Barrel Movement- Part 5

Changing Bullet weight or design

As mentioned in an earlier article, changing components will affect performance. Bullets of the same weight but different brand or shape will produce different velocities with the same powder charge. This is due in part to differences in jacket thickness, bullet bearing surface within the bore, gas seal on the base due to shape, hardness of core material, etc. If you change bullet brands in your load, you will need to determine if the new bullet's velocity is above or below the previously identified nodes. Once you do that, all that should be required to make it shoot much better is to adjust the powder charge so the velocity is within the range your barrel likes.

Change in Primers

Changing primers will also produce different velocities. Once you have identified what load your gun likes, it is simply a matter of adjusting the powder charge if you change primers. In the final accuracy analysis, the only substantial difference between primers may be the thickness of their shells. There is, however, a difference in velocity they produce with a given powder charge,sometimes 50 f/s or more. Is there a real difference when charges are adjusted to deliver the same velocities? For all but the most die-hard reloaders, probably no.

Standard deviation and tracking data

Most shooters are familiar with the measurement called "standard deviation." Standard deviation (SD) is simply a statistical measurement of the uniformity of a sample of events. The standard deviation data provided by most modern chronographs will help identify good loads. While it is not always true that loads with the lowest SD produce the very smallest group, generally a good load will have a low shot to shot velocity deviation.

Velocity deviation can sometimes be controlled by seating depth, neck tension, flash hole deburring, etc., and these are things you can worry about after you get a good load and have some free time to play around with the really fine tuning. The bottom line is, however, that if you don't have data for comparison, you cannot fine tune your loads.

Wrapping Up

Obviously, the degree of care in loading, component quality, and shooter ability all impact group size. And, of course, not all guns are capable of shooting one hole groups, except, apparently, those owned by some of the gun magazine writers. (Besides, if you want a 1-hole group just shoot once!) But, once you identify node velocities for a particular firearm with a chronograph, much of the guesswork in working up loads can be eliminated and a wide range of components can be made to provide satisfactory results. If you are happy with the load you can stop here.

If you are a stickler for details and want to wring the very best out of your loads you can go a little further with some detail work to insure maximum uniformity of your final working load. Weigh 10 unprimed pieces of your brass and find the average weight. Then weigh all the cases and cull any cases that deviate by more than about 3/4 gr from the average weight, or sort them into batches by weight. Uniform the flash hole using a tool like Midway's controlled depth flash hole deburrer. Insure that the primer pockets are of uniform depth (but don't ream the too deep) by using a pocket uniforming tool like Sinclair's. And then there is selecting the very best bullets.

In the end, how much work you put in to finding the most accurate load for your reloading brass is fully up to you. Remember, never move to fast and always take baby steps when adjusting a load. Just the smallest adjustment could yield the results you’re looking for.