As a couple people mentioned, Aerospace Specialty Products has great
MMX kits. They're all minimum diameter unless I've missed one. Walt linked above to ASP's T-MM tubes, which all their kits are based around, and they sell some nosecones for scratchbuilding.
The
Fliskits MMX I've seen are not quite minimum diameter but they're very close. A motor tube goes inside the body tube with very thin centering rings.There's a
build thread here with photos that show what I mean.
The MMX rockets I've seen from New Way Space Modeling (the square rocket people) are even closer to minimum diameter, albeit square: A motor tube sleeves in to insulate the 3D printed (PLA) body tubes, but that's it. There's
a picture here. They're sold by
eRockets.
The SemRoc MMX I've seen are bigger, though smaller than BT-5. I don't know if they have minimum diameter MMX but it wouldn't surprise me. They're available from
eRockets.
My
R5 Hawkeye is minimum diameter. I don't currently carry tubes, but do also have a reasonable range of
T-MM nosecones for scratchbuilding.
Two things to keep in mind:
- The minimum diameter rockets do really accelerate away from the pad, but they don't necessarily go higher than other kits. MMX rockets are artificially limited by the momentary delay burn in the motors. Doesn't really show up w/ Quest's plastic rockets, but you can see it w/ a lot of others: The rocket's still coasting upward when the ejection charge blows.
- Having that altitude limitation is... probably ok? Successfully retrieving a minimum diameter MMX rocket is frequently an iffy proposition. They're. So. Damn. Small. And they go so high, so fast. Extremely hard to track in flight. Then when they land they're easily hidden by anything but closely cut grass. If the engines let them hit their true possible altitude they'd be real close to one-shot throwaways in all but the most favorable launch sites.