308 Subsonic - suspiciously low velocity?

[Ritterbruder]

I recently took on a project to develop a subsonic load for my Ruger American Predator 308 with a 1 in 10 twist and 18" barrel. I cooked up some loads and chronoed them today, and my velocities are low to the point of being suspicious.

I say suspicious because I have been looking around at other peoples' loads and velocities just to get an idea of what charge weights to try. I know it's hard to find someone with exactly the same barrel length, using the same bullet and powder, and living in the same altitude and climate area. Still, despite this, my velocities seem very low.

Load:
Lake City Brass unmodified
CCI #200 Large Rifle Primers
Hodgdon Trail Boss
Hornady 180gr SP
Bullets seated to ~2.800" to fit in a magazine
No crimp

Conditions
Sea level near Texas Gulf Coast, 80 degF, humidity 65%

Charge weights and velocities:

10.2 gr:
-862 fps
-855 fps
-819 fps
-864 fps
-871 fps
Mean 854.2 fps S.D. 18.3 fps

10.4 gr
-884 fps
-857 fps
-859 fps
-868 fps
-878 fps
Mean 869.2 fps S.D. 10.5 fps (not bad consistency)

10.6 gr
-836 fps (??)
-906 fps
-Error
-1054 fps (??)
-1052 fps (??)
Mean 962 fps S.D. 94.3 fps (what the @#$% happened here!?)

10.8 gr
-949 fps
-951 fps
-944 fps
-902 fps
-909 fps
Mean 931 fps S.D. 21.1 fps

11.0 gr
-953 fps
-950 fps
-972 fps
-947 fps
-952 fps
Mean 954.6 fps S.D. 8.9 fps (I will take that consistency any day!)

I am using a Magnetospeed Sporter with the sensitivity set to high. Do these velocities "seem" a bit on the low side given my setup? I have already researched a lot of subsonic loads, including the ones on this forum. I would greatly appreciate it if anybody experienced with loading subs could shed some insight and opinion. Is there something I could be doing wrong? Thanks!

 

[Gunfighter14e2]

Your charge weights are within the norm but your are about 200fps slower across the board than what I've seen, given the same weight bullet with approx same bearing surface. Using a 180 Sierra pro hunter 10.4 of trail boss gets me 1080 fps and 10.2 w/a 175 smk is the same speed, using a win case an WLR primer. I'd check the crono. Have you shot any of your loads with a can on because I think the 10.8 & 11.0 grain load would give you the sonic crack, depending DA.

 

[Ritterbruder]

Thanks for the reply. I had my suspicions. My can is still in NFA jail. I was just out to do some preliminary chrono, grouping, and stability tests before I get my stamp back. I was considering just resorting to testing by ear and listening for the crack.

That might explain the weird readings I got at 10.6 grains. Maybe that was the bullet right at the sound barrier, and things were going crazy.

My my chrono has appeared to work fine in the past with supersonic loads. Maybe it just isn't accurate at such low velocities. I'll contact magnetospeed about it.

 

[Gunfighter14e2]

If you can shoot in or into a large enough valley/drainage if it's sonic, you will hear the crack still lingering after the bang goes away.

 

[JCCinOhio]

I had some questionable numbers too, but using 220gr bullets with TB. I asked on some forum if TB was position sensitive, and the general response was no... but haven't verified that yet. If you have easy access to shooting (out your back door or something) maybe try pointing the barrel straight up before shooting on a few shots, and then point the barrel straight down for a few shots, and see what happens with the speeds.

 

[dirthead1]

Tag for later

 

[spife7980]

Set your sensitivity to normal, I tried high on my sub load when chronoing a couple weeks ago and it gave me readings 100 fps slower than it did on normal. I think they high is for super small bullets, the chrono isn't going to be struggling to pick up 220 grains of lead passing over.

 

[Gunfighter14e2]

I had some questionable numbers too, but using 220gr bullets with TB. I asked on some forum if TB was position sensitive, and the general response was no... but haven't verified that yet. If you have easy access to shooting (out your back door or something) maybe try pointing the barrel straight up before shooting on a few shots, and then point the barrel straight down for a few shots, and see what happens with the speeds.

I use 700X, Red Dot, an other fast pistol powders, Trail Boss is not my favorite sub powder. It's great for those just getting into subs as a double charge will promptly, stand out. Done a lot of sub testing over the years an found you get a increase in speed w/ loose powder over having the powder packed on top of the primer. In fact it you install a wad to keep the powder at the flash hole you can lose as much as 170FPS vs having it just laying everywhere in the case.

