If its the type where the handguard attaches to the barrel nut with probably 6 screws ? then they should have used shims with the barrel nut to index it correctly , sometimes that can be a pain, but not impossible. if the handguard simply clamps onto the barrel nut then its easy to fix. Pics will tell us.
Barrel nut needs to be either tightened a hair or loosened a hair and then the hand guard re-attached. It's the most pain in the rear process of a building an upper or changing a barrel. Sometimes "good n tight" has to be good enough because tightening with a torque wrench to "spec" mis-times the threads and the hand guard comes out crooked.
If it's too loose backing the nut off a little, you will want to add a spacer washer or two to the barrel nut to correct the thread timing.
Exactly , the shop should have indexed it correctly, they should have had a shim kit if that is something they do more than once, even with a shim kit it sometimes can be frustrating, the last railed forend I bought simply clamped around the barrel nut, making the line up much easier, that's likely the type I'll use going forward.
You can remove those six screws, slide the fore end forward, and either tighten or loosen ( assuming it isn't already loose ? ) the barrel nut slightly to get the rail to line up . mil spec torque specs is 80ft lbs , but in all honesty it doesn't have to be that tight, initial torque to about 35 ft lbs, and then tighten to get things to line up, mine usually end up around 50 or 60 ft lbs.
Wonderful question as to why they didn’t do it. Partially my fault for noticing until i went to shoot with my BUS. The gunsmith not wants me to pay again for his error. I have ALL tools. I’ve just never messed with never messed with a barrel nut
Find out what pitch the thread is and calculate shim thickness [eg at 40 threads per inch 1/4 turn is .00625" etc.] So for 1/16 or 1/32 turn you would need a .0015" or .0008" shim.
Kitchen aluminum foil is around .0008" or .0012" for the Heavy Duty grade and makes great shop shim stock.
Or as stated just forget the exact torque and align, fix with threadlocker.
If you have BUIS this misalignment will cock the top of front sight post way to one side, you can run out of windage on rear sight easily. Ask me how I know . . .
is the gas tube even lined up properly? If you paid with a card protest the charges. I would also order the shim kit from the hand guard maker to make the install easier if you do end up doing it yourself.