Here is another link to primer tests, www.fr.frogspad.com/miscelld.htm
showing variance in pressure and velocity. Interesting discussion below that about changing seating depths.
For example in a 300 WSM seating out another .100 drops about 4500 psi for some bullets.
Q. How do changing various components affect chamber pressure and velocity?
Changing components can definitely affect things as the tables below show for the .308 Winchester cartridge. As you can see from the data, a change of almost 8,000 cup or more is possible simply by changing one component. If multiple changes occurred the effect could be even greater, and if the initial load was at or near maximum one could easily get into dangerous territory. This is especially true of small capacity cases in high intensity rounds like the 9 x 19 mm where case capacity can vary greatly. Note: These results were typical under the conditions tested. Your firearm and ammunition may behave differently so don't use this as loading data.
Changing Bullet BrandsBullet Wt
(cup)168SierraLCW120IMR-489540.5 gr262652.2k"Speer """"262551.2k"Hornady""""262551.2k"Nosler""""262452.1k
Changing Primer BrandsBullet Wt
Changing CasesBullet WtData courtesy NRA.
Most reloaders standardize on a particular brand primer and bullet for a given load and the factor most likely to change is the cartridge case since most shooters will pick up anything that looks like brass. To give you an idea of how things can vary look at the difference in case weights for representative 9 mm and .308 Winchester cases that I have measured. The weights are based upon the average of 5 fired and unprimed
(but otherwise unmodified) cases. Your cases may differ so don't count on these figures. (If you can supply data on other cases I'd greatly appreciate it
.308 Case ComparisonBrandCase Wt.
(gr)Federal (old)158.2WW (Old)159.9Hirtenberger '84161.3*Norma165.6Remington170.5Lapua171.8Sako172.0Lapua(recent)173.2Federal (Current)175.4Federal Nickel (Current)177.1WCC 67177.4LC 76179.0Samson Match179.2WW (Current)179.2LC 77179.3LC 79 MATCH179.4LC 67 MATCH179.5LC 74179.9IMI 308 Win181.0FA 65182.7FNM '82194.1* These case have a much smaller than normal flash hole US Specification for the
7.62 x 51 NATO case is
190 +0, -20 gr
In the 9 mm cases the difference in case weights translate to tremendous difference in case capacity. There is an old rule of thumb that for equivalent pressure, charge weight varies by about 12 percent of the difference in case weight. While this rule of thumb is based upon .308/.30-06 size cases it hold true fairly closely for other case sizes and can at least keep you out of major trouble.
For example, a load developed in 9 mm Winchester brass listed above would have to be reduced by about 1 gr in Remington cases and a whopping 1.3 gr to maintain the same pressure in TZ cases. In a cartridge like the 9 mm where typical powder charges run 5 to 6 grains that's a big 22 percent change. In the .308 examples above the change incurred from the lightest to heaviest cases would be about 4 grains. With a typical 45 grain charge weight that's about an 8 percent change. It should be noted that average case weight can vary from lot to lot (usually no more than about 2 gr) within a particular brand of rifle cases. With .308 size cases differences in average weights of about 1.5 gr can probably be ignored safely. In 5.56 the charge difference can be around .6 gr which with an average charge of powder (27 gr) would about a 2 percent change
You should also be aware of the fact that the burning rate of your powder can change slightly from lot to lot. Although the lot to lot consistency these days is quite remarkable there is still the chance that things can change. (This is one reason why big-time match shooters buy large quantities of a single lot of powder).
No wonder the reloading manuals caution you to drop at least 10 percent from any maximum charge data and slowly work up. A good rule of thumb for the amount of an increment to use when working up from a starting load is to fill the case to the base of the neck with the powder you are using, dump and weight the powder, and to then use 1 percent of that weight as an increment. As an example, if your case would hold 55 grains of the powder you are using a good "increment" would be .5 (5/10) of a grain. Once you get "in the ball park" for accuracy (see the article on load development
) you could then cut that increment to about half (say to .2 gr in this case) to fine tune things.
Remember. Play it safe. If you change anything drop your load 10 percent and slowly work back up.