We all know(?) that H110/W296, AA1680, Lil Gun, etc are the "best" powders for the Blackout, but unfortunately most are difficult, if not impossible, to find. In a way, this is good because it forced me to find alternatives.
IMR4227 is "out there" if you look hard enough. I've found that it works great, especially with the 150gr FMJBT. Using "converted" brass, fully functional and accurate loads seem to be easy to obtain. For MY rifles, 17.5 gr, CCI 450 primers, shoot nice round groups at 50 yards (I have a windy range so I prefer 50 yards to minimize drift). This group is fairly typical seven shot group, velocity is AVG 1920fps, SD 13 from a 16" carbine gas system barrel. As always, start low and work up - I'd suggest 17.0 gr as a starting load, but I'm not a "professional", so use with caution!
When I started loading for the 300BLK I couldn't find any Lilgun, A1680 or any of the other top performance powders, so I picked up some IMR4227. It is a good all-around practice powder with regard to pressure and feeding. However, in my experience it is VERY DIRTY! The difference between the residue on fired cases between it and Lilgun is dramatic. I will not use 4227 again because I don't like running dirty powders in a gas gun.
I'm getting 2100fps with 155gr Hornady OTM's and CCI 450's in a 16in BHW 1:8.5 with no pressure signs, 100% load density and perfect reliability. The cases (converted 5.56) come out amazingly clean for a gas gun.
I got lucky. Check the suppliers every morning and caught one with it in stock. Two hours after I bought a jug they were out of stock again.
Lilgun seems to be the optimum powder for the Blackout up to 155gr bullets. My load was with once-fired LC brass. I tried it with mil surp FC and got ejector swipes. FC brass appears to be significantly softer than LC. Higher percentage of copper which is very apparent when annealing. This performance is obviously "pushing the envelope", which is why I did not post a charge weight. It is too hot for a practice load, but is my S.H.T.F. load with new brass.
I strongly recommend, as usual, that everyone start with manufacturers min charge weight and do their own ladder.
The major advantage of Lilgun is that you use a lot less of it to get the same performance as slower powders. Less powder means less heat and residue in a gas gun.
Lil' Gun has been as difficult to find as H110, AA1680, and a host of others. Supplies seem to be catching up with demand, but I always like to have a "Plan B" (or "C", or "D"). It's interesting that you found 4227 to be "dirty"; I've found it to burn "clean" to "average". I guess "YMMV" applies.