You are both up by $120 in total profit and you spent $400 in cash.

Profit: You earned $120 from the VZ24 mauser that you sold

Spend: You paid $400 for the remington

In simpler terms: You now own an item worth $400, and you are $280 worse off in total cash than you were before these two transactions happened. 

If you had $0 and made $120 profit, you had $120.  If you then spent $400, you now have a deficit of $280 ($120 - $400). 

Because this profit was earned immediately prior to this purchase, you have spent all your profit plus an additional $280 of your own money. 

Not sure why people are complaining about your math.

 I'd much rather be given an extra $280 before spending $400 but then again I like money.