Recently picked up a 5” M&P9 and mounting a X300U-B. Using the “P” mounting adapter for picatinny the light doesn’t sit properly because the mounting adapter is too far forward to fit in the slots and the rear of the light is forced down because of contact with the trigger guard.

If I use the “U” adapter for universal then it barely fits. The adapter is loose in the actual mounting slots on the gun but still makes contact on the trigger guard. (By loose I mean the adapter doesn’t actually fit the entire slot).

I’m assuming this is a common issue because of how popular the gun and light are. The limited links I could find said to dremel the trigger guard. Personally I’d rather not do that currently.

Mounted with the “P” (picatinny) adapter. You can see how much the light it tilted up and doesn’t mount properly. Also notice the contact on the trigger guard.
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Mounted with the “U” (universal) adapter. Here the light sits fine while still contacting the trigger guard.
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Here is the “U” (universal) adapter by itself showing where it sits and how little of the slot it actually engages with when mounted.
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You use the universal insert in this case.  The fact that it only bears against the front of the rail slot is normal, as that's what keeps it from sliding forward and off the rail.

Use the Universal adapter. With a B model Surefire you really don't need a Picatinny adapter. The TLR-1HL will fit perfectly with a 1913 adapter.

This, it's fine with the u cross bar if that's how it fits best, literally have thousands of rounds on that same set up and similar.

SIde note if you get an A- model it will fit with the 1913 slot but you have to give it a good whack to seat it.

I have the A model.  I could get it to seat with some effort but it was canted up.  So instead I dremeled off a couple mill from the front top of the trigger guard and opened up the first 1913 slot just a hair. Tested it with a couple hundred rounds and it works well. I didn’t want to spend more money on another light.

use the "P" adapter and get a small punch to gently tap out the 4 very small stainless steel pins to remove the spacer that is in between the toggle switches and the light will fit perfectly...