There may be some confusion about the ammunition. Your rifle can fire 22 Short, Long, or Long Rifle (LR) in your Speedmaster. The "magnum" label is usually associated with a different cartridge - the 22 WMR (Winchester Magnum Rimfire) which will not fit into your rifle.

CCI manufactures a 22LR round that is named the "Mini Mag" which is safe to fire in your rifle. The Mini Mag is just a trade name and the round is a LR that is nearly identical to the Winchester brand Super X. Both are high velocity 22 LR rounds that are very good for your purpose. Most fellows like high velocity ammunition for their squirrel hunting as it hits a bit harder than standard velocity ammunition, others like the standard velocity ammunition as it does not break the sound barrier and give that sharp crack sound when fired. The high velocity ammunition is usually favored by shooters of semi-automatics like yours.

22 rifles are funny things. Some will shoot well with one type of ammo while an identical rifle will not. Try different brands of LR if you can and see which one shoots the best in your Speedmaster. There is currently a shortage of 22 ammunition due to people hoarding it, but if you look around you can probably find a couple of boxes of different brands to try.

Find a likely spot and sit quietly. I don't think it matters much to the squirrels what clothes you wear, but they have good hearing and can spot movement very well.

I figure on about one squirrel per person - with biscuits, mashed potatoes, and squirrel gravy for a fine meal.