Here are three examples of Sierra's multiple BC values.
Bullet #2145, 0.308, 165g, SBT GameKing's BC goes from 0.404 above 2400 fps and increases through one intermediate step to 0.419 under 1600 fps. G1 BC increases as velocity drops.
Bullet #2140, 0.308, 165g, HPBT GameKing's BC goes from 0.363 above 2600 fps, to 0.355 between 2600 and 1800 and then back to 0.363 under 1800 fps. G1 BC dips at midrange velocity.
Bullet #2200, 0.308, 168 HPBT MatchKing's BC goes from 0.462 above 2600 fps and decreases through two intermediate steps to 0.405 under 1600 fps. G1 BC decreases as velocity drops.
Looking at the above examples it's clear that no single drag function can best match those three bullets, which is why Sierra is sticking to their multiple BC technique. In a way they are giving you a custom drag function for each of their bullets. Unfortunately, most apps can't accommodate Sierra's multiple BC values, at least not correctly.
As for other software, Ballistic Explorer does all the drag functions including Sierra's multiple BC technique, and it also has by far the most accurate conversion of BC's from one drag function to another drag function of anything I've found. Likely that's because the conversion is based on research published by Ken Oehler. It can even convert Sierra's multiple G1 BC values to any other drag function including back to a single G1 value. Here are some examples.
Assuming a MV of 3000 fps the equivalent single G1 BC value for bullet #2145 is 0.408 and the G7 value is 0.208. Relative to the multiple BC value, drop relative to LOD out to 1000 yards is within 0.08 MOA for the single G1 BC value and within 0.39 MOA for the G7 BC value. G1 is a better match for this bullet than G7.
The same experiment for bullet #2140 gives an equivalent single G1 BC value of 0.359 and an equivalent single G7 BC value of 0.186. Relative to the multiple BC value, drop relative to LOD out to 1000 yards is within 0.01 MOA for the single G1 BC value and within 0.68 MOA for the G7 BC value. G1 is a better match for this bullet than G7.
The same experiment for bullet #2200 gives an equivalent single G1 BC value of 0.442 and an equivalent single G7 BC value of 0.224. Relative to the multiple BC value, drop relative to LOD out to 1000 yards is within 0.27 MOA for the single G1 BC value and within 0.10 MOA for the G7 BC value. G7 is a better match for this bullet than G1.
This all assumes Sierra's measurements of BC are accurate.