I've been reading elsewhere on the use of Trail Boss for loading subsonic rounds. I haven't tried Constructor's loads for the 130 gr bullets yet because I haven't remembered to pick up the Reloader 7, or when I did try, it was out of stock at the places I buy powder.
Trail Boss is a high bulk, low pressure powder, so I bought a few containers (9.0 oz. in a "1lb can") . Downloaded the instructions from the IMR website http://www.imrpowder.com/PDF/Trail-Boss-data.pdf , then adapted the instructions for the 6.8 SPC. The projectile I used at first was the 115gr Rem FMJ. I loaded up rounds along a roughly 3.0 grain spread, and after firing a low and high charge round to check for bullet stability at 50 yards by firing them through a cardboard box. No key holing!
Added the suppressor, and then started shooting from lowest to highest charges, listening for the supersonic crack. Found it about 1.4 grains up from my starting point. None of the rounds cycled the action, which is fine, because the action cycling just adds more noise when your goal is to be quiet.
Headed back to the bench and then the range. (a 1/2 mile from my house) Zeroed in on the supersonic crack with my rifle and its characteristics using 0.2gr adjustments on the loads. I'll fine tune for accuracy now that I know right where the limit is. Accuracy offhand with a holosight at 50 yards was pretty good, about 2" for relatively rapid fire, about 1 round per second. Impact was lower than where my 110gr TTSX's impact by an inch or so.
Loaded up test rounds for 100gr JHP, 130gr PSP, and 150 gr PSP so that I can estimate where supersonic is, but I have yet to fire those.
Please read the instructions if you try this, and remember that you need to measure/estimate where the base of the bullet is within the case, as that is your 100% load. I just cut the neck and shoulder off of one case, then set the empty case with a seated bullet, and a bullet without case next to the others and marked the case there. Filled the cut case to the mark with powder, weighed that, then reduced by 30%. Example (not actual): If my powder to the mark weighed 10gr, then multiplying 10 x 0.7= 7.0 grains. If the weight was 8.0, then the starting load would be 8.0 x 0.7 = 5.6grains. Interesting process. My main concern would be sticking a bullet in the barrel as I head up in weights. We'll see.
I was nowhere near the 100% load, but the highest loads showed a prisitine primer without any signs of pressure.
Last edited by dogfish; 01-07-2013 at 02:54 PM.