either .223 or .224. Have the barrel slugged.
Not recommended
There is a web site entitled the model 99,which should be able to date your savage model 99.
look under the buttplate - should have 4 numbers 42 43 made between 1942/43
If it was manufactured before 1/1/1969, it probably doesn't have one. If manufactured later, it should be clearly marked on the receiver.
NO!! Although the bullet diameter is the same, the case length is different and should not chamber in the 380 caliber handgun.
If it was made after 12/31/68, a serial number should be clearly marked on the receiver. If made earlier, it may not have one or it may be hidden almost anywhere.
You should slug the barrel to determine the actual bore size (lands and groove diameter) to ascertain the actual diameter of bullet the rifle uses. Selecting a resizing (swaging) die for that diameter will give you the best accuracy. All sizes of swaging die are available through RCBS.
It is going to be late 1950's through mid-1960's. It should not have a serial number. The Savage 340 was an updated version of the earlier Model 325. They are primarly found in .30-30 caliber and are popular with reloaders since they can use a spire-point bullet. Also chambered in .22 Hornet, .223, .225 Win. I used to be able to buy these regularly for around $150, but now typical asking prices are twice that with the smaller calibers bringing the most. sales@countrygunsmith.net
The Kilbourne Hornet is an "improved" cartridge. The original "Hornet" was called the .22 Winchester Center Fire, was a black power cartridge and fired a lead bullet at about 1500 fps. The cartridge was first released in 1888. Some time in the early part of the last century, P.O. Ackley took a liking to the .22 WCF, married it with smokeless powerd and renamed it the .22 Hornet. Some short time later Lysle Kilbourne decided to push the shoulder forward on the cartridge and it became the K-Hornet or Kilbourne Hornet. You convert a .22 Hornet into a K-Hornet by purchasing a reamer from a company like Clymer. The reamer is a cutting tool used to reshape the chamber of the .22 Hornet into the K-Hornet shape. You should not try this operation yourself, unless you are a competent gun smith. Once your chamber has been bored using the reamer you must "fire form" your brass. The K-Hornet can fire factory .22 Hornet bullets. After firing those bullets in a K-Hornet chamber the powder capacity of the case is increased and the "improved" K-Hornet can be hand loaded to higher velocities. The cost for a reamer as I write this is about $120.00 US. The cost for a gunsmith to ream the chamber is about $80.00 US. Using a rented reamer, ($26 - $36), makes the process even more affordable. The results are worth it. :) Just did one myself today.
You should ask the Savage company that question.
The chambers of the sample HW5 which I inspected were bored the same diameter (cartridge case diameter) straight through. Chambers of good quality revolvers (e.g. Smith & Wesson and Colt) are reduced to bullet diameter at the front end (barrel end) of the cylinder to guide the bullet squarely into the barrel. With the HW5, the bullet must jump from the neck of the cartridge case to the barrel with no guidance to ensure that it enters the barrel perfectly in line with the barrel axis. This is not conducive to good accuracy. Bored-straight-thru chambers were common back in the days of outside-lubricated bullets having the same diameter as the cartridge case (like the .22 rimfire) but the 32 S&W is an inside-lubricated cartridge design and should have reduced-diameter chamber throats.
You should contact Savage arms directly,they should identify the year of manufacture for your rifle.