Something I didn't mention when harping on buying the best calipers you can find. I've bought a Sheridan slotted gauge in every caliber I load for (that they're offered in at least):
SLOTTED or NON-SLOTTED AMMUNITION GAUGES.
sheridanengineering.com
I don't necessarily use it for the measurements that it'll do because I have other tools, but when you're starting out it's a great tool for visualization of what you're doing since you can see everything through the slot. Stand the gauge up vertically and use a straight edge and you get to see the cartridge settle on the different headspace measurements inside the chamber.
Case gauges can do different things, so you have to check each different brand to make sure it does what you want. An example is LE Wilson gauges are sized large in the body so they can be used to measure case trim length and headspacing on fired brass. Hornady is reamed to the SAAMI body size to check that a round will chamber in factory spec rifles. Very similar looking tools that do two very different things.
Here's some decent measuring stuff to start with. Higher prices really does tend to mean better quality on measuring tools, but compare part numbers across multiple vendors. You can buy top quality measuring tools from machinist supply stores. This list is the first stuff I pulled up on Midway and are middle-tier. Perfect for someone starting out that wants to get good measurements but not go insane chasing down the final 0.0005".
Calipers:
Direct read 6" caliper offer sharp, clear graduations of .001" and .100" per one revolution. Knife edge contacts for both inside and...
www.midwayusa.com
Mitutoyo 500-196-30 Advanced Onsite Sensor (AOS) Absolute Scale Digital Caliper, 0 to 6"/0 to 150mm Measuring Range, 0.0005"/0.01mm Resolution, LCD: Amazon.com: Industrial & Scientific
www.amazon.com
Bullet Comparator:
The Hornady Lock-N-Load Bullet Comparator is perfect for the reloader looking to quickly and precisely measure seating depth of loaded ammunition from...
www.midwayusa.com
Headspace Gauge:
The Hornady Lock-N-Load Headspace Gauge is a great tool for increasing the accuracy of your loaded ammunition. The Headspace gauge measures from the...
www.midwayusa.com
Cause Gauges:
L.E. Wilson has been making precision reloading tools for over 80 years. This gauge allows reloaders to measure case length from the base of the...
www.midwayusa.com
Every reloader should know exactly how much your Full Length Sizing Die is pushing back the shoulder. With the L.E. Wilson Case Gauge Depth Micrometer...
www.midwayusa.com
Hornady Lock-N-Load Cartridge Gauges allow the user to check their cases and ammo to ensure a correct fit in SAAMI chambers. Cut to SAAMI maximum...
www.midwayusa.com
Examples of higher quality would would be Starrett also makes a higher quality digital caliper that's $315 on Midway, $308 at Grainger, and MSRP is usually more like $285 elsewhere. AccuracyOne makes a universal digital comparator that costs $115. So basically double what's on the list above.
This 6" precision caliper features easy-to-read numbers, long wearing carbide faces and a hardened stainless steel body for a lifetime of service....
www.midwayusa.com
The Accuracy One Seating Depth Comparator measures bullet seating depth, ensuring consistency. Compatible with 22 to 30 caliber cartridges.
bullettipping.com
Edit: Links show the wrong calibers, but 300 Win Mag is in the drop down menus.