All,
I live in the communist state of NY where AR pistols are limited to fixed magazine builds. We aren't even talking about bullet button builds....literally fixed magazine. For example, some will use a bullet button and then epoxy the button hole closed. Anyways, I am not here to discuss fixed mag options but rather to discuss barrel length options.  I already own a 300 blackout with a 8.5inch barrel which so far runs flawless  and is a blast to shoot but it is also quite costly to shoot (even with reloads) so im looking to buy an upper to use on my already assembled fixed mag lower.  Since this will not be for any kind of application involving living critters, I am not concerned about ballistics. I will be using it at the range ONLY, likely with fixed sights or maybe a red dot out to 50 yards.  I have heard that 10.5inch is the "sweet" zone for a 5.56 build. I think this is for ballistics, muzzle blast, reliability with keeping a relatively short barrel...etc.  Since I do not care about ballistics nor muzzle blast, I was contemplating going shorter barrel like to my 300 blackout 8.5inch. what are your thoughts? it seems that if I ran into reliability issues, most of the time going with a lighter weight buffer spring OR cutting a coil off a regular buffer spring will remedy this. That would mean if I wanted to change from 300black to 5.56 upper I simply switch uppers and buffer springs. No big deal. Am I missing anything else? Should I just go with a 10.5inch build? Thanks!

anyone have any thoughts? I am leaning towards a 10.5in since that seems to be the recommendation even though this will be pure plinking toy. I don't think I will get an adjustable gas block since ill be buying a pre-assembled PSA or something similar. I don't want to invest much since again it is just a range toy so it seems just for the reliability factor I guess the 10.5 is the way to go. Is it still fun to shoot or is a 10.5inch barrel starting to feel long on a pistol?

I have a 5.56 pistol with a 7.5 inch, 1/ 7 twist barrel. With my current handloads of 69 SMKs, I can keep all the rounds inside a sheet of typing paper at 100 yards, without a rest. If I put it on a rest, I can keep them within a 5 inch group at 100 yards. No keyholing.

I've loaded some other weights to test fire. Hoping I can find something that groups a little better.

You should be fine with a 10.5". Generally that's the shortest you can go without it being finicky.