I had my local gunshot install a Daniel defense modular tube on my upper. I’m noticing the rail is slightly off with the rails on the upper. How big of an issue would this be if I ran iron sights? I hate to seem petty and take it back but it’s annoying me.
Depends what slightly means. You paid the man to do the job. He needs to make sure it’s done right. If it’s bothering you now, it’ll only get worse.
Depends. I don’t know how many people use a level while installing a rail. If it is canted too much you will may hit the threshold of how much windige right or left you can adjust your sights to. I don’t think it matters if the rail is canted a little.
You’re going to need to post a picture, we can’t know how far off it is.
It can take a bit of extra effort to make sure the rails flush up with some handguards. Usually fixing it is nothing more than a matter of loosening up whatever fasteners hold it in place, giving it a bump in the right direction, and tightening. Sometimes tolerances stack in the wrong directions and it will never flush up perfectly. Unless it’s interfering with how you’re mounting your optic (it shouldn’t), or unless it’s really ****ed up, it’s not going to have any effect on zeroing.
Or maybe he completely screwed it up and it’s not even installed correctly. You’re going to have to post a picture to know what we’re supposed to be looking at.
double post
You paid a professional to do a job you could have done yourself and he messed it up. Make him fix it.
Unacceptable in my book, but if it is canted to the right (looking down the sights) you may be able to use a good strap wrench to line it up. Going the other way (loosening barrel nut) would make me a little nervous. Good luck.
Unacceptable. And clearly this idiot is not used to doing this type of work. Simply using a carry handle to hold the rails in alignment will prevent or minimize this.
That was totally my thought when I read the OP. I’ve always used an optic mount to bridge the upper & handguard. It’s self aligning. I figured that was common knowledge.
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Yeah… there’s people out there working on this stuff who can mechanically get the parts installed. But doing the job right is a little more than that. I’ve not done one of these rails specifically, so I don’t know if the nut requires timing, etc. But either way, it’s not right.
Admittedly, I can be a bit anal at times but I’ve always felt that the only thing that should ever be cocked is the hammer. 
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I’m envisioning it low or high, relative to the receiver top rail. Not canted. But only the OP can help us understand the problem.
I’ve installed handguards that simply will not flush up, no matter how much effort you give it. It’s been my observation that it’s generally a rail dimension mismatch: one or the other machines on the large side (usually the receiver), while the other comes out at the smaller end of the spec, or is possibly even out of spec.
Most rails don’t even carry through to the receiver, and many of those that do don’t actually offer any useable slots. We generally don’t span the gap with an optic mount, so it’s really nothing more than aesthetics at that point.
Ahhh… that’s possible. I re-read the OP and he could mean exactly that. In that case, there’d really be no fix.
If this is the case, a front post elevation change should suffice…
Oh yeah, I see that now. I was only going off of what markm said. Dammit, pay closer attention next time, will ya. 
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Don’t be too hard on yourself… I really couldn’t ascertain that info from the OP. As we all know, it’s usually a canting issue that falls in-line with his description.
Well, it’s been day, and the 1 post OP has yet to respond. Wonder if we’ll ever find out what they meant by canted and/or if it’s been otherwise resolved.
Assuming this is a Daniel Defense MFR, the barrel nut is not timed. The hand guard is secured via 4 set screws that press on a ridge in the barrel nut. Loosen the 4 set screws, align the rail however you please, and re-tighten. That’s it.
If there’s a step down or up between the upper receiver and hand guard, then one or the other is out of spec.
This. Another reason why I like to do the work myself.
Curious as to how much you paid as well. These things are really easy to do on your own. I’m sure you can get enough guidance by reading here or watching YouTube.