Interesting workup Kevin.
I've enjoyed using Trail Boss but have put it on the back shelf now and am using it for lighter loads only.
I decided to try to work up a hunting load in my 45/70 Guid Gun and it is a pleasant, accurate powder. I used loading information from IMR. IMR had told me TrailBoss was safe in ANY cartridge if you measured the seating depth of the bullet and subtracted it from the depth of the case. Weigh that amount of powder and that is your Maximun load. Deduct 10% and that's your starting load. I did that and worked up with no pressure signs.
I got good groups at 100 yards even with 17 grains.
I had cast 405 grain bullets hard using WW and 2% tin.
While discussing the load on castboolit, one member reminded me that TB wa as fast or a tad faster powder than Unique and I was pushing the limit for the GG.
Your load of Trail Boss is probably above the milder levels suitable for most rifles. It's a relatively large volume of fast burning powder (your charge is noticeably above manual recommended loads) under a relatively heavy bullet. Depending upon how it's used, Trail Boss approximates the burning range of Red Dot on the fast side to Herco on the slow side.
The amount you're using is quite generous, and if it is indeed producing 1500 fps (which I rather doubt) pressures would be very high to get that "estimated" velocity. FWIW, 16 grains Unique gets 1260 fps with a 405 grain Lee bullet in my 22 inch Marlin and that's not really a Trapdoor load either, as it's producing above the level of pressure safe for that type of rifle. Your stubby Guide Gun barrel would get less than this.
I know you've got a stronger built Guide Gun, but the load likely isn't as mild as you think.
I'd say IMR's estimate is rather off, and I'd also counsel some amount of caution, in the sense that you're operating well above the advised levels in the Hodgdon manual, which gives 25,600 CUP with 13 grains of Trail Boss under a 405 grain bullet to obtain 1007 fps. So you can see why I very much doubt 1500 fps with 17 grains, and also why I say that you're operating on the rather high side. FWIW, 13 grains Red Dot gets 1120 fps in my 22 inch barrel, and is a close approximation of the Hodgdon load levels and velocity.
I don't think 17 grains is necessarily well advised, let alone 18. I suppose there's some room to go above the 25,600 CUP level of the 13 grain load, but given that the pressure limit of the Marlin is around 40,000 CUP, and you're 31 percent above the 13 grain charge with 17 grains, and 38 percent above that with 18, you're skating very close to the edge, if not over. 31 and 38 percent more powder will give more than 31 to 38 percent more pressure as the pressure levels of fast powders rise disproportionately to the amount they're increased.
If you really want 1500 fps, Trail Boss isn't gonna get you there safely. Given its proximity in burn rate to other fast powders, I'd suggest not leaning on the powder measure too much. The gun probably isn't gonna blow up, but a low pressure plinker load it ain't.
Since I still hadn't hit the magic 1500 FPS point, I switched back to 4198 and kept the TB for target and plinking loads. It's a shame too because it is a nice clean powder.
