-Gadsden- said:
When Lyman gives data for a 120gn lead bullet, does this data correspond for a 124gn bullet that you might buy at an online retailer?
Yes, but there are reloading variables to take into account (bullet nose shape/ogive, bullet seating depth, OAL used, etc. - more on this below).
I have found most jacketed bullets can vary by 1+ grains and I use load data for 124/125 bullets interchangeably when using OAL longer than published.
I have found most plated bullets can vary by 1-3+ grains and I use load data for 124/125 bullets interchangeably when using OAL longer than published.
I have found many lead bullets can vary by 3-5+ grains and use load data for 120-125 bullets with some consideration. If I was pushing 125 gr bullet at near max load data, I may do my initial work up to .1-.2+ gr below max charge depending on the bullet seating depth.
Here's an example.
When I used 125 gr lead RN bullets with a step like Dardas/ZCast bullets shown on the right in the above picture, W231/HP-38
Hodgdon load data for 125 lead CN bullet and 1.125" OAL worked well.
125 gr LCN W231/HP-38 OAL 1.125" Start 3.9 gr (1009 fps) 25,700 CUP - Max 4.4 gr (1086 fps) 31,200 CUP
1999-2005 Winchester load data
When I switched to Missouri 125 gr RN, I got leading at 4.2-4.4 gr charges during my powder work up. I initially wondered if the transition from 22-24 BHN to softer 18 BHN made that much difference but found the real cause with the bearing surface and bullet seating depth.
If you look at the comparison picture above, the shiny part of the bullet base represents the bearing surface that rides the rifling (except for the stepped base of two RN bullet on the right). As you can see, Missouri RN bullet has shorter/rounder nose profile (ogive) than Dardas/ZCast RN bullets with longer bearing surface (BTW, Dardas sells both nose profiles). During my barrel drop test, I found I needed to use shorter OAL of 1.080"-1.100" for the Missouri bullet so the bearing surface of the bullet won't hit the start of rifling.
Using shorter OAL with different nose profile (ogive) caused the bullet base to be seated deeper in the case neck which would increase the chamber pressures. Lower 3.8 - 4.0 gr charges indicated by 1999-2005 Winchester load data did not cause leading.
Here's the seating depth calculations I used (OAL - bullet length = bullet seating depth). For calculation purposes, I used 1.125" as OAL for the CN bullet (.620" in length) and 1.080" OAL for the RN bullet (.565" in length) - but always measure the bullet lengths as bullets from different company may vary in OAL (case in point, SWC bullets from MBC and Dardas on the left in above picture).
1.125" - .620" = .505" bullet seat depth for the CN
1.080" - .565" = .515" bullet seat depth for the RN
So based on this example, bullet base got seated .010" deeper with the Missouri RN bullet with 1.080" OAL as compared to CN at 1.125" OAL.
So why use one nose type over another?
Well, longer the bearing surface, more bullet base to ride the rifling to rotate and stabilize the bullet in flight. More specific to lead loads, IME, longer bearing surface helps with sealing the bullet to barrel (obturation) to produce more consistent chamber pressures which translates to more consistent muzzle velocities which produces greater accuracy. Another bonus? I can use less powder charge and still produce accurate loads.
So why use stepped RN with shorter bearing surface over shorter/rounder Missouri bullet with longer bearing surface? That would depend on the barrels used. My aftermarket Lone Wolf 40-9 9mm conversion barrels for Glock 22/27 were reamed at the factory and allows for longer OAL for the barrel drop test. The new Lone Wolf conversion barrel I got for the Glock 23 has much quicker start of rifling with almost no leade like my Sig 1911 TacPac barrel.
Consequently, I needed to seat the Missouri 125 gr RN bullet to much shorter OAL to pass the barrel drop test and at the shorter OAL, bullet base would compress the 4.0 gr Promo/Red Dot powder charge I usually use (for 1.080" OAL, 4.0 gr of Promo is the max charge before I start compressing the powder charge - The shorter OAL would even compress the lighter 3.6 gr charge of Promo). Using the stepped RN bullets from Dardas/ZCast with 1.120" OAL passed the barrel drop test and did not compress the 3.6-4.0 gr Promo/Red Dot powder charges.
BTW, The quicker start of rifling/almost no leade Lone Wolf barrel
ABSOLUTELY provides faster chamber pressure build up and better obturation as even after several hundred rounds of shooting, the barrel is shiny clean with no smearing even at the chamber end! Life is good.