Interpolating a load data for a bullet that's not listed in the charts or using a different COAL (or primer, or case for that matter) that what's used in the load data charts is where you leave the safe side of the street and start taking some (hopefully small and informed) risks. You
have to make some assumptions and you have to keep a sharp eye on each piece of spent brass in between shots to see whether there's any signs of overpressure being shown. And even then you're still always making a guess from that brass' appearance, because reading spent brass is a lot like reading tea leaves: some brass will show signs of pressure while other for the same bullet/primer/powder charge will not.
Doesn't mean you can't approximate the load data, but it means you have to be careful and err
on the side of caution at all times. People DO develop loads outside of the manuals, after all, by starting with informed approximations, minimal powder charges and increasing it one small step at a time. They also have balls of brass, those guys, because making a major mistake can result in a ruined firearm and serious injuries.
For me, the first rule is to use the load data entries for a bullet of the same weight (at plus or minus one grain, like 125gr data for 124gr bullet). Then I look for bullets of the same material and construction: lead vs polymer-coated vs copper-plated vs copper-jacketed. And when the first two criteria are met, I look for bullets of the same general geometry (RN, JSP, JHP, SWC, etc). What I don't
ever do is to substitute a powder that doesn't appear in the load data chart for one that does, even if the bullet from the chart is the same exact one that I'm using in the press. If I don't have the exact brand/model of powder on hand which is specified by the load data, I just don't reload. Perhaps one day I'll grow those brass balls I was speaking about, but
not today!
Your LRN bullet is coated with a polymer called Hi-Tek to act as a lubricant. That coating is apparently claimed to be at least as though as the plating of a copper plated round. So I think you'd be right to treat it as a copper plated or jacketed bullet of the same weight, since the geometry is so similar. Checking what I could find on the Hi-Tek coating FAQs on the web, it seems those bullets will simply be a little faster down the barrel than the FMJ or LRN for the same amount of powder in the cartridge. No issues there that I can see.
Fiddling with the COAL does demand a little caution, though. The SAAMI specs call for 9mm Luger to be anywhere between 1.000" and 1.159" in length, so you have real leeway. From my understanding of internal ballistics:
1. when you increase the COAL, the peak chamber pressure at firing will be slightly lessened, so you can keep using
up to the max powder charge specified by your load data.
That is,
unless you make that COAL so long that you actually jam the ogive of the bullet into the leade of the throat when you're chambering it. Then the resistance it encounters as the burning powder tries to force it down the rifling will
increase the chamber peak pressure. By how much? We can't know, it depends of the geometry of the throat, of the geometry of the bullet, of the material of the bullet, the burning rate of the powder, the primer used, etc... Here's a diagram of what I mean, from a website about the 38 super that has an interesting page on COAL (
http://www.38super.net/Pages/Overall%20Length.html):
2. When you decrease the COAL, the peak chamber pressure at firing will increase. That means you need to be careful if you were planning to use the maximum charge allowed by the load data. Depending on the density of the powder you use (i.e. how much space it occupies in the casing), you might also be causing a "compressed load" which
can be harder to properly ignite by the primer. Rule of thumb, I avoid shortening my 9mm COAL, and if I can't avoid it I don't use the max powder loads figures from the load data: I cut it down by about 5% (in my case, about 0.2gr) just to stay safe.
So in the diagram, to properly chamber that flat nose (aka "truncated cone") bullet without jamming it into the leade, they had to reduce its COAL, meaning increase the chamber pressure a little bit at firing time. That's what I meant by "fiddling with COAL" and keeping in mind the effect it may have.
To make things more interesting, while the max pressures allowed by your load charts probably conform to the 9mm normal pressures (35k PSI), it's possible that your pistol may be rated to safely handle +P loads that create 10% more peak chamber pressure (38.5k PSI). In which case... well, you're working with a safety margin that's 10% greater to begin with.
One last caveat, and it's an important one:
don't let my writing style fool you, do not take anything I wrote up there as pure gospel. Check with other sources. I told you honestly about how I go about reloading my 9mm, but that does not mean it's the only, much less the best
or safest method to reload! All I can say is that my firearms haven't blown up in my face...
YET.
