>With the same bullet weight, load, and OAL, pressures will be higher with a JHP than a FMJ because the JHP will be longer and sit deeper in the case
If the JHP is loaded to SAME COL as the FMJ-RN, it may be too long to chamber as you have moved the bullet forward due to the lack of the RN ogive segment. It all depends on the ogive geometry and the bearing surface. COL is determined by the length where the ogive makes contact with the lede/rifling, and that is VERY dependent on the bullet's geometry. I have never noticed any pressure problems with JHPs and, for the SAME load data, Hornady calls out a COL of 1.100" for the FMJ-RN and 1.075" for their JHP for 115gn bullets. For 124gn bullets, they used a 1.150" COL for that FMJ-RN, 1.060" for the HP-XTP, and 1.050 for their FMJ-FP, so this would indicate that if the JHP or FP bullets were loaded to same COL (which would be LONGER), pressure would be lower.
You use the GD data for FMJ and start at the start load and work UP. You determine the COL that works best in YOUR gun's chamber with that bullet and you don't assume the manual knows anything about what COL will work in your gun.
If you have no idea how to determine the COL range that works in your gun and simply have to have a COL from a manual, look in any manual for FMJ and use THAT COL. So, looking in a few manuals, COLs are 1.075-1.135". The very few FMJ-RN I have loaded over the years have been at 1.135-1.163".
As discussed below, first load a couple of inert dummy rounds and find the COL that feeds and chambers before you ever start to load a round with a new bullet.
The reason manuals have start loads is that YOUR components aren't the same as THEIR components and you need a safe place to start to work up the load. Powder lots vary, bullets vary, COL varies, the chambers in different guns vary, and even the case and primers can have an effect.
Re: COL
Per Ramshot:
"SPECIAL NOTE ON CARTRIDGE OVERALL LENGTH “COL”
It is important to note that the SAAMI “COL” values are for the firearms and ammunition manufacturers industry and must be seen as a guideline only.
The individual reloader is free to adjust this dimension to suit their particular firearm-component-weapon combination.
This parameter is determined by various dimensions such as
1) magazine length (space),
2) freebore-lead dimensions of the barrel,
3) ogive or profile of the projectile and
4) position of cannelure or crimp groove.
• Always begin loading at the minimum ‘Start Load.’"
Your COL (Cartridge Overall Length) is determined by your barrel (chamber and throat dimensions) and your gun (feed ramp) and your magazine (COL that fits magazine and when the magazine lips release the round for feeding) and the PARTICULAR bullet you are using. What worked in a pressure barrel or the lab's gun or in my gun has very little to do with what will work best in your gun.
Take the barrel out of the gun. Create two inert dummy rounds (no powder or primer) at max COL and remove enough case mouth flare for rounds to chamber (you can achieve this by using a sized case—expand-and-flare it, and remove the flare just until the case "plunks" in the barrel).
Drop the inert rounds in and decrease the COL until they chamber completely. This will be your "max" effective COL. I prefer to have the case head flush with the barrel hood (or a few mils higher than where the head of an empty case aligns with the barrel, as all cases are too short and I prefer to minimize head space). After this, place the inert rounds in the magazine and be sure they fit the magazine and feed and chamber.
You can also do this for any chambering problems you have. Remove the barrel and drop rounds in until you find one that won't chamber. Take that round and "paint" the bullet and case black with Magic Marker or other marker. Drop this round in the barrel and rotate it back-and-forth.
Remove and inspect the round:
1) Scratches on bullet--COL is too long
2) Scratches on edge of the case mouth--insufficient crimp
3) Scratches just below the case mouth--too much crimp, you're crushing the case
4) Scratches on case at base of bullet--bullet seated crooked due to insufficient case expansion (not case mouth flare) or improper seating stem fit
5) Scratches on case just above extractor groove--case bulge not removed during sizing. May need a bulge buster.
