I just finished loading 100 9mm Hornady 115gr XTPs.  I adjusted the seating die
to 1.125" per my manual and made/measured several test cases, no problems.
While seating the loaded rounds, I noticed some cases required quite a bit more
force to seat the bullet.  On those cases that required more force, the OAL was
noticeably longer than the rest.

I measured the loaded rounds and found that the OALs varied from 1.110" to 1.145".  
Brass is mixed headstamp once fired, no one headstamp is having issues.  I could
also tell that many cases seemed bulged where the bullets were seated as well and
others weren't.

I've loaded XTPs in .38 spl before without issues.  I'm using a Lee 50th press
and lee 4 set dies.  I just loaded 400 RMR 124gr RN this weekend without
a hiccup, so I have my doubts that the seating die is my issue, but who knows.
I almost exclusively load Hornady projectiles and have had nothing but fantastic
results until now.

Any ideas as to the inconsistencies in OAL?

It sounds like the bullets are having trouble starting squarely into the case mouth. Check your mouth flair with one of the XTPs you may need more with its flatter base. Today many cast bullets have a bevel base that make them easier to seat.

Next check your seating plug to see if it fits the XTP's tip good. If it don't it may be trying to start the bullet at an angle.

The fact you are getting bulged loads indicates a seating problem to me.

Big question is,, what powder and how much ??

Could very well be you is trying to compress the powder more than it'll actually compress...  If I recall, XTP has a bunch of bullet in the case before making desired OAL

Bulging may well be 1 bullet diameter + 2 case wall with the different manufacture cases you say you is using

I'd also like to know what powder? If it's a compression issue then it's a good thing you're not using hornady recommended oal of 1.075"

5.9gr unique.  Max load is 6.3gr.  Case is pretty full with charge, but all the cases are equally filled.  My issues are hit or miss.  Im sure the load is compressed to some degree, but no crunch is heard.

The cases are belled enough that i can easily start the bullets by hand.

Ill take a look at the seating die this evening.  No damage/deformation on my loaded rounds though.  I didnt have any issues when i made my load work ups either.

I had the same problem on my 550 with 115gr. HAP's(same profile as XTP I think).  Once fired mixed brass.  Dillon dies. I think it is a combination of the seating stem profile and the brass length.  Some of the shorter brass wasn't belling enough. Measured the brass and it was quite different in length. I reset the bell for the shortest brass in the mix and it seemed to solve the problem.

For 9mm I try to not run mixed headstamps. I load a lot of 115 XTP, usually over 4.5-4.6 of VV N320,  and my OAL is set at 1.145 (VV calls for a longer OAL IIRC).

Try segregating your headstamps and run in batches.

I'm relying on memory, which isn't reliable 100% of the time, but I believe Hornady's manual lists 1.070" as their OAL with 115 grain XTP's. I wouldn't go over 1.100" for reliable feeding.

Seeing a noticeable bulge in the case where the bullet is seated is good, it means you have plenty of neck tension.

Belling the case mouth slightly more may ease bullet seating issues. Check the seater stem to see if it is the rounded (fmj) style or if it is the flat nose variety. I think the round profile may give you a more consistent seating depths. The flat nose will mate against the highest point on each hollow point causing inconsistent seating depths.

I can't see any issues with the seating die.  It has the rounded profile for RN style projectiles.
I just can't wrap my head around why there's such a difference in OAL.  I know I was getting
a full stroke on all rounds, so I wouldn't think it would be powder compression causing the issue.

Either way, if I test all the rounds and they all chamber in my XDm, is there any foreseeable
dangers to firing this ammo?  I'm a lot less worried about the shorter rounds now since several
have said Hornady recommends 1.070" seating depth.

ETA: All 100 drop in my XDm barrel freely.

When you get ready to run the rounds through the die that bells the mouth, use a light shot of One Shot case lube, or equivalent,  on all case mouths.  Helps the die be more consistant as it opens up the case and bells the mouth.  Will not hurt the powder or primer.  This will help even out your OAL problems.

I sometimes put One Shot, or actually my own case lube mixture, on a q-tip and wipe each case on the inside, about 1/8" inside the mouth.  Yes, adds another step but the results are worth it IMO.

Then when you are seating the bullets, push the bullet into the case just enough to start the seating process.  Then lower the ram and turn the case 180 degrees and finish seating the bullet.  

You will find a good combination of how far to start and how many time to turn the case to prevent or at least minimize the bulge, you are getting.

If your sizing die is on the small side of tolerances, you may still get a noticeable bulge ring around the case at the base of the seated bullet.  Doesn't hurt a thing.

Also make sure there is no build up of crude under the shell holder.  This area should be spotless.

When you said bulged I was thinking on one side only.

You will not have a powder compression problem with Unique and the 115 XTP. The 147gr XTP is much longer and it has no problem.

I bet your OAL difference is due to bullet tip deformation. I use the 147s and can defiantly see a difference in the size of the hollow point due to it being squeezed (crushed if you will) by the seating plug. I have also noticed a big difference in seating pressure when loading mixed head stamp brass.

Do a little test. Kinetically pull 2 that are the farthest apart in OAL and measure the bullets themselves.

Should I have any reservations with the safety of these rounds?

Do you mean because they are .035" difference in OAL? Your load is .4gr under max. If one at "normal" length shows no sign of high pressure then the ones .035" shorter should be fine. This is my opinion.

Precisely what i meant.  I know oal plays a factor in pressure, just wasnt sure how big of a factor.

Thanks for the tip, just might give that a try.

Just to point out hornadys recommended oal is with their specific load data. Be mindful of this. You don't want to use powder charge data from one source with oal data from another source.

This doesn't relate directly to your specific question, but I thought I might mention it because you might not ever think of it.
I always do a visual check of the opening in the tip of hollow point bullets after the seating die and the crimp have been adjusted.  I have seen examples (in my own work as well as with the work of others) where the hollow point opening has been compressed/crimped smaller diameter because of an over tight case mouth crimp.
In fact, I use this metric as one guide to whether I have achieved a maximum crimp.  If the hollow point opening is getting crimped, then my seating die/press is putting too much force on the bullet.  So, always compare the hollow point opening of a seated bullet with an unseated, new from the box bullet after setting up your die.

I've seen some threads here where peoples hollow points were getting crushed/closed
like you're mentioning.  I'm seating and crimping in two separate steps.  None of my
tips were deformed/closed/etc that I could tell.  Only differences I could tell from one
to the other was different amounts of case tension and OAL.

I suggested this about 7 posts ago. I see it all the time with 9mm XTPs. I see no sense pulling a bullet out that is short because the tip is crushed.