I recently changed the barrel on one of my ARs and after I checked zero I was a little over 4mil high from my old barrel. Seems like a large difference between barrels. Anyone else experience this? That's almost like adding a 20moa mount.

How do the profiles compare?

The barrel I removed was a 20inch 32oz barrel and replaced with an 18inch 40oz barrel(SPR profile). A decent amount heavier.

4 mils is pretty extreme but not unheard of. Two different top tier barrels can easily have a 1 mil difference in POI just because no barrel is every perfectly straight, and then depending on where the barrels end up indexed also effects it. Then add any runout in the threads from machining and you have stacked tolerances too.

For production grade barrels 4mils would still be pretty extreme assuming the same bullet at a similar velocity, but it wouldn't surprise me. The production stuff is made on a CNC so it's not going to have to curve of the bore indexed like some gunsmiths will when hand cutting a barrel, and either way the precision of the equipment will effect the tolerances of the machining and then you're screwing it into a barrel extension that's also machined to a certain tolerance so more tolerance stacking.

Did you check your receiver bore diameter? Did you check both barrel extension diameters? Depending on how they fit together can change how the extension sits in the bore of the receiver and any runout or mis alignment of the receiver face can also be amplified. This is why it's best to lap the receiver face so that it's concentric to the bore and shim the receiver extension if needed to get clearance as close as possible as well as bedding the barrel to eliminate the clearance beyond what shimming can accomplish.

The barrel weight has less impact than you'd think, especially if the weight is from a heavier contour. Even hanging a heavy suppressor on the end of a barrel doesn't really cause much or any droop, it's not uncommon for a POI shift with a can on to be high, left or right when it should go down if it's dropping the barrel.

I did not measure the extension and bore diameter. The barrel did slide in but and didn't have much wiggle to it at all. I also have not lapped/squared the face of the receiver.

The windage stayed almost exactly the same, less than 1/2mil off. The barrel shoots fantastic as well.

As long as the new barrel is consistent, accurate, and meets expectations, I wouldn't worry about the shift between barrels. Lapping the receiver face could more than account for it.

That's kinda what I'm thinking.

20moa is about 5.5mil and you can easily see that in a 20moa rail for a bolt gun back to front. So I'd think if the receiver was out of square that much my barrel and rail would be angled up from the receiver.

Did you square the receiver face? Are you sure all parts were clean when the barrel was installed?

Usually all misalignment issues are the result if the barrel extension/receiver face not being true to the centerline of the upper receiver.

I mentioned dirt because misalignment usually stays the same when installing any barrel on the same receiver if it isn't trued. If some crud is trapped between the barrel extension and receiver face it's not sitting flush.

Also a gas tube binding in the upper receiver can cause issues. Double check to insure it's centered in the barrel nut passage and is floating (not being tensioned) through the hole in the upper receiver.

If you have a muzzle break remove it and check for bullet impact through the exit port. Clean or ream it with the next larger drill bit size to gain the clearance you need.