Im not sure if there’s a proper term for this, but is there an ideal distance between the end of the muzzle and the first blast baffle? Specifically for a .22LR. I built a quietbore kit that seems like it’s got a pretty nasty first round pop, but the threaded adapter I’ve got on my pistol seems like it’s longer than it needs to be.

Maybe a better question is, is there a “bible” for form 1 can design?

The short answer is "it depends"

For .22 LR, you want it very close,  1/8" or so.

The general consensus seems to be minimal blast chamber for subsonic applications.

That said, when I was ignorant, I built a form 1 .22 can with at least 1" between the muzzle and first baffle, and there is no discernible FRP, rifle or pistol, it sounds identical to a Spectre II. I do have a pending form 1 for another .22 can and I'll be building that one with the muzzle very close to the first baffle.

My buddy built a .30 cal can with the intention of using it on 6.5cm and subsonic .300BO, so it has a decent blast chamber, it has a very pronounced FRP on .300BO but sounds incredible on subsequent rounds.

This is it.
22lr needs NO blast spacing and so any you design is being wasted, where another baffle or 2 could be utilized.

The only reason to make it a bit larger (than 1/8") is to account for mount and barrel threading. If your mount's threads are .440 long, but your muzzle threads are 1/2", you'll want to make sure the blast baffle doesn't come too close.

The adapter Volquartsen sent with their carbon tension barrel, for my 10/22, had long threads. I couldn't use it on my AAC Pilot w/o a spacer because the threads would touch something, inside the silencer, and throw accuracy out the window. I cut those threads back a bit and the issue went away. Just make sure you're not creating a similar problem.

Well it sounds like that explains my FRP. The first baffle is .700 and my adapter is .450. Guess I need a longer rather than shorter adapter.

Thanks all.

Do you think that a distance of 1.75” is sufficient for 556 muzzle to blast baffle distance?

Depends on the overall design and tube sizing.
But, in general, no. You'll want something closer to 2.5in

Is there a "too close" number? I'm shooting a .22LR setup that has about 0.013 clearance between muzzle and first baffle.

That sounds very close. I'm wondering if the first baffle is able to do much being that close. I'm sure others with more experience can chime in though.