Let's talk about antimony. Antimony is added to lead shot to make it harder. The amount of antimony in lead shot can differ greatly and not "all lead shot has a couple percent of antimony in it". Generally, US manufactures use less antimony as the pellets get larger.
For example, common US antimony levels for "hard" shot (a subjective term) are:
- hard #7 - #8, 5%-6% antimony
- hard #4 - #6, 3%-4% antimony
- hard #2, 2% antimony
- hard #9 skeet, 3% antimony
Not all commercial loads contain "hard" shot. Antimony costs more than lead so, generally, cheaper loads contain less antimony and are not "hard". Very cheap loads usually contain much less antimony and are "soft".
Are weight differences in shot with differing antimony levels significant? We know that 1% and 2% antimony shot is denser than 5% antimony shot. The specific gravity of 1% antimony shot is 11.259, 2% antimony shot is 11.18, and the specific gravity of 5% antimony shot is 10.94. At first that might not look like much of a difference. But round ball ballistics are influenced by the size of the ball and the weight of the ball. With equally sized balls, #7.5 in this case, the difference in weight effects the ballistics. The denser, heavier shot retains more velocity because it is heavier for a given spherical size.
We can relate that velocity difference to energy. Consider the downrange energy of two #7.5 (2.41mm) pellets, one with 2% antimony and one with 5% antimony, leaving the muzzle at 1200 fps:
Energy of 2% antimony pellet at:
- Muzzle = 4.75 ft lb
- 20 yards = 2.61 ft lb
- 30 yards = 2.09 ft lb
- 40 yards = 1.66 ft lb
- Muzzle = 4.65 ft lb
- 20 yards = 2.54 ft lb
- 30 yards = 2.01 ft lb
- 40 yards = 1.59 ft lb
- Muzzle = 2.1%
- 20 yards = 2.7%
- 30 yards = 3.8%b
- 40 yards = 4.2%
- Muzzle = 2.9%
- 20 yards = 3,8%
- 30 yards = 5.2%b
- 40 yards = 5.4%
The down side is that softer pellets deform more easily. Deformation creates additional drag that will reduce the velocity of the pellet. Deformation acts to counter any density advantage a softer pellet has. Realize, though, that deformation is not confined to "soft" pellets. "Hard" pellets also deform. They just deform less then softer pellets do when subjected to equal forces. When shooting birds, we should do what we can to reduce pellet deformation.
The greatest cause of deformation is initial setback of the shot when the shell is fired. Wads are used to buffer those forces and reduce deformation. Higher velocity loads subject the pellets to more initial deformation than lower velocity loads. So, one way to mitigate initial setback is to use lower velocity loads. Deformation also occurs when pellets are jammed into each other in the choke. A tight choke tends to deform shot more than an open choke. So, another way to mitigate shot deformation is to use an open choke.
"This purposely exaggerated illustration shows the rearward rows of pellets flattened by pressures of gas behind them and inertia of shot weight in front. Here the load is entering the constriction of the choke, but many of its soft pellets have already been deformed by setback forces at ignition" - from Shotgunning by Bob Brister
Like everything, it is a matter of trade-offs. You can only do so much to reduce deformation of shot, especially if the shot is very soft. You can reduce velocity, but reduce it too much and you'll start crippling birds. You can shoot with an open choke but you still need to have enough choke to ensure adequate pattern density or you will miss or cripple birds. It goes the other way too. You can use harder shot to reduce deformation but very hard shot will shed velocity faster. To overcome that you can use bigger pellets and increase velocity but more velocity will subject the shot, hard as it may be, to more setback. Using bigger shot means fewer pellets per load and you will need to increase choke to maintain a good pattern density for hitting your birds.
On one extreme you can have very soft, pure lead shot fired at low velocity and no choke. On the other extreme you can have bigger, very hard shot, fired at high velocity through a tight choke.
Everyone must find a compromise that suits them. The loads I usually use on pheasants illustrate the compromises I have chosen. One load fires 15/16 ounces of #7.5 in 2% antimony shot at a velocity of about 1180 fps. I usually shoot that out of a Paradox gun that chokes it like a light modified choke would. The other load is 1 ounce of #7.5 or #6 in 3% antimony shot fired just over 1200 fps and usually fired through a light modified or modified choke. Both of those loads give excellent performance on pheasants and, I will add, chukar and forest grouse.
The fellow shooting 1-1/2 ounce of #4 shot at 1450 fps out of a 3" magnum shell has made his compromises, too. In his situation, setback is exacerbated by the heavy shot charge and by a high muzzle velocity. The high velocity is needed to compensate for the higher drag of deformed shot but the extra velocity deforms the shot more so he needs bigger shot and more velocity to compensate. It's a catch-22. He also needs a tight choke to keep those big pellets from leaving voids in the pattern. That beats the shot up even more and causes more deformation. However, in the end, his load will send a tight enough pattern out to the birds with more than ample energy to ensure clean kills. But his choice means he will be carrying a heavier gun, experiencing more recoil, and buying costlier ammunition. He will be killing no more birds than if he was using one of the more efficient choices in pheasant loads. And he won't care that those loads are effective and efficient, are suitable for a light gun instead of needing a heavy gun, and are pleasant to shoot. No, he won't care because he still thinks he needs #4 shot at 1450 fps to kill a pheasant.