The internal shape of the hull, amount and bulk-density of the powder, shape and size of the wad, and the amount and size of the shot-charge interact to create a load that must fit well enough together to get a firm crimp that is neither too deep nor too shallow. This wasn't much of an issue when reloading lead-shot because 1) volume of the shot-charges did not vary as much with shot size as with steel shot, and 2) wad pressure could be adjusted to either leave the wad fully extended or compress it to get whatever space was necessary for a good crimp. But when reloading steel-shot, larger shot sizes take up a lot more space than smaller sizes and the wad cannot be compressed to get more space. Consequently, steel loads often require use of filler wads to occupy extra space.

The most common filler or spacer wads are made of hair, felt, wood fiber, or cork. Felt spacers or wool-felts are made of soft, compressible, hairy material, card wads feel like very dense cardboard, cork wads are simply discs of cork, and fiber wads are made of a very light, crumbly material with a felt top. All of these wads are placed in the base of the shotcup, under the shot-charge, to elevate the shot column to appropriate height for a good crimp. However, you must be cautious not to elevate the shot column too far out of the shotcup such that set-back pressure upon ignition won't push all pellets back into the protective sleeve before traveling down the barrel.

The ballistic effect of filler wads is rather confusing to me. BP says that you can use whatever combination of filler wads you need without influencing the ballistics of your load significantly. RSI says that if the recipe calls for a felt spacer and you substitute a card wad, pressure will increase 500 PSI. My favorite duck load was from MEC and although the recipe did not call for a card wad, I always put one in the base of the wad to eliminate shot bridging and sticking in the base of the wad. When I sent this load to be tested by Ballistic Research Inc., a note saying that addition of a card wad when the load data did not call for it, leads to reduced peak pressure and velocity accompanied the test results. So it seems clear that filler wads do alter the ballistics of a load if not exactly what is called for in the recipe, but how significantly is not clear.

I am an advocate of using a filler wad in the base of steel-shot wads in every load where one will fit. I have patterned a lot of loads in my search for the mythical "best" load for me, and I have picked up a lot of wads with pellets stuck in the base of the shotcup. SAM1 wads have been particularly bad for me, but it was not uncommon with MEC or BP Ranger wads also. For a long time, I had a wad on my desk with 16 steel #2 pellets lodged in the base. Sixteen pellets in a load with only 140 is significant. Adding a felt or card wad in the base of the shotcup nearly eliminates this problem and has given me more consistent patterns. However, be very careful about putting a felt or card wad ON TOP of the shot column. I did this once, with my favorite duck load, when I bought 20-gauge instead of 24-gauge card wads, and they would not fit in the base of the shot-cup but fit very nicely on top of the shot. Patterns went from an average of 69% with the card wad below the shot to 42% with the card over the shot. I did the same thing with a felt spacer when advised to do so over the phone by RSI using load #13 in their manual with a Federal plastic-based 3-inch hull. The dished-crimp load averaged 72% while the beautifully-crimped load with the felt on top of the shot did only 66% with less uniformity. Filler wads placed on top of the shot may give you beautiful crimps, but they can interfere with pattern quality.