Tuning in regards to reloading:
Tuning loads for your guns. No matter the platform, tuning a load to your gun does these things.
reduces the recoil, maximizes the accuracy, maximizes life span of firearm, brass, and your reloading resources.
will make the best accuracy for your rifle with the greatest speed, lowest arc of flight, and with the right selection of slugs, will make ammunition that can not be beat by anything bought off the shelf.
For other styles of shooting:
there are certain ammunition that is not readily available like 7.62X61 NATO, 7.62X51 NATO, and other old military cartridges. The 7.62X61 and the 7.62X51 have modern brothers that have higher operating pressures and will blow up your old military rifles. I am speaking of the M-1 Garand and the M-14/M-1A. These are still used for PALMA style competitions. These two guns need a specific pressure limit that is far below their commercial brothers the 30-06 Springfield and the 308 Winchester. For these two guns you must tailor/tune loads for your gun to perform properly.
Tuning loads involves customizing length for your chamber (headspace), lead, and jump. (These terms will be discussed next)
Definitions- chambering and chambers:
Headspace as defined is the distance between the breach face and what ever stops forward movement of the cartridge. This could be the shoulder, the belt, the rim, or the case mouth depending on case design.
Lead: consists of two parts; the free bore for the slug before it engages the lands and grooves of the barrel. The throat or forcing cone; this is the tapered part of the chamber that transitions the slug from the free bore to the lands and grooves so that the slug will stabilize outside the barrel.
Jump: is the length from the slugs Primary diameter to the forcing cone/lead of the chamber. This can be set if you know how to measure it. Optimally 0.005″ to 0.030″. What ever your rifle likes best.
The lands and grooves: consist of two parts, primary bore: this is the original polished hole down the barrel and is the nominal bore size (caliber) for example 30 caliber or 0.300″. The second part is the groove diameter, the grooves are cut into the barrel larger than the bore diameter so that we have a spiral down the barrel so the projectile has spine/rotation so rotational energy will keep the slug from tumbling down range after it leaves the confines of the barrel. We rate these spirals in “twist rate” these usually come in revolutions per a designated length in inches. For example 1:8″ (translated one revolution per 8″ of barrel length). A 30 caliber has a nominal bore diameter of 0.300″, and a groove diameter of 0.308″ the grooves are cut 0.004″ larger than the bore diameter to have enough surface area to hold on to the copper jacketed slug and impart rotation to it.
Lands: the small part of the bore, the original/primary diameter of the bore
Grooves: the larger part of the bore. Consisting of either cut rifling or rotary hammer forged.
You have 6 case designs:
rimmed, semi-rimmed, rimless, rebated rim, rimless magnum and belted. Each has its advantaged and limitations.
Examples of rimmed cartridges are lever gun cartridges where the extraction rim has a bigger diameter than the body of the case. Most all revolver cartridges. 38 SPL, 41 Colt, 44 SPL, 22 LR, 7X61, 30-30, 32-40, 25-20, 32-20, 30-40, Nitro Express rounds,
example of rimless: most all pistol cases (9X19, 9X21, 9X23, Super 38 Comp, 45 ACP, 10MM, 40 S&W and the alike) most bottle neck rifle cases. 30-06, 308, 270 win, 7X57 Mauser and other modern Mauser cases.
Belted cases: these are your magnum cases. 300 H&H, 375 H&H, 416 Remington, All the Weatherby cases, These cases are identified by the thick belt just above the extraction cut that is larger in diameter than the rest of the body of the case.
Rimless magnum cases are a more recent design for magnum cases. These are thick walled, headspacing on the shoulder, cases that are easier to headspace and have less of a chance to have a case head separation. Examples of these cases are: Winchester Short Mags, Winchester Super Short Mags, Remington Ultra Magnums, Remington Short Action Ultra Magnums, 404 Jefferies, 338 Lapua, and Sherman Short Mags. One big advantage of this case is superior accuracy over the belted magnums.
There are very few rebated cases, most notably the 284 Winchester, the new 375 and 475 Bishop Short Magnums for the AR-10 platform. These are identified by their case body larger than the diameter of their case head below the extraction cut.
The semi rimed are very old turn of the 1900’s cases. These cases have a semi deep extraction cut with a slightly larger than case body diameter extraction rim. The cartridges you will find semi rimmed are 25 ACP, 32 ACP, 38 ACP, Super 38. No others come to mind right now. These are all colt cartridges and John M. Browning designed guns and cases.