Meh.
I have a 26.5mm "flare rifle." Bought it for $200 a couple years ago.
Contriving 26.5mm rounds that do other things can't be THAT hard. I'm not interested in experimenting right now and have no supplies or intentions to do so currently, but the inventiveness of the human mind is awe-inspiring.
Grenade launchers, primitive javelin-style anti-APC rockets, IED's... I'd rather have the ability to purchase quality units manufactured by experts, but the alternatives aren't that difficult.
Yes, and several of us got them, some with an eye towards getting the DD stamp for ours so we can play.
However 26.5mm is generally too small to make individually registerable munitions that are actually somewhat combat effective. It took ATK several years and a few million bucks to do it in 25mm for the prior OICW project and the current XM 25 system. And even then there are still detractors who question the actual lethality or effectiveness of the 25mm explosive payload.
You can certainly go over the 1/4 oz NFA limit on explosive filler and run up against the letter of the law, but unless it's RDX or something, it won't do much but make noise. Low order explosives, BP etc. just don't have the kick.
30, 40mm it gets a bit easier since the payloads are substantially larger and go up quickly with the diameter, but to really do it right with high brisance explosives on a semi-production basis is still going to be expensive for you
somewhere, if your cost basis isn't that high on the actual rounds, it still is on the logistics and production end so it's do-able without getting yourself killed.