Just got my first .300 Blackout done, 16-inch pistol-length gas, but I am not in a position to roll my own yet.  What I want to know is will a 125-gr OTM open up reliably out to 300 yards?  Specifically I am looking at using the new Sig .300 Blackout ammo for white tail and mule deer.  Any experience with this ammo is greatly appreciated!!!

http://www.brownells.com/ammunition/rifle-ammo/elite-performance-match-grade-ammo-prod79552.aspx?avs%7CCartridge_1=CCP_300%20AAC%20Blackout

open tip match ammo is not a god hunting bullet . about the only thing thats going to still expand at 300 yds is the barnes 110 grain .
zeroed at 100 yds out of a 16 inch barrel my come ups ar 3 moa at 200 yds 5 moa at 250 yds and 7 moa at 300 yds .

OTM is best at being match ammo. Can you hunt with it? Yes but there are better bullets out there that will give you better expansion at longer ranges.

One of the best is as mentioned above is the Barnes 110gr Black tip that is specifically made to fit into the magazine and load easier into the rifle. It also provides great penetration and expansion. Barnes has tested it to penetrate 20 inches of gel at 300 yards and still get a full half inch expansion.

If you like the 125 grain bullets there are some nice ones made for hunting that will give you reliable expansion out to over 220 yards.

I run a small time NV and thermal hog hunting operation and utilize the 125grn Nosler ballistic hollow points with great success on hogs. With 300 supers you just need to figure out your drop at 50 yard increments out to what ever range you deem and go from there. My rule is if it'll put a hog down it'll easily put a deer down. My farthest shot has only been 185 yards though.
300blk is a great round inside of certain parameters, and I think you are within them so you should enjoy success.

Look at the PNW and the gorilla ammo 125 noslers, both do their job well.

The Barnes factory Vor-Tx loaded ammo with the 120gr blacktips is a fantastic round along with the 110 gr version. They would be my go to selection for ANY deer hunting using supersonic ammo.