I use a chronograph to choose primers. I take the load and for each test primer I run ten with the powder back over the primer and ten with it forward over the bullet. The primer that produces the lowest average spread is igniting that charge weight in that cartridge and bullet combination most consistently, so I choose it.
I use a chronograph to adjust for changes in powder lots. I keep back enough of the old lot for at least 30 rounds. I fire those and (after a pressure confirming work-up) the same number of rounds loaded with the new lot to that same charge weight. I compare the mean velocity values. I change the charge of the new lot in inverse proportion to the ratio of those two velocities and check for a velocity match. If the matching velocity charge weight differs by more than two percent, I additionally model both in QuickLOAD to get matching velocities by changing the burn rate for the new lot, and then adjust to match barrel times rather than velocities, as that holds me closest to a sweet spot.
I use a chronograph to determine the ballistic coefficient for a cast bullet I have no reliable published number for. In the last couple of years, I've used the Labradar for that, as it supplies velocities at the muzzle and four other ranges for each shot. I fire 30 rounds and see which one produced the highest BC and figure that is the one that had the least initial yaw and so its BC better represents what will happen with the bullet as it gets beyond the Labradar's range.
(I used to use conventional optical chronographs for the above. I have two, an Oehler and a CED that track well, and I'd set the Oehler at a close range and the CED near the target and use their reading pairs to get velocity loss over the range between them. The Labradar is much quicker to set up and use and the only way to get multiple ranges from the optical chronographs is to fire enough rounds for a reliable average and then move them and repeat. It's a lot of work.)
I use a chronograph to choose field ammunition velocities. All bullets have an ideal impact velocity range. Get your bullet maker's data on this. Apply the minimum and maximum ranges you expect to shoot and the bullet's ballistic coefficient to a ballistics range table calculator. Find the muzzle velocity that ensures impact at your maximum range is still within the manufacturer's recommended minimum velocity and that it doesn't give you too much velocity at your closest expected range. Once the computer has given you that muzzle velocity, the chronograph lets you produce a load to match it.
jmr40 said:
If you look at a book load showing 60 gr of powder should yield 3000 fps with a 150 gr bullet as the max load, then 3000 fps is the important number, not the 60 gr of powder. Without a chronograph most people keep adding powder and comparing group sizes while looking for traditional pressure signs. The problem with that is that traditional pressure signs don't show up until you reach 70,000 PSI, well over where you should be.
Without a chronograph you might incorrectly conclude that your 60 gr load is perfectly safe since it is showing no pressure signs. You could be shooting 3150 fps with 60 grains and still not be showing pressure signs. But you're certainly over pressure.
By using a chronograph you can monitor muzzle velocity as you work up your load. When you start getting close to 3000 fps you are approaching a max load. You may find that some combo's of rifle, brass, primer, and bullet will reach 3000 fps with only 57-58 gr of powder. That is where you stop adding powder.
In other cases you may find that 60 gr of powder is only getting you 2900 fps with some rifles. Technically it would be safe to keep adding powder over the book max of 60 gr and it would be safe in THAT rifle. I strongly advise against this because those loads could end up in another rifle where they could be dangerous. You're just going to have to accept that 2900 fps is all that rifle is going to do with that powder and bullet.
"First contemplation of the problems of Interior Ballistics gives the impression that they should yield rather easily to relatively simple methods of analysis. Further study shows the subject to be of almost unbelievable complexity."
Homer Powley
One of two reasons many chronograph instructions say not to try to use the instrument to assess pressure is that interior ballistics are complicated enough that it is easy to get turned around. Everything in Jmr40's post sounded very reasonable, but, alas, it is turned around.
If you need more powder to achieve book velocity in a same-length barrel, you have lower peak pressure, not higher peak pressure. This is because the larger quantity of powder is making more total gas. That extra gas holds the pressure up higher past the peak value, raising muzzle pressure. This means a larger portion of the bullet's total acceleration occurred after the pressure peak was done than was the case for the book authors. Since the total acceleration to get to that matching velocity was the same, it follows that a smaller portion was done by the peak value, which means the peak pressure was lower, not higher.
If you need less powder than the book level to achieve the same velocity in the same length of a "fast" barrel, then, because the lower total gas quantity has lowered post-peak and muzzle pressure, more of the total acceleration had to occur during the pressure peak. That means your peak pressure, even with that lower charge, is already higher than book peak pressure and you should lower the charge and accept a lower velocity if you wish to avoid exceeding the book peak pressure. How to figure that out gets into the real complexities of the relationships, but it can be estimated.
There are, in fact several ways to estimate the differences. Probably the easiest way is to put the same load into QuickLOAD and adjust the burn rate of the powder until you have a matching velocity in the test barrel length. The pressure it gives won't exactly match the measured pressures from Hodgdon or Lyman or other sources of measured pressure data, but if you change the adjusted burn rate powder's charge weight in the program to match your actual required change, QuickLOAD's resulting peak pressure will change in very close proportion to the actual change in pressure your charge made. So you can multiply the book pressure by the ratio of the pressure changes in QuickLOAD to get a very close estimate of the actual pressure peak value you are getting. QuickLOAD then also lets you change barrel length so you can see what you should expect from your barrel based on the expansion ratio differences for your cartridge.
