want to cover:
On a more serious note: 5 gr. of powder in a '06 case is almost nothing.
Do you use some stuffing, like a bit of cotton fluff or paper on top of
the powder to hold it against the primer for consistent ignition? Or
does it just not matter in your application?
And, do you find that if
you reduce powder charges too much, you get incomplete ignition
resulting in a squib? Honestly, I have never seen anyone going down in
velocity to such a degree, but you seem to have a reason for it, and it
does get you out to the range. And that is reason enough.
Covering this in some detail,
You're right on 'almost nothing'. The standard M2 Ball load in .30-06 is a 150-grain jacketed bullet over (originally as I recall) 48.0 grains of IMR4895 powder for a velocity of ~2800 feet per second. Compared to that 5.0 grains is tiny. And this uses a fast-burning pistol powder which takes up less space than 5.0 of 4895 would*. I've read a lot over time of people using something as a filler with small powder charges to keep it in place near the base of the case for consistent ignition(from "You must use'" to "Don't be stupid, don't do it!"); not just on tiny charges like this, but reduced charges like you'd use in a Garand using cast bullets. Some cartridges/propellants it's not argued, you need to do so; some propellants will not ignite and burn consistently if you don't. Some it's optional unless you're going for maximum accuracy, it can make a difference. And for some, doesn't matter(one reason for The Load for many cartridges being 16.0 2400 under a cast bullet 150 to 180 grains; 2400 isn't position sensitive so no filler needed). Bullseye(or Unique or several others) seem to ignite and burn quite well without being so confined, and serve the purpose well loose in the case. Also, that's one reason a lot of people recommend drilling the flash hole out**, to make sure ignition will be as good as consistent and even as possible.
One of the warnings in some of the Cast Boolit threads on the subject was 'Yeah, if you want to try taking it as far down as possible, be sure to take a range rod with you 'cause sure as hell you will get one stuck in the barrel at some point.' In the spirit of "Just how slow can I take this?" I might try that sometime, but right now I was looking at, in no particular order,
at least decent accuracy,
not having to beat something out of the bore.
Partly out of "This sounds neat, I'm gonna try it!", partly "How would this work for cheap practice/pest control out of a centerfire rifle?" And I've got answers; with a bullet your rifle likes, you can get pretty good accuracy out to 50 yards with low-to-nonexistent recoil and much-reduced noise.
Also, so far I've seen no evidence of unburned powder granules, and the bores have stayed nice & clean, no sign of lead fouling.
*Don't EVEN think of using that small an amount of such propellant. Ever. Just don't.
**Remember, you should mark and keep the cases for these loads separate from what you use for full-power loads. ESPECIALLY if you drill out the flash hole, NEVER use those for anything but the very light loads.