Just for reference, here are some helpful numbers calculated from Lowry's ballistics program. All figures are in an 8# gun:

1-1/8 oz 1145 fps 18.0 ft/lb recoil
1 oz 1252 fps 18.0 ft/lb recoil
7/8 oz 1383 fps 18.0 ft/lb recoil

1-1/8 oz 1200 fps 20.4 ft/lb recoil
1 oz 1200 fps 16.1 ft/lb recoil
7/8 oz 1200 fps 12.3 ft/lb recoil

Pressure is something entirely different. The chamber pressure for a 1/2 oz .410 AA target load is almost twice that of the 12g AA "Low recoil Low noise" 15/16 oz load which is just less than 6000 psi.
The smaller the bore, for the same fps and shot weight, the higher the pressure. I think that's the Bernoulli's equation but somebody smarter needs to confirm Rolling Eyes

This speed/pressure chart from B&P does at least show the relationship between speed, shot weight, and pressure (bar and psi):
http://www.bandpusa.com/files/db_trio.pdf

See this thread for some infro regarding psi vs. bar:
http://www.16ga.com/forum/viewtopic.php?t=2422