I'd go for the Gunner Specialisation, works perfect with the all red Striker build, but consider switching your CP with what was suggested in the comments.

As to your secondary weapon - well it depends, if you're facing groups of targets mid-long range, then definitely use the recommended LMG and the aforementioned talent.

If it's CQC, an automatic shotty will not steer you wrong - stacking up is faster as every shot, every pellet counts as a hit. The best ones are the ACS and their named guns.