Welcome to the site Glowin. I own the same model Colt and mounted the Walther PS55 sight on mine. Works well. The only other addition to mine is a vertical foregrip so, I really couldn't tell you anything about Magpul parts fitment. Is the little Colt popular in the UK?
That's one helluva deal! Wholesale order price from Ellett Brothers Wholesale is $276.00. They must have purchased a huge quantity in order to offer that price. :thumb: I say "YES", the friendliest word in the world (unless you're asking the old woman if she's been running around on you, then not so much). ;D
Muzzle brakes on rimfire rifles, I think, are purely cosmetic. So the rifle looks the tactical part more completely. On rimfire handguns though it's a different story. In rapid fire competition they can make a measurable improvement.
I had a Russian full auto bb pistol. It was very cool. Used batteries to facilitate the action and the bbs were propelled by CO2. Only problem, CO2 is very cold and on full auto the pistol would freeze up and grind to a stop. Then you had to wait for it to thaw and start the cycle again. Maybe with the HPP this one will be ok, but I doubt it will work worth a damn on CO2.