RN vs. RNFP overall cartridge length

[Sonnylee 20]

I've started to load .380 ACP with FMJ RN from hornady 100 gr. at a OaL of .980

I ran out of round nose and have RNFP extreme bullets, I seated the first bullet and my overall length is .950.

Not sure if I should ajusting my seating die or continue with this Oal.

The two bullet have a .039 in length difference. FP shorter.

Called xtreme they said .950 was a good starting length.

Would this increase pressures in the cartridge being its a smaller case?

[Tunaman 549]

It certainly would increase the pressure. Don't guess at your OAL. Do a plunk test in your chamber. Make some dummy rounds with no powder or primer. Make sure to back the bullet off the land a few thou at least. This should be your proper oal for that particular gun. I push down on the round and spin them at the same time. You can feel when it no longer touches. Once you find the right length then make a few and throw the dummy rounds in your mag and cycle them out by hand. If they work you should be good to go. And don't forget to start with lead data for those plated rounds.

[T Bill 649]

The 380 COAL max is .980, so the FMJs are at their max already. Do you have a 100 FMJ head to measure the length?  Measure the FP, what is the length here?  Difference will give you an idea of the seating depth of each in the case.  If the seating depth is near the same in the case, pressures will not change that much.  It is the seating depth of the head in the case that determines the pressure curve, not the COAL provided the head does not touch the lands in the barrel.  Accuracy maybe affected by how much freebore there is from the bullet head to the barrel lands.

[Tunaman 549]

I should have mentioned to check the both bullets for equal length first. Max OAL is .984