I've used the MyWeigh UltraShip 35 scale for years now to weigh gun parts, complete rifles, shipping packages, and whatever else needs weighing. Always works great and comes with holders to secure round objects on the scale such as barrels. 35 lb capacity, very accurate and always repeatable. Measures in various weight units. Battery or AC adapter cord operation. Tare function for zeroing scale with other objects placed on it such as holding or securing devices. Also has a detachable face so you can still see the weight clearly if you need to position the scale in a way that would make it difficult to see the reading. 30 year manufacturers warranty. Anything goes wrong, just send it to them and they'll get you a new one. Cost of the scale is only about $30.
*AC ADAPTER IS NOT INCLUDED (for some crazy reason). DON'T FORGET TO PURCHASE THE AC ADAPTER FOR AN ADDITIONAL $10 IF YOU WANT TO USE A RECEPTACLE TO POWER THE SCALE.*
They also make an UltraShip 55 with 55lb capacity, an UltraShip U2 with a 60lb capacity, and an UltraShip 75 with a 75lb capacity if you need to weigh heavier parts or packages.
Here's the link to check out the MyWeigh UltraShip 35 scale I use.
http://www.oldwillknottscales.com/my-weigh-ultraship35.html