Miscellaneous Questions #6

This section continues the discussions of various ballistics and shooting related topics as requested by correspondents. If you have a question you have been trying to find an answer to (keep 'em ballistics or shooting related--see your minister for the mysteries of life) email me by clicking here and I'll do my best to find the answer for you and if it is of general interest, publish it here. If you can contribute additional input to one of the answers I'd would appreciate hearing from you too.

On this page:

What is a balloon head case?
What effect does canting the rifle (not holding it vertically) have on the point of impact?
What are the ballistics of modern Soviet Bloc military rifle and SMG cartridges?
What are the color codes used on Soviet Bloc ammunition?
How long will my barrel last?

Q. What is a "balloon head" case?

A. Balloon head cases were an early design for the rapid production of cartridge cases. The first cartridge cases were formed from a disc of copper sheeting that was folded into shape.  However, this "folded head" design would not stand high pressures, even when reinforced with an inside cup.

The earliest cartridge cases were rim fire--that is the priming compound was around the edge of the rim. Later the priming mixture was placed in a cup like insert that was fitted into the base cartridge case and held in place with a crimp. Cases of this design are known as "inside primed."  Neither of these designs were reloadable.  When separate primers were developed they were originally used in a folded case design called a balloon head which had an integral pocket for the separate external primer. Because the folded balloon head design could not withstand high pressures it was subsequently reinforced with a separate internal cup at its base. Unfortunately, this proved both difficult to do and insufficiently strong.  

Newer technology allowed the forming of cartridge cases from a hollowed out brass slug and these balloon head cases (also referred to as the "hollow bar case" gave greater strength, but were still insufficiently strong for the pressure levels generated by modern smokeless powders. The balloon head design was superceded by the solid head (also called the solid bar head) case design which is used in all modern ammunition.

Balloon head cases are rare today and if found should not be reloaded.

Top Row: Rim fire, inside primed, folded balloon head.  Bottom Row: Reinforced balloon head, solid balloon head, solid head

Q. What effect does canting the rifle (not holding it vertically) have on the point of impact?

A. Canting, or failure to hold the sights vertically induces a deflection in the point of impact.  The amount of the deflection is determined by the degree of cant, the curvature of the bullet's trajectory, and the height of the sight line above the center of the bore.

In the 1800's a quality rifle was often equipped with a front sight level. Long range shooters of the day understood that the highly curved trajectories of the cartridges of day made it necessary to hold sights perfectly vertical. Smokeless powder and modern boat tail bullets introduced at the turn of the century shot much flatter. Since a typical shots rarely exceeded 250 or 300 yards, well within acceptable point blank ranges for most hunting rifles; sight levels disappeared as common long range shooting equipment and most shooters forgot about the effects of canting.

Modern varmint and long range shooters are now shooting targets at ranges beyond 1000 yards once again. At these ranges even modern loads again have curved enough trajectories that serious shooters have rediscovered the importance of sight cant. New variations of the old timer's sight levels have been adapted for use with scopes and some new scope designs have internal levels.

Canting a firearm only a few degrees from true vertical can significantly change the bullet's point of impact. Canting to the right moves the point of impact to the right and to the left moves the point of impact left.  It should be noted that there is a lot of erroneous information floating around on canting including some "big name" software programs.  They err because their canting calculation are based upon rotating the firearm about the bore axis rather than around the line of sight which is what you do when you cant. (RSI's Shooting Lab software gets this correctly.)

The table below is based upon the M80 Ball bullet with a 1.5 inch sight line and shows what happens to the point of impact when the rifle is canted 5 and 10 degrees to the right. (As a visual reference consider that the angle between minute marks on a clock face is 6 degrees so you can see just how little cant can start affecting things.) Note that while the drop stays fairly close to the original position the deflection starts to move off target. 

If a firearm is held in the same canted position with every shot, but zeroed at a specific range, tight groups can be maintained at the zeroed range but the arm will shoot right or left of the LOS at other ranges.

At long ranges, canting only 12 degrees (equal to 2 minutes on a clock face) may open a group horizontally as much as a 10% powder charge variation does vertically! Even if you shoot to distances less than 300 yards there are aspects of cant that may be important to you. If you shoot quarter size groups at 100 yards and never bang away at distant targets, 12 degrees of cant can still add 50% to your horizontal group dimension.  As you can see from the table the affect of cant is much more pronounced at longer ranges, when velocities fall off and the trajectory becomes steeper. Even if cant does not move a 200 yard big game shot out of the vital zone, the affect can be compounded by normal freehand shooting error. The cumulative error can easily put the shot outside the vital zone.  Be a "straight" shooter!

Thanks to Jim Ristow for much of the above information.

