Overbore Addiction

Overbore Addiction

The Big .224’s

 

 

This is an overview on a subject that has been dubbed barrel burner cartridges. You know the talk “you’ll replace that barrel in 600 rounds, better have a good supply of extra barrels on hand, and the barrel makers will love you”.  As avid varmint shooters (that’s why you are reading this…right?) we all know about the effects of too much heat to a barrel will end up with an eroded throat, the 220 Swift was an example of this however shooters proved that otherwise with proper loading recipes and keeping barrel overheating in mind.

 

While shooting my 22-250 and 22BR I eventually became interested in the Big .224’s, sure I can push my 22-250 to 4000fps with 40grain woodchuck medicine but out to 500 yards bullets are taking the beating effects of wind and drift.  I had a copy of “Wildcat Cartridges Vol 1&2 by wolfe publishing” and became intrigued with the examples of .224’s pushing 80 grain bullets well into the 1000yard range.  These dedicated purpose cartridges are developed for long range shooting where long and heavy .224” bullets are used to buck the wind better and retain there energy out to extreme distances.  How this is done is sort of confusing to the reloader since fast twist barrels will most likely shoot best with fast burning powders and like wise for slow twist barrels and slow powder.  What I have found out is that to obtain the desired results from an Overbore case design is to load up with a slow burning powder, heavy bullets and a fast twist barrel.  The results are the longer the pressures push down those 26+ inch barrels enough velocity is produced to send the heavy .22 caliber bullets down range effectively.  Another factor that becomes apparent is the need to scrub out that bore more frequently (some as frequently as after 10 rounds) the high velocity produced in a small bore capacity tends to copper foul quicker and degrade accuracy.  A word of caution when working up loads is to always look for high pressure signs and stop shooting any load that produces such indicators.

 

There are numerous cartridges that fit this category and I will only just touch on a few.  Some need extensive case preparation and forming while others need only to be necked down, I consulted Quick Load for max case capacities, overflow grains in H2O and will list them from least to greatest:

 

 

 

 

  1. 22-250 A.I.           = 46.90
  2. 22-243                  = 50.00
  3. 22 Cheetah MK1  = 54.00
  4. 22 Cheetah MK II = 56.00
  5. 220 swift A.I.        = 57.00
  6. 22 Clark                = 59.00
  7. 22-284                   = 61.84

 

22-250 A.I.

 

Another P.O.Ackley success with his classic blown out case and 40 degree shoulder, velocities will push to 4000fps and has an effective range out to 500 yards. Reduced bolt thrust and case stretch along with the ability to chamber and shoot factory 22-250 loads make this extremely accurate round a versatile choice.

 

 

 

 

22-243

 

This cartridge was developed by Paul Middlested of San Diego, California it is based on the standard 243 Winchester case necked down to .224”.  The body dimensions remain unchanged only the shoulder angle is increased to 30 degrees giving a longer neck ( .300” vs .241” ), case forming is accomplished by running a 243 case thru a 22-243 full length sizing die. Velocities have been reported to exceed 4000fps while keeping groups well under ½ MOA @ 100yards, this cartridge excels with heaver bullets in the 68 to 75 grain range with rifling twist rates of 1-9 or 1-8. Other versions include the 22-243, 22-243 Middlested and 22-243 A.I.

 

 

 

22 Cheetah

 

The 22 Cheetah was developed by Jim Carmichel, shooting editor of outdoor life magazine and Fred Huntington of RCBS. The case is a full length Remington 308BR case with small primer pocket and necked down to .224”. There are actually two versions of this case one a MKI with a 40 degree shoulder and the MKII with a 28 degree shoulder. This cartridge has proven to be superbly accurate and very effective out to 300 yards.

 

 

 

220 Swift A.I.

 

P.O. Ackley’s Wildcat, blown out with a 40 degree shoulder. The three best known versions are the Ackley, Weatherby Rocket and the Kilburn, they share the mechanical advantage, extract better and cases last longer for the hand loader. Again better ballistics can be obtained over the standard case by the use of slow burning powder and heavy bullets.

 

 

 

 

 

 

224 Clark

 

Timothy Clark of Madera California designed this case for long range use around a flatter trajectory and harder hitting with greater retained bullet energy. The case is a 257 Roberts necked down to .224” with a steeper shoulder angle and expanded body, the 257 Roberts cases were found to be stronger as compared to reforming 6mm Remington brass. The small bore and relatively large case capacity and velocities ranging in 3800+fps to 4100fps can be a combination for short barrel life. Twist rates of 1-9 or 1-8 should be used to stabilize the long heavy bullets.

 

 

 

22-284

 

This variation of the 284 Winchester case is simply necked down to .224 (one way to accomplish this is with a 6mm-284 forming die then a 22-284 FL die) the 35 degree shoulder and body taper remain unchanged. We have arrived at the case capacity of diminishing returns where larger amounts of powder don’t necessarily result in greater percentages of increased velocity.  Accuracy will be best with heaver bullets as several conversations with gunsmiths have resulted in a preference of fast twist barrels and 70+ grain bullets. Reported velocities have been in the 3800fps range.

 

 

 

In closing there are several others not mentioned here such as the .224 Vais, and 22-6mm.  A look thru volumes 1 & 2 of Ackley’s Handbook for shooters and reloaders reveals quite a few and provides good insight on ballistics. Pressures can spike quickly with Overbores so a careful eye should be kept on spent cases along with sticky bolt lift. There is abundant information on these cartridges and with a little research you will be well informed on any specific cartridge. So if this small sampling finds you wanting more as it did for me grab your computer, some books and brew up a pot of coffee! Soon you will be chambering for that Overbore Addiction.

 

 

 

overbore 003

 

 

 

This is a handy chart I found on a cartridge forum posted from an unknown source. Interesting thou.

overbore table

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