Has anyone developed subsonic 308 with 220 gr SMK bullet? I did some load development yesterday and 13.0 grains of Trailboss gave me 1000 fps in DPMS 18" barrel, 1 in 10 twist. I use standard brass (LC) with CCI BR2 primers. However, the SD was 32. BTW, no keyholes which was a concern (stability factor).
With 175 gr SMK using 10.8 grains of Trailboss I get 1050 fps and a SD of 12-14. It's very accurate and consistent.
Has anyone else done this and seen a higher SD using the 220 SMK? (which decreases accuracy). Thanks.
subsonic 220 SMK in 308
I actually have a theory on why the 220 grain SMKs resulted in a higher standard deviation (SD). The 220 SMK is long, 1.497", and I seated the bullet deeper, so that they were magazine length. I think the bullet bottom was past the neck and shoulder of the case, with it seated that deep.
I think if I seat it closer to the lands so that I have an Ogive length jump to the lands of say 0.015", then it will result in the bullet bottom not extending into the case. This might give a better SD, and more accurate subsonic round too.
I think if I seat it closer to the lands so that I have an Ogive length jump to the lands of say 0.015", then it will result in the bullet bottom not extending into the case. This might give a better SD, and more accurate subsonic round too.
Did you test accuracy with those loads? SD does not necessarily correlate to accuracy, as many handloaders and benchrest shooters have found. SD of 32 isn't that bad if accuracy is acceptable, especially for a subsonic round.
I doubt seating the bullet base below the case shoulder affects the SD. If you need to improve SD, pay attention to case prep, bullet alignment, and particularly neck tension. Also pay attention to any sizing lube inside the case neck.
You may have done all this, in which case it's a matter of load development. I haven't used trailboss, but I've read that velocity can become erratic when pressure gets too high. This powder's meant for low pressure loads so you 'could' be approaching that region with the 220 SMK, I don't know.
I doubt seating the bullet base below the case shoulder affects the SD. If you need to improve SD, pay attention to case prep, bullet alignment, and particularly neck tension. Also pay attention to any sizing lube inside the case neck.
You may have done all this, in which case it's a matter of load development. I haven't used trailboss, but I've read that velocity can become erratic when pressure gets too high. This powder's meant for low pressure loads so you 'could' be approaching that region with the 220 SMK, I don't know.
Thanks for the advice.
I was doing load development, to discern the exact amount of Trailboss I needed to achieve 1060 fps with the 220 SMK. My target was only at 50 yards, during chronographing and load development. While a SD of 32fps is not terrible at full power loads, I think anyone can understand that vertical stringing is more pronounced/exaggerated at Subsonic speeds (as compared to full power). Three groups had 2 shots touching or in the same hole and one vertically stringing 3/4" higher. So the 32fps SD did result in vertical stringing.
I use the same prepared brass for subsonic as I do full power loads. My full power loads have single digit SD's (about 7-8fps with 175gr SMKs). I achieved that by going so far as beveling the inside of the primer hole (which interestingly did improve and make the SD's consistent). I don't tumble my brass, I use an ultrasonic cleaner and full size before cleaning. So there is no chance of lube or other issues.
I'm going to try seating the bullet farther out, say .015 off the lands, and see if that makes any difference. It may not matter at all, but it's worth a try. I guess I could also think about wire brushing inside the neck before full sizing, to have a consistent smoother inside neck to the brass. Maybe I'll try that if changing the seating depth doesn't do anything.
I was doing load development, to discern the exact amount of Trailboss I needed to achieve 1060 fps with the 220 SMK. My target was only at 50 yards, during chronographing and load development. While a SD of 32fps is not terrible at full power loads, I think anyone can understand that vertical stringing is more pronounced/exaggerated at Subsonic speeds (as compared to full power). Three groups had 2 shots touching or in the same hole and one vertically stringing 3/4" higher. So the 32fps SD did result in vertical stringing.
I use the same prepared brass for subsonic as I do full power loads. My full power loads have single digit SD's (about 7-8fps with 175gr SMKs). I achieved that by going so far as beveling the inside of the primer hole (which interestingly did improve and make the SD's consistent). I don't tumble my brass, I use an ultrasonic cleaner and full size before cleaning. So there is no chance of lube or other issues.
I'm going to try seating the bullet farther out, say .015 off the lands, and see if that makes any difference. It may not matter at all, but it's worth a try. I guess I could also think about wire brushing inside the neck before full sizing, to have a consistent smoother inside neck to the brass. Maybe I'll try that if changing the seating depth doesn't do anything.