8-Handed. UTG straddles, UTG2 calls, CO raises to $20. A reg at BU has 27BB. All other players have 100BB+. With what hands will he goes all-in here?

At NL2/3, I think the 3-bet range is: TT+, AQs+, AQo+. How close this is?

How about at NL2/5?

Well hes getting called by someone because of the straddle and open so your ranges are probably close enough but how did he get to 27bb?

If he lost a big hand in the same orbit he is a lot wider.

Is it 27bb or 27 straddles? How big is straddle?

In your example, you said the raise was to 20, what were the blinds, what was the straddle?

Sorry. The game is NL2/3. UTG straddles to $6 or 2bb. 27bb is BU's stack size, not 27 straddle.

If this player has had a frustrating session and is whittled down, to "double up or go home" territory then I think something like:
77+, ATo+, AXs, KTs+, KJo+, QJ is an approximate range. There's a lot of dead money in there, it's a good gamble if they get it heads-up.
Adjust the range tighter if this is more of a player who likes a "made hand" before they get it in

There are preflop charts on GTO Wizard for short stacks in tournaments that are probably pretty good examples of what to do. I wouldn't look to much at the spots eith limps though even if there are limpers because short stack strategy in tournaments with limps incorporates a lot of strong hands in the limp range. At 14bb (always consider the straddle to be the bb in these spots, actually bb is not very relevant in the presence of straddles), facing an EP limp and CO ISO, we are actually supposed jam very tight in theory, like TT+, AQs+, AKo, and mix jam and foldwith KQs, A5s, and 99.

And then the initial limper is supposed to call off very tight, calling off with QQ+, AKs, and mixing call/ fold with JJ.

In practice, the initial limper's range is not going to have traps (yeah every once in a while someone does, but for the most part they don't). Also, they are probably going to call off lighter, despite the fact that the action is opened up for CO to jam. Mid and maybe even low pocket pairs, AJ type hands, sometimes two broadway type hands will call off, sometimes fold depending on the player.

All that said, probably something like 55+, and suited AX, A9o+, KJo+, JTs, QTs+, KTs+ is probably fine. Only thing to consider is how tight is the iso raiser and how likely are they to call off. We jam QJs assuming that sometimes everyone is going to fold and sometimes we will be flipping.

That said, I have seen people at 1/2 call off $300-400 with QTo, J2s, J7s and I'm not sure why. Our threshold for value can get pretty low in those cases.

Hopefully your bankroll is healthy and you can experiment some with this and just reload if it doesn't work out. There's a 1/2 game where I live that has a 75% match the stack, and I used to sometimes play while I waited for a bigger game and go all in for a few hundred dollars when there was a bunch of dead money from cold callers. The fold equity wasn't as surprising as how often I would get called and be ahead with A4s, K5s. And then you get action when you have it too.

Obvously villain dependent, but given typical 2/3 players this is probably about right, though theoretically I'm sure he should probably be wider. He could of course be very wide if tilted and in a go-home mode.

In practice he will also call a bunch of hands he shouldn't call with. Not clear he should even have a calling range here.

I originally read this as how wide should the reg be jamming.

But how wide does he actually jam? That depends and varies villain to villain. Your range suggested is on the tighter estimate and mine is a bit wider. If I had to guess, your range is in the top 25 percentile tight, mine is in the top 25 percentile loose. Actually ranges are one average somewhere in between, but occasionally tighter, occasionally looser.

One thing I will note here is that regs don't play for 27bb very often. They usually play 100bb + and top up if they get too low.

At any rate, we need to make a judgment call. Is he loose, tight, on tilt?