A wildcat is a cartridge that has had the dimension changed and usually includes a change in caliber. One of the most versatile cases used is the venerable 30-06. The 30-06 has been stuffed with bullets from 17 caliber to 40-caliber. First, we need to define a term: Overbore or overbore capacity is a case with a relatively large case volume or case capacity with a relatively small diameter bullet. Two things to consider is that as the overbore capacity increases so does barrel wear and that the efficiency of powder charge begins to drop off requiring more powder per velocity increase. The formula is Case capacity is measured in grains or milliliters (ml) of water.:
O ratio= Volume case / Area bore. The 30-06 has a capacity of 4.42 ml compared to the 308 Winchester at 3.64 ml.
Let’s take a look at what has been tried and those that were so successful they became commercial offerings:
1. 17-06, this was a 30-06 necked down to .17 caliber. Way overbore capacity for a .17 caliber. Was not very successful.
2. 22-06 for a .22 caliber cartridge this round is still overbore. It was found that this round was without any practical benefit. With 220 Swift based on a 7×57 mm Mauser filling the bill for high end .22 caliber rifles.
3. 6mm x 06 ( or 243-06) can push a 105 grainer out the barrel at 3200 fps. The 6-06 is more inexpensive when considering its brethren, the 240 Weatherby Magnum or the 6mm-284. Still overbore, but more practical than either the 17-06 or 22-06.
4. 243 Catbird is a necked down 270 Winchester with the shoulder blown out to 35 degrees. This can push a 70-grain bullet out to 4000 fps. A barrel burner you only git 1500 rounds before barrel replacement.
5. 25-06 is the 06 necked down to take .257 bullets. In 1969 Remington introduced this as a commercial cartridge, the 25-06 Remington.
6. 6.5mm – 06 offers ballistic performance between the 25-06 and the 270 Winchester. A-Square standardized the round as the 6.5 – 06 A-Square in 1997. At this time, it is not commercially loaded. Included in SAAMI listing through March of 2018. As of 2015 the ANSI/SAAMI is no longer listed.
7. 270 Winchester is a 27-caliber bullet in a 30-03 case and one of the most popular sporting cartridges in the US.
8. 7mm-06 originated as an experiment it is a 7mm projectile loaded into a necked down 30-03 65mm case. The 30-06 case is 63mm in length and its progenitor’s 30-03 is 65mm in length. Hence this became the 280 Remington.
9. 8mm-06 came about as the end of WWII dumped a lot of 98K Mauser into the US market, but no ammo. So, the expedient was to have the Aught 6 case neck to 8mm bullets, and making the Mausers shootable again.
10. 338-06 necking up to .338 caliber produced a good combination of readily common brass and allowed for the shooting of heavier bullets. 338-06 A Square became the designation, but no commercial ammo is available the A-Square round is no longer in the ANSI-SAMMI listings. Nosler still produces loaded ammo for 338-06 A-Square. This is one of the best Aught 6 wildcats out there.
11. 35-06 was a 06 necked up to 35-caliber and became the commercially available 35 Whelen. This cartridge can take on any game in the US including big bears.
12. 375-06 was made by necking up to the .375” bullets.
13. 400-06 has never be popular because there is so little shoulder that there have been severe headspace problem.
Ackley & Gibbs Improved Wildcats
P.O. Ackley improved many of these wildcats as well as the parent cartridge by sharpening the shoulder and with the case sides nearly parallel became Ackley improved cartridges and where very common. R.E. “Rocky” Gibbs also improved many of these wildcats and the parent cartridge by ‘fire forming’ the cases much as Ackley did his. So, the 30-06 has a fairly well-established progeny, some that were commercialized and many that shoulder on as hunting weapons. But with introduction of the 7.62×51 mm or 308 Winchester the 30-06 surplus brass is not as plentiful as it used to be, probably why a whole bunch of our hunters carry .308s and 308 wildcats.