How do you measure barrel lenght?  Is 18" the minium lenght allowed without a permit

Make SURE it's unloaded.
Close the action.
Put a cleaning rod or dowel rod down the bore until it contacts the breech face.
Mark the rod even with the muzzle.
Remove and measure the rod.

Measure again to be 100% CERTAIN.

The minimum FEDERAL length is 18 inches for the barrel, and the entire gun must be longer than 26 inches.  Some local laws may be different.

A smart person never cuts a barrel LESS then 18 1/2 inches.

I kinda messed up when I cut the barrel it is exacty on the line of 18".  Do you think I will be fine or should I just get a new barrel?? Thanks

Quoted: I kinda messed up when I cut the barrel it is exacty on the line of 18".  Do you think I will be fine or should I just get a new barrel?? Thanks

I guess it really depends on who is measuring it. From what I've heard, the ATF uses a dowel like described above. I guess there's a possibility that their dowel could be off by 1/16th or something and that could  get you into trouble, but I doubt it. For one, you'd have to give someone a reason to believe you're not in compliance. Even 1/16th less than 18 inches looks an awful lot like 18 inches, so I don't think you'd have a problem.

But remember, it'll be you that has to answer if for some reason it's under legal length and someone were to find out. Cutting barrels goes back to the old adage of "measure twice and cut once".

I'd get rid of it and buy a used 18" barrel unless you can permenantly attach a short muzzle brake or something to it for less money.  Better safe than sorry in this case IMO.  
If I lived somewhere SBS friendly, I'd pay the tax and keep on cutting as soon as I got the ok

If you're using a good measuring device and it's RIGHT on the line, you're OK.
Remington for many years sold shotguns with barrels EXACTLY 18" long.
If someones going to start using a micrometer to measure your barrel, you're probably already in trouble for something else.

In other words, AS LONG as you really are on the line, don't worry about it, lawmen have more important things to worry about than a shotgun that "might" be off by 1/32" or so, and no ones going to be measuring it anyway.

Quoted: If you're using a good measuring device and it's RIGHT on the line, you're OK. Remington for many years sold shotguns with barrels EXACTLY 18" long. If someones going to start using a micrometer to measure your barrel, you're probably already in trouble for something else. In other words, AS LONG as you really are on the line, don't worry about it, lawmen have more important things to worry about than a shotgun that "might" be off by 1/32" or so, and no ones going to be measuring it anyway.

Tell that to Randy Weaver!

Bob

Randy Weaver wasn't busted for the short barrel, he was busted for short WOOD.
The guns barrels were 18" long.  It was the overall length that was 3/8" shorter than the legal 26".

His lawyer Gerry Spense, basically won his case when he held up the wood butt stock and told the jury that "Three people died over a tax dispute about a few cents of cheap wood and a bad ruler".

Spense wrote about this in his book on the case.

In any case, Weaver really wasn't busted over the guns, the guns were the EXCUSE.  The ATF wanted him to infiltrate the Neo-Nazi's in the area and the gun bust was "Leverage" to force him to do it.