Hey all - after researching for an answer to my question, or a definitive direction, I decided to register and see what you all think. I'm sorry in advance for the wall of text. I also posted this over on the high road and got some much appreciated good info, but I'm casting a wide net where I can.
I purchased a M84 SRC Trapdoor from TrapdoorCollector and I'm interested in loading smokeless for it. I bought some HSM "cowboy" loads for it, but they leaded the barrel something terrible. I purchased a box of HSM hard cast lead bullets from SportsmansWarehouse (.459, 405gr, single lube groove and loaded them over 24.3g & 25g of A5744) and they, too leaded the barrel. I've decided that perhaps the bullets @ .459 were too small, but they were absolutely too hard. The leading seems to start at the breech and walks forward towards the muzzle. It's leaded up and the groups walk out after less than 5 shots. By 10 shots the group is LARGE.
I slugged the bore and got .460+/-.
The factory HSM cowboy loads measured at OAL 2.535.
The max OAL to the lands with the HSM bullets for my reloads measured at 2.585.
I loaded the HSM bullets to a 2.580 OAL. Not a problem, that's more than the min OAL listed for the 5477 powder I was using, so I knew I was well under (ideally) pressure limits at the min load.
I've since bought a couple more types of bullets to try and sort the leading out. The two that arrived today have me wondering about OAL, pressure levels, and what's safe...
I bought two models from GTBullets (nice people - great communication and friendly) - a 390gr FlatPoint HollowBase and a 425gr FlatPoint flat/solid base, both cast @ .460. They use a 96-2-2 alloy, which seems plenty soft (can dent with a HB pencil - I couldn't even scratch the HSM bullets with same pencil), and each have two lube grooves.
My issue comes with the OAL when I seat the bullet(s) just touching the rifling (using cleaning rod method).
For the 425gr, I get a OAL @ impact of rifling of 2.4575.
For the 390gr, I get a OAL @ impact of rifling of 2.5225.
For the 390gr the closest load for A5744 in my Lee manual is the 405gr - which shows a min OAL of 2.550 at the "never exceed" load (@16.1kcup). I want to load .020 off the lands, so that puts my OAL of a finished round at 2.5025, which is .05 shorter than the min OAL listed (2.550). To compound my concern, the 390gr is 1.0295 long, whereas the 405gr HSM was 1.009. The longer bullet will be seated further in the case AND is .02 longer, meaning even MORE lead in the case and less open case volume (HOWEVER - there is a 385gr bullet listed with a 2.505 min oal @ max charge for a LOT of other powders, so I'm not as concerned with this bullet as the 425gr I list next).
For the 425gr there is a 420gr A5744 load in the Lee manual, but the min OAL is 2.600 (@16.1cup). Additionally, the 425gr measures 1.0345 long vs the HSM @ 1.009. (Again, the longer length (to me) means not only is the OAL shorter putting the bullet further back in the case, the bullet is deeper in the case than the HSM would've been at the same OAL because the bullet is longer.) If I go .020 off the lands, it puts the OAL for the 425gr @ 2.4375 - a LOT less than the "min oal" for the never exceed load.
I want to start on the LOW end of the scale, and try and get a consistent, accurate load between 1150-1250fps with each of these bullets. I'll pick the most accurate load within that velocity range, if more experienced folks think that's a safe approach. To be clear, I'm using fps waypoints because I don't have any other way to gauge pressure, and the concern I have is that the shorter OAL is skewing the pressure curve in a meaningful way. I need a safe start - and stopping - point.
Because my OALs are SHORTER than published for max loads (most notably the 425gr @ .1625 shorter), I plan on starting the 390gr projectile @ 22gr (405gr min is 24.3) and the 425gr projectile @ 19gr (420gr published min is 25.6). I came up with those starting points by reviewing what AA does to the 5744 powder charge at differing OALs in the 45-70. It's rough, but it's the best correlation I could find.
Last note - if I set either bullet crimped at the groove, I end up in the rifling pretty good, although then the COL issues I have may be a non-issue, I'd then be trading the COL issues for the bullet set into the lands pressure spike issue. As I related on another forum, I don't have experience with that other than what I've read.
Here's the questions...
1) Using A5744 (or H4198 which I also have on hand), is the shorter OAL a point of concern for an original TrapDoor at minimum load levels as long as I a) stay well away from "never exceed" loads and b) monitor velocity during testing and keep a reasonable goal of 1150-1250fps?
2) Can I assume, for my practical purposes, that given the "known" weight of the projectiles and moderate FPS goal, that I can develop safe loads even with the shorter OAL using FPS as my waypoints?
3) If FPS can work, should I adjust my "target", or is 1250 reasonable for a high end? (… understanding I may settle on a load slower than that depending on which load tests out most accurate.) Could 1150 for 425gr and 1250 for 390gr be those reasonable high end target velocities?
Who knows - after all this contemplating, they may not shoot well at all, and that I can handle. I just want to be safe while I work through this.
Thanks all.
Edit: I also sent a check off for 150 bullshop 405gr hb lee cast to my barrel dimensions per his recommendation. These loads are only for the GT bullets - as the bullshop bullets should be fine with the factory lee & accurate loading data.