Chamber casting is easier than reloading on a single stage press or bullet casting. Buy a chamber cast ingot (usuallycalled Cerrosafe but other vendors offer casting alloys) from Midway/Brownells/Mid South/etc. Push a tight fitting patch into bore from muzzle using a brass jag about 1/2" to 1" shy of rifling lands. Want cast to include beginning of chamber body, shoulder, neck, freebore area and first bullet length of rifling. Want to measure inside diameter of throat (especially match chambers so can outside turn and inside ream cases to fit neck with minimum clearance but not bind), diameter of chamber at base, where shoulder begins, throat length and distance to lands. Will allow you to examine transition from freebore area to engaging threads for burrs that may require lapping plus helps in setting up bullet seating depth for precision loads and sholder angle but most importantly measure to ensure chamber dimensions are in spec.
This is a very simple process that is almost impossible to mess up. Coat chamber with just a fine dusting of graphite, heat your Cerrosafe in a ladle of some sort like plumbers ladle, gutter installers ladle, lead dipper (I use a older cast iron bullet casting ladle for casting muzzle loader bullets over a fire with a spout) that is big enough to drop a Cerrosafe ingot broken into two pieces) using your stove, propane torch, camp stove, etc till melts the Cerrosafe to 200° F and pour into chamber holding barrel plumb with muzzle down till Cerrosafe fills chamber and mounds up just a bit so get full cast but not let overflow over sides so does not make removing as difficult. If your ladle does not allow an easy pour a prewarmed aluminum funnel can be a help.
As the Cerrosafe cools it shrinks just a tad allowing you to tap it out without undo difficulty using a wooden dowel from the muzzle or if need stiffer rod because cast cools fast an aluminum or brass rod will not damage rifling of a steel barrel unless your just stupid but be careful not to nick/damage crown. Cerrosafe is made of a nifty material which shrinks a thousandth or so as cools so easy to remove but returns to original size as cures (During the first 30 minutes of cooling cerrosafe shrinks. At the end of one hour it should be "exactly" chamber size) giving user a very accurate cast of their chamber, throat and rifling. There is slight size change with time based on brand/alloy of chamber cast material but this chart gives average expansion factor fairly accurately as it cures overy time..
Contraction - Expansion Factor VS. Time
2 minutes -.0004"
6 minutes -.0007"
30 minutes -.0009"
1 hour +-.0000"
2 hours +.0016"
5 hours +.0018"
7 hours +.0019"
10 hours +.0019"
24 hours +.0022"
96 hours +.0025"
200 hours +.0025"
500 hours +.0025"
Once you have your properly cured cast can use a myriad of tools to take all types of measurements. Using anything from a chamber checker to just check your cast fits, remember cast will be bigger than a loaded shell and likely the same dimensions as a SAAMI chamber dropping the cast in a device same sizes as cast it will be very snug to coming just shy of fully dropping in but will give a quick easy idea of how close to spec your chamber is. Allows easy measurements of neck for length, concentrity, free bore, diameter at case head, shoulder, etc.
If do not want to keep a chamber cast of your chamber can remelt and use to cast another chamber. I have some rifles such as 22 CHeetah, 22 Nosler, 22-250/22-250 A.I., 240 Weatherby Mag, 6mm-284, Lazzeroni 257 Scramjet, 7mm Rem Mag, 7mm Practical up to 300 Win Mag and 300 RUM plus other throat smoking hotrods had built over the years which take a chamber cast every 400 to 600 rounds to help determine if time to lap the throat or time to replace barrel. My 240 Weatherby is set up nothing like the factory rifles as smith who built it set it up to run with little to no free bore.
How Weatherby gets velocity without blowing up rifles is a lot more free bore than most so bullet has nearly cleared case when engages rifling and thus pressure drops much quickly plus the bit of extra gas bleeding back reduces stuck cases. My 240 after blueprinting the action lapping bolt lug engagement and cryofreezing barrel and action to relieve stress points in steel smith says to load to full 240 Weatherby specs or beyond, seat bullet where ogive engages lands when close the bolt or has minimal freebore and pretty much grantees can't blow it up unless use fast pistol powders. Have several turn bolts built by same smith that he advises what would be dangerous distances from ogive to rifling in factory rifles but his are built to take these pressures.
That said have rifles with 1,000 round to 2,000 round barrel life before see steep accuracy drop off unless they are lapped as needed. Have a bore scope that does video capture so able to compare still pics and video of throat when new against current round counts. When start to see rough areas and burrs can use Tubbs Fire Lapping Rounds or hand lap the throat thus extending the bores life. Some of these rifles may start off with 28" to 36" barrels and when throat is smoked smith cuts anywhere from 1/4" to 2" off back of barrel, recuts chamber, replaces barrel and can sometimes get three rechambers out of barrel before rifling is eroded past use.
Will take a chamber cast before fire first round if smith does not provide with a cast then take a new cast every 200 or so rounds. When see odd wear or divots/burrs will lap and then when lapping no longer brings back accuracy take to smith to decide if cutting and rechambering is needed or new barrel. Have a 22 CHeetah on its second barrel which have the casts from first barrel as souvineers and recently took second cast on new barrel. Get a really good feel for when rifles will show accuracy fall off without firing a round on paper as barrels with 1,000 round throat life other than verifying scope as needed just shooting paper to check accuracy as a habit reduces the number of varmints or long range bigger it kills before buying a new barrel.
I purchased a bunch of barrels in end of year clearance from a popular AR parts company several years back. Like eight of one, five of another, etc. They had a reputation for one particular stainless barrel having chamber issues which had purchased four of. I took a cast of all four, two of which throat was very rough and another throat was rough and lands were mangled. Sent email with photo of casts plus video captures from bore scope and they sent an RMA number same day saying that replacement barrels would be shipped next day rather than having to send back for inspection. To date have only built two of the four barrels, had I not had this habit could have built the two good ones first and not discovered the bad pair till a decade after purchase and doubt warranty would have covered them even if had just been removed from boxes and wrappers.
I can't keep casts of every chamber of every rifle along with comparisons as round count increases. As 5.56, 6.8, 458 SOCOM, etc tubetube ome in will take a cast, measure and check for concentrity, obvious issues and if passes add to parts locker. If doesn't get it replaced and check replacement. My varmint rifles and extreme hotrods will keep original, casts taken through its life then when smoke it use the old casts to make new casts for other rifles. Only AR 15 cartridge I keep original cast are 22 Noslers, AR 10s keep 6XC and 6mm Creedmoor casts.
Delved a bit deep but taking a chamber cast is the easy way to ensure there is no issue with chamber as have a barrel right now to return where reamer shattered while chamber was being cut and inside looks like bomb exploded. Find more issues than would think but I buy lots of clearance barrels at two to a dozen units per purchase if cheap enough. Purchased eight Noveske 6.8 barrels in one lot second hand but sealed in wrapper so need to know they are right before go in locker if may sit years before built. Also don't want to assemble a single unit and discover chamber is fubar after all the labor of assembly.