The MJG Firewire initiators will fit the nozzle of D12s and larger BP motors. I use them in a model that uses 3 D12s. You should hook one up to your launch system to ensure they continuity check safely. I fly off of the DARS 12v system and have never had an issue with them.
Haven't had a chance to fly this year, but hope to fly the model with three boosters in separate tumbling stages (Moh, Lari, and Coily - The Three Stages).

I bought a box of 80 MJG Firewires from Chris Short at Chris' Rocket Supplies.

Chas

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Charles Russell, MSgt,USAF (ret.)
NAR 9790, Lvl 1
SAM "Balls Three"