Looking at a 4.5" for a Mark II.

I don't know but every Ruger .22 auto I ever had shot better then most rifles. I have had 6 and still have 2 left. I won Ohio state .22 silhouette with one and had no sight settings at the start with the new gun. My spotter seen the misses on the first pig, turkey and ram. I got all the rest plus the shoot off chickens at 100 yd's. 57 out of 60 plus 10 shoot off targets.
I shoot squirrels all the time with mine.
The ammo is going to be the biggest problem. I find CCI Blue Tag hollow points to be the best now. (Mini Mags.)
Ammo has gone downhill for many years.
I would not spend money on a barrel when the gun is as good as it can get as is.
Even a Browning Buck Mark will suck with some ammo.

Thanks, which 2 did you keep?

I have 3 5.5's and a Slabside, but wanted something a tad lighter/shorter/handier, looking at a Micro Buck Mark SS, OR perhaps chop the bbl and possibly flute on one of the 5.5's.

Any sight or trigger mods? I did recently add Volquartsen sights on 2 5.5's.

A buddy of mine has one that he uses a suppressor on and it shoots very well.

I have the 9-1/2" blue and the SS 6" slabside target.
Browning makes a great gun too.
Long ago I had the Ruger standard with fixed sights. It was light and super accurate. My brother in law talked me out of it and he still has it.
There was a clone to the Ruger out for a while but I don't remember the name. It was BAD as far as accuracy. My friend has a Magnum research .22 auto and I don't like it. It is light but doesn't hold a candle to a Ruger.
I had one of the original Bearcat revolvers too and it sucked.
.22's have come and gone all of my shooting life but the Ruger auto will always be one I will keep.
I have had or shot S&W 41's, High standards, Brownings, K .22's, single sixes, Bearcats and about every other ever made.
My Rugers stay here.