Yes! If you want to trade one caliber for another just click the "SELL" button on that caliber in your INVENTORY. There is a 5% transaction fee on the sell side only.
Once your ammo is sold at the current market price that credit will go on your account and can be used to buy more of a caliber you are subscribed to.
To BUY simply click on the BUY button on your Inventory page. (Special Note: if a particular caliber is on backorder then the BUY button will be greyed out.) By default any credits are used for your regular subscription payment first - so if you have a subscription go through we'll pull your credits then charge your bank card the remainder. If you don't want to use your credits for this you'll have use them before your regular subscription payment runs.