.220 Thunderbolt


Anyone familiar with this wildcat load ? Did a search on the IAA Website & came up empty.

Any details about what it is, parent cartridge, specs, designer, status ?

Keith Pagel


I have notes on a “.22 Thunderbolt” cartridge, a wildcat based on the standard .30-06/8mm Mauser head. It is also known as the “5.6m/m Cook,” that evidently being the name preferred by the designer, Roy Cook. The round evidently was designed not too long after WWII. I don’t know anything about this man Cook, but he may have been an Australian, as my one blurb on this round is from an Australian Cartridge Collector’s bulletin. Stupidly, I forgot to write the bulletin number and date on the copy. A specimen made from a 1938 Polte-made 7.92 x 57mm case is described as having the following specifications:

Projectile Diameter: 0.222" 5.60mm
OAL: 2.096" 53.22mm
Neck diameter: 0.254" 6.42mm
Shoulder diam.: 0.428" 10.86mm
Head diameter: 0.466" 11.84mm

I don’t know if this is the same round as the “220 tunderbolt” or not, but it all that I have a file on under anything resembling that name.


Keith & John

I’ve posted an inquiry on another Forum devoted to wildcats and if I learn anything I’ll let you know.

If your description is correct John, it would be barely distinguishable from the many 22 wildcats based on the 06 case and, after 1955, the 308 W. The only difference between it and the 22/308 or 22/243 would be in the shoulder diameter and bullet diameter. The 7.62x51 case was being wildcatted, even before it was adopted, by enterprising shooters who managed to get their hands on pre production experimental brass. I have a couple in my collection that are headstamped FA 52.



Thanks Ray & John. I believe it’s different than the .22 Thunderbolt. This is supposedly more recent, and on the 6.8mm/.35 Rem case. The designer is a John Scandale, PA Army Guard, Armed Forces Reserve Shooting Team.



Just a small nit
The 6.8SPC is baised on the 30 Rem case (not the 35 Rem, which has a larger case head than the 30 Rem does)


yep. You’re right. typing too fast.