I have XS Big Dot on my Shield.

I called XS and spoke with customer service.

XS informed me they are required to have more steal than currently on rear sight to encase the tritium vial and if they built up the rear sight they would also need to build up the front sight and per XS you would run into holster fit issues.

I went XS Big Dot for my Shield for the low profile rear sight because I pocket pack and with the height of the Shield the stock 3 dot would catch on my pocket.

I have no problem in low light with my XS Big Dot. The only time a tritium rear sight would be useful would be in complete darkness and in that scenario you are going to be belly to belly with the bad guy making sights useless.

Russ