Keep in mind that the Handi is around four inches shorter than a bolt-action or semi-auto rifle with the same length barrel, so a 22" Handi is already about as short as most 18" rifles.
If you do chop it to 18.5, you may want to handload with a faster powder. I use IMR 3031 in my 18.5-inch Model Seven in 6mm Remington. It's a much faster powder than the IMR 4350 that's typically recommended for the caliber. I get 2825 fps and sub-MOA accuracy with a 100-grain Grand Slam.
Right. I have a Remington 788 in .308 with an 18 1/2" barrel I bought new in 1981. For years my favorite whitetail load in this rifle was the 165 gr Speer BTSP over 50.5 gr 760. Incredibly accurate, and has taken many deer over the years, great bullet. I finally bought a chronograph, and found I was losing about 200 fps from the book velocities with this slow powder. So I experimented with a bunch of medium burning powders-- 748, H335 (lots of flash, recoil and good velocity, but pressure signs), IMR4064, AA2460, AA2520, and a few others. I also settled on 3031, which gives me the same accuracy, and only gives up 100 fps from the Speer book velocites, with no pressure signs. Loads avg near 2600 with the 165, which is not bad at all from the .308 and a short barrel. And the Speer 165 Boattail at that velocity hits like a hammer, trust me.
A friend at the hunting club has an old Rem 742 in .30-06 with the 18 1/2" barrel, and that is one LOUD rifle!
Rick