 

[Gunfighter14e2]

[No message]

 

[JCCinOhio]

Do you use wadding on subs thru a suppressor? If so then I assume the wadding incinerates? Wouldn't want it building up in the suppressor. What do you use for wadding?

I use 700X, Red Dot, an other fast pistol powders, Trail Boss is not my favorite sub powder. It's great for those just getting into subs as a double charge will promptly, stand out. Done a lot of sub testing over the years an found you get a increase in speed w/ loose powder over having the powder packed on top of the primer. In fact it you install a wad to keep the powder at the flash hole you can lose as much as 170FPS vs having it just laying everywhere in the case.

 

[Gunfighter14e2]

Do you use wadding on subs thru a suppressor? If so then I assume the wadding incinerates? Wouldn't want it building up in the suppressor. What do you use for wadding?

Don't use any type of wadding, nor would I recommend using any type ever!! The only time I use wadding as a norm is when making 45ACP shotshells from anything with a .473 case head buts that a different story, for a different time.
I just tried wads I made/cut from a Styrofoam cup to test the theory about loose low charge weights of fast burning power being unstable. I had read a lot about "S-E-E" (secondary explosion effect) in low charge weights and wanted proof that was what was happening. W/O going into detail I am convinced it was double or triple charging the cases that was the issue. I even went on to weld shut the barrel of a 30 cal to try an duplicate S-E-E and yes it blew up. So knowing what happened I'd believe the projo prior never exited the bore before they shot another. I got into subs in 1957 via my grandfather, but it only been the last 20 years I've really stepped it up. All my sub projos now are bullets I cast an push via 700X, Red Dot, Clays, or Bullseye, with 700X the best. I have to weight that powder as it jams up in my thrower, but when the bullet has to be in a precise place, it's the powder of choice.

 

[Ritterbruder]

Update

I got my can out of jail and ran all the listed loads through my can. I didn't hear a supersonic crack on any of them. My first shot was with the highest charge (11 gr) and it was significantly louder than the rest, but that might've been first-round pop. All other shots fired sounded the same to me.

 

[spife7980]

Did you try the chrono with your sensitivity set to normal yet?

 

[Ritterbruder]

Your charge weights are within the norm but your are about 200fps slower across the board than what I've seen, given the same weight bullet with approx same bearing surface. Using a 180 Sierra pro hunter 10.4 of trail boss gets me 1080 fps and 10.2 w/a 175 smk is the same speed, using a win case an WLR primer. I'd check the crono. Have you shot any of your loads with a can on because I think the 10.8 & 11.0 grain load would give you the sonic crack, depending DA.

What does a sonic crack sound like when the bullet is barely supersonic? I tested my through a can today, and I noticed that there was a "whisper" that lingered for about 1-2 seconds after the report. It's not loud, but it is definitely distinct and it occurred pretty consistently past 10.8gr.

Was I hearing a sonic crack there?

 

[spife7980]

It sounds loud. 800 fps is close to movie quiet, when I had my bo 220s going that fast my bipod spring ringing was as loud as the shot it seemed. If you're close to hearing a loud report then you're drifting into that area. Shoot them over the magneato but set it to normal sensitivity. High is for small stuff, not big stuff moving slow.

 

[Gunfighter14e2]

What does a sonic crack sound like when the bullet is barely supersonic? I tested my through a can today, and I noticed that there was a "whisper" that lingered for about 1-2 seconds after the report. It's not loud, but it is definitely distinct and it occurred pretty consistently past 10.8gr.

Was I hearing a sonic crack there?

No such thing as barely supersonic, it's either or at the muzzle. What you may have heard is the oxygen bang in the can on first rd, an echos from your A/O. (Knowing how echos work can be an advantage at times.) Keep a eye dropper bottle filled with water an before you install the can, put 3-4 drops into the blast area. This will negate first rd pop an will also reduce your cans db for about 4-5 rds as well. If your trying to hide don't use more than 1 drop. All kinds of other stuff on the market but water works well. Different can designs work better on subs. A can that is great on high speed projo db reduction, normally sucks on subs, an vise-versa. I'll give up 5-6db on high speed for the extra 10-12db on subs quick. Done right a can dedicated for subs an to stop FRP will still work on high-speed but not as well as one designed for HS from the get go.