CurryCornDog
Member
- Joined
- Mar 2, 2023
- Messages
- 33
When pressure testing is done to publish load data, actual pistols/firearms are rarely used. Instead, "universal barrel fixtures" that accommodate multiple caliber barrels are used which are single action that do not feed from the magazine.
So OAL/COL listed on published load data simply means that was the cartridge length used for pressure testing that day for the particular test barrel. But using this length does not ensure finished rounds will fit our pistol barrels nor reliably feed from magazines.
If you are loading for revolvers, you can use the listed OAL but more often, you will use the OAL that will allow roll crimp into the crimp groove unless you are using taper crimp.
But if you are loading for semi-auto pistols that must feed and chamber from the magazines, we must first determine the Max OAL allowed by the barrel then the Working OAL that will reliably feed and chamber from the magazine and this is the cartridge length we use for conducting initial powder work up. (And if the Working OAL, or rather bullet seating depth, is shorter than published, we can consider reducing the start/max charges by .2-.3 gr to compensate, depending on the difference in bullet seating depth, calculations for which we already addressed in previous posts)
And if powder charge that produces smallest groups is not at max charge and you want to squeeze a bit more accuracy, you can incrementally decreased the OAL/bullet seating depth by .005" to see if accuracy improves/group size decreases. If group size gets smaller, use the shorter OAL. If not, use the longer OAL.
These are load development steps match shooters use to determine accurate match loads. The last step of incrementally decreasing the OAL is further used by bullseye match shooters (Or those that want to pursue the accuracy rabbit hole) to where powder charge/shorter OAL results in lighter recoil loads to require reduced recoil spring rates.
An example of this is my 9mm reference load development using 115 gr FMJ RN and W231/HP-38:
- 4.6 gr and 1.150" will reliably cycle the slide and start trending accuracy of group size
- 4.8 gr and 1.150" will produce smaller groups
- 4.8 gr and 1.140" will produce smaller groups
- 4.8 gr and 1.130" will produce even smaller groups and this is my current 9mm reference load (But there are other powder loads that are even more accurate using Bullseye/Titegroup/WST/Sport Pistol/WSF/BE-86)
- 4.6 gr and even shorter 1.110" will produce comparable accuracy if savings of powder is desired (When you are loading tens of thousands of rounds, .2 gr savings per round adds up)
- I will not load 9mm with 115 gr FMJ RN shorter than 1.100" because due to tapered case, neck tension will start to decrease with shorter OAL and bullet will simply drop into the case
Man, practical application of theory sure does bring about real working knowledge over time. It all clicks now. This is good news.
It makes sense why you asked me about setback. These are simple things to know, and now I'm understanding the "why" when you put it all together.
It's a chess game. Each move must be criticized in order for every part to move freely for the best yield possible. Something out of whack makes everything out of whack, hence the setback question. I'm starting to see it now!
Deeper seating has the chance of flattening the taper crimp, thus potentially decreasing neck tension which may push the bullet all the way back to bad day territory each time one enters the chamber. If one wants to truly know their margins, they must know the size of the cartridge at the last moment before sending the bullet.
If powder charge is too high in a set back chamber= kB. So, in a way, powder+ NOT case chamber size start, BUT post setback case chamber size after loading = your actual, true-to-life working OAL from round to round. Or thereabouts.
Such a simple concept.