As someone who has served over 21 years in uniform and now a reservist, having reached the rank of Chief Warrant Officer, I would like to say that 210 rounds goes very fast in a sustained firefight, and if that firefight is away from base, all the ammo you have is what you can carry, plus your water, food, etc etc etc..
A couple of years back in Iraq, a British patrol was running low on ammo (and instead of running away) they fixed bayonets and charged the Iraqi insurgent positions and won the day. Some of those soldiers said that as they "stuck it" to the enemy, they could see the look of disbelief and fear, and the insurgents that did not get "stuck" ran away. They also wished their bayonets were longer because with bullpup rifles, they got closer than they wanted to and pulling the bayonet out to 'stick it' to another was cumbersome.
Perhaps that Sergent had a point - excuse the pun ...
From first hand experience, I would recommend Leatherman Wave and a good sturdy fixed blade, max 6" blade, such as Buck Strider Spear Point if you can still get them, if not as suggested above, Ranger, TOPs, Glock, etc. And yes weight is an issue.
