We already knew this, but I just worked through an example to show just how important it is to minimize the dispersion of muzzle velocity when shooting subsonic. I posted the details here, but here's the summary:
I shot a box of Remington Express 220gr at 100 yards. Average muzzle velocity was 960fps, but the values observed for this sample ran from 883 to 1013 fps, which represents a 14% extreme spread. At 100 yards that spread equates to over 5 inches of drop! The actual vertical spread on the target was 6.8 inches.
If we control for the variation in muzzle velocity then this embarrassingly tall group turns into a typical round (Gaussian) group:
Handloading helps with greater consistency. With handloading I've only had one subsonic bullet ever move that much on me and it is no longer produced. The new version of the bullet has given me no issues.
So my question would be how exactly does one get a tighter control on mv variances? My subsonic 175smk .300blk loads shoot extremely tight. While my 175smk .308 sub loads exhibit large vertical stringing. The strange thing to me is the .308 loads using trailboss are all hand weighed for powder and bullet weight, while the 300 blk using lil'gun are machine dropped charges and non sorted bullet weights.
I have found that with such low powder charges that the .2gr variance built into the typical powder scales along with powder throw devices, greatly impacts accuracy. I now use a jewllers gem scale to weight each charge intended for accuracy along with the cases and bullets. The Gem250 scale is accurate to within + or - .02gr, that is significantly better than the + or - .1gr of powder scales. I also have a analytical scale that is certified accurate to within 1/140th of a gr to verify. Accuracy is not an issue when you go all anal on the weighing. It is very slow and laborious but worth the efforts.
I took an Oath to preserve protect and defend the Constitution, I never took an oath to defend the Govt!!!
As talonxracer stated, small variance in charge weight makes a huge difference with sub-sonic loads.
While weighing things to as close tolerance as possible helps, the problem is not with the charge weight.
(Here is a hint)
This big difference is not present when using cast bullets.
Robert touched on it earlier in this thread.
Lapua took a page from the rimfire side of their business and use lube on their subsonic loads.
If you look closely on their 200 grain subsonic bullet,
you will notice two recessed bands and a canneluer.
g22rmr wrote:So my question would be how exactly does one get a tighter control on mv variances? My subsonic 175smk .300blk loads shoot extremely tight. While my 175smk .308 sub loads exhibit large vertical stringing. The strange thing to me is the .308 loads using trailboss are all hand weighed for powder and bullet weight, while the 300 blk using lil'gun are machine dropped charges and non sorted bullet weights.
Presumably you're shooting 175 .300BLK in a bolt gun, right?
Trailboss probably needs a heavier bullet to reach high enough case pressure for consistent burning. I.e., for subsonic speeds you'd need to be looking at 220gr bullets (and not too long a barrel).
Last edited by dbooksta on Fri Jan 03, 2014 12:30 pm, edited 1 time in total.
300Blk wrote:Lapua 308 subsonic has bullet lube that they claim helps. I have never tested that, but someone may want to experiment. You can try Moly also.
Makes sense -- those 220 OTMs have a very large bearing surface!
I have some hexagonal Boron Nitride (hBN) for lubing bullets. I'm going to lube some 220s and see what those do.
dbooksta wrote:
Presumably you're shooting 175 .300BLK in a bolt gun, right?
Trailboss probably needs a heavier bullet to reach high enough case pressure for consistent burning. I.e., for subsonic speeds you'd need to be looking at 220gr bullets (and not too long a barrel).
I'm shooting .300blk with 9'' acc upper with the same 30p-1 suppressor that I use on my 23'' GA Precision bolt gun. Sound from the 23'' is amazingly quiet. I never sought out a dedicated subsonic bullet, just used what I had on hand and went with it. I'd imagine the aac 1-7 twist rate has alot to do with it vs. the 1-11.25 twist GAP.
