I’ve got some old C5s I acquired with some B14s, but never used any of either. I plan to try both this year.
But, I do like C11s. They may be oversized for some small rockets, and a tad overweight (about 10g heavier than 18mm Cs), but price is within pennies on a per motor basis.

AC Supply sells C6s for $7.07 for 3, or $2.36 per motor. The C11s are $4.79 for 2, or $2.40 per motor.

I’m currently building a 24mm booster for a scratch-built Farside-X. I’ll likely fly it on a C11-0 booster first flight, just to give it a nice kick at liftoff, but keep altitude reasonable.

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Lee Reep
NAR 55948

Projects: Semroc Saturn 1B, Ken Foss Designs Mini Satellite Interceptor
In the Paint Shop: Nothing! Too cold!
Launch-Ready: Farside-X, Maxi Honest John, Super Scamp