Q. What are the ballistics of modern Soviet Bloc small arms?

A. The table below shows the approximate ballistics of 20th century Soviet Bloc military rifle ammunition. The velocities are based upon information from several sources and are from the standard infantry weapons. Those for the 7.62 x 54R are given from the M91/30 rifle with a 28" barrel. (Velocities in shorter 21" barreled rifles will run about 120 f/s slower.) The 7.62 x39 mm and 5.4 x 39 mm velocities are from their respective AK rifles.  Velocities for the 7.62 x 25 are given for the PPSh 41 SMG and the 9 x 18 mm are given for the Stechkin APS pistol. It should be noted that there is a wide variance in the published statistics of Soviet ammunition but this data is believed to be accurate.  Some lots of Chinese and Czech 7.62 x 25 have been chronographed at 1875 f/s and higher but Soviet technical data seem to indicate specification was 1640 f/s. If you can supply verifiable data please contact me clicking here.

A sight height of 1" is assumed, with the exception of the AK47 / AK74 ammunition which uses the 2.0" sight height and the APS which is .7". (For those of you who may be interested the line of sight for the telescopic sight on the M91/30 sniper rifle is reported to be 3.4 inches.) A zero range of 300 yards (typical combat zero) is used except for the SMG ammunition.  Where known, the actual measured G7 ballistic coefficients are used and where not known  equivalent modern commercial bullet G1 equivalents are used. 

Q. What are the color codes used on Soviet Bloc ammunition?

A. The color codes used within the Soviet Bloc nations have varied from nation to nation but their generally accepted standards for small arms ammunition are given below.

Q. How long will my barrel last?

A. Barrel life depends on several things, including the operating pressure of the cartridge used, powder type and quantity used, rapidity of fire, type of bullet used, proper care and cleaning, the quality of the barrel's steel, and the degree of accuracy degradation you will accept. In the .22 rim fire with its soft lead bullet, and small powder charge it is not unusual for the barrel to last almost indefinitely as will most pistol barrels if used with lead bullets.  With some high intensity "magnum" rifle cartridges useful barrel life may only be a couple of hundred rounds--the smaller bore diameters being the most sensitive. The Remington 7 mm Magnum, the Winchester .264 Magnum, and the new Winchester short and super short magnum cartridges are among those that have a reputation for being hard on barrels.  Several years ago a friend's new 7 mm Rem Mag chronographed at 3182 f/s  using a 140 gr handload when brand new.  About five hundred rounds later that same load was chronographing at 2958 f/s (not much more than I was getting from a .280 Remington with the same barrel length and burning 10 grs less powder) and his groups had opened up about an inch.  While still more than accurate enough for field use he had lost a lot of the vaunted performance.

With medium intensity rounds like the .308 and .30-'06 useful barrel life seems to average between 5,000 and 10,000 rounds or more, assuming one doesn't "rattle-battle" them. The thing that ruins the barrel is the erosion of the throat area due to the heat of the burning powder. Tests conducted at Frankford Arsenal many years ago using the ammunition acceptance test barrels, found that peak accuracy was obtained between at 500 and 1000 rounds and that "acceptable" test barrel accuracy lasted for several thousand rounds.  As the barrels wore, the accuracy began to decline and the velocity began to decrease due to the lengthening and roughening of the throat area and the first few inches of the bore. Their standard was to stop using the barrel to test ammunition after an arbitrary 8,000 rounds.  However, tests showed that they gave acceptable (though not up to test barrel standards) accuracy to almost 23,000 rounds. (Unfortunately, I do not have any data on the accuracy decline.) These old barrels were made of carbon steel which was much softer and less erosion resistant than the chrome-moly and stainless steel barrels in common use now.  Thus modern barrels have the potential to have a longer useful life.

Interestingly, as the throat area wore, if the bullets were seated long (to give the same distance from the lands to the bullet ogive as standard ammunition in a new barrel of the accuracy) the accuracy came back to normal standards.

While competition shooters may go through barrels the average hunter is more likely to ruin a barrel by improper cleaning than by shooting. As mentioned on "Page 3" more barrels have been ruined by improper or over cleaning than by shooting. Use a rod guide and clean from the chamber end using a clean, properly fitted rod, bore brush, and tips.

Avoid maximum loads, don't over do the rapid fire, and clean carefully, and your barrel may outlast you.

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Disclaimer

As far as I know all the information presented above is correct and I have attempted to ensure that it is. However, I am not responsible for any errors, omissions, or damages resulting from the use or misuse of this information, nor for your doing something stupid with it. (Don't you hate these disclaimers? So do I, but there are people out there who refuse to be responsible for their own actions and who will sue anybody to make a buck.)

Updated 2008-03-17