I know Ill be ok on thickness, but what about gas hole location? Can the 20" HBAR be made to proper M4 specs? I should probably just trade it, but its a like new colt that I got a good deal on.

The reason you got a good deal is because you can't do what I think you want to do....

The gas port on 20" rifles and 16" or less carbines in in a different spot.  You are better off turning the 20" habar down to A2 "government" profile to save weight, then buy or build a carbine upper.

Of course they are in different spots.  Tell me this where is the gashole located (in inches) in a std 20" barrel compared to a short barrel. And for less than $150 for an exc cond colt 1x7 barrel I think I got a good deal whether I turn it down or trade it off.

You'd be better to sell/trade the 20" barrel if you want a 16" M4-profiled barrel. I figure by the time you pay to have the barrel cut down, have it turned down to M4 specs, move the front sight/gas block back, and refinish it all you will probably have more in it than it's worth.

Think you guys are right, probably better just to trade it. Thanks.

The center of the gas port is measured from the front edge of the shoulder for the FSB on the barrel. That distance is the same for either length barrel, 0.295"+/-0.005"

The front edge of the shoulder is measured from the front face of the barrel extension.

The dimension for the shoulder on rifle length sight radius barrels is 12.125" +/- 0.005" and is 6.750" +/- 0.005" for the carbine length sight radius barrels. I bought several Colt M16A2 barrels (w/lugs, A2 contour, and 1X7") for $100 per but this was 4-5 years ago.