If lets say a Canadian such as myself just wanted to go shoot in the US with either a LE colt M4 or a brand new SIG PE90 (SIG 550), would it be possible to bring it over just for shooting? do the laws apply? What if I move to the US, would it no longer be legal for me to own any LE AR's or weapons banned under the current import laws?

Back in '90-'92 when I shot IPSC at Tonawanda Sporstmans Club with the "Western New York Practical Pistol League (north of Buffalo + east of Niagra Falls), Canadian fellas would bring their AR's, AK's, AUG's and other misc stuff to shoot the side matches. Just be sure to have a copy of the match/program when entering the USA. Usually Customs did not hassle them, but that was 10 yr ago. Things change.