American Handgunner May/June 1977

Factory cannelure (top) and homemade can- Case damage due to excessive Typical case mouth and body damage asnelure (bottom) are the best way to keep pressure is reflected by the broken caused by the rough firing and ejection patbulletsfrom receding into the cases. rim & blown primer cup seen here. - - terns of certain number of the autoloaders.Also, the design of most swinging-armpriming setups makes it pretty easy to seata primer cocked, or to peel one edge of iton the case head.MOUTH EXPANSION: Up to now,what we've covered applies equally tocases intended for revolvers and autos.Mouth (neck, to some) expansion reallyshould be done differently for autos. Duringfeeding, the bullet strikes hard againstthe feed ramp (sometimes also against thebarrel tang or chamber wall) and can bedriven deeper into the case. This may alsooccur in the magazine due to recoilforces. From either cause, a receded bulletcauses feeding problems and may bevery inaccurate.To insure bullet stability, the casemouth should first be very tight upon thebullet; second, expansion should extendinto the case only to a little bit less, say '132-inch less, than the depth to which the bulletwill be seated. This leaves a shallowinternal shoulder in the case, which supportsthe bullet against recession. By stoppingmouth expansion just a bit short ofseating depth, the bullet base does thefinal forming of the shoulder as it is seated;thus the bullet is certain to be solidlyagainst said shoulder.To accomplish this, most expanderplugs must be altered. To do this, first runa plug into a case until its conical portionproduces the correct amount of mouthflare. This is the amount of flare requiredto allow a bullet to just barely be fingerstartedin the case. If a square-base bulletenters more than about '/3~-inch, themouth is flared more than necessary.Next, measure down from the case mouththe distance the bullet will be seated intothe case, less '/m-inch. Below this point, reduceplug diameter enough so that itdoesn't touch the inside of the case. Jointhe reduced portion to rest by roughly a45degree angle. This is best accomplishedon a lathe with a toolpost grinder,ibut you can do it, albeit laboriously, bykhand. with abrasive cloth and stones.Note that to do this right, an altered expanderplug is suitable for only one bulletseatingdepth. This means, that to someextent, you'll need to prepare different expanderplugs for different bullets. Theplugs will never wear out* a practicalmatter-though, so it's worth the effort.Expander-plug diameter (working portion)is also important. For hard lead bullets,it should be (in my experience) about.003-.004 inch less than bullet diameter,for jacketed bullets it can be up to .006-inch less, the more the better, so long asthe force needed, to seat the bullet doesnot damage the bullet nose or change itsshape. Remember the object of all this isto give the case as tight a grip as possibleon the bullet. This is not onlv a kev to re-liable feeding, but heavy, consistent, bullet-pullimproves ignition and combustionof the propellant, and, consequently, willgive better accuracy. Greater velocity,also, other factors being equal. You mayencounter an occasional batch of brasswith unusually thin mouth walls. After resizing,the inside diameter of the mouthmay be greater than the diameter of theexpander plug. Unless re-sized in undersizedies (available from RCBS), suchcases can 't hold bullets tightly enough.However, assuming they are tight enoughto keep the bullet in place during normalhandling, they can be modified to preventbullet recession. This consists of rolling acannelure into the case to support the bulletbase. Both C-H and Corbii make excellentcanneluring took I've used foryears. Just don't overdo it and make thecannelure too deep; about ,005-inch protrusioninside the case is plenty. Moremerely weakens the case. Adjust cannelurelocation so that the bullet base isforced tightly against the internal shoulder,;"p-"