Seating depth for subsonic loads

[mgunderson]

Question - Does seating depth work in subsonic loads? Are the seating depths similar to supersonic loads at almost 3x the speed.

Looking for practical experience before I go down another rabbit hole.

 

[RegionRat]

I will assume we are discussing rounds like 300 BO or 7.62x39 to be used with a suppressor.... then...
I look at them like pistol loads.
Yes, there are times when the groups (or velocity stats) respond well to a different seating depth, and times when it doesn't seem to help.

 

[Hoser]

I was playing with 300 Whisper and 230 Bergers out of a bolt gun a couple years ago. Bartlien 7 twist in a Bighorn/Zermatt TL-3.

Vhit N-110 gave the best ES/SD of any of the powders I tried initially. Once I found a promising load I stared playing with seating depth.

At 100 yards a so-so 3/4 inch group went below a 1/2 inch group with a little seating depth playing.

I sold the rifle and ended up with a DTA bullpup with a Brux barrel and repeated it. Same reamer. Same end result.

With the 300 it worked for me.

Next project is my 510 Whisper. I just need the time and energy to work up a load. Ironically, Vhit N-110 also showed the most promise so far.

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[mgunderson]

Is the n110 better than trailboss? Just curious if you have tried soft seating and getting the bullet up against the lands? Not sure how to get the best combustion with the light charges.

 

[Dusty Stevens]

I was playing with 300 Whisper and 230 Bergers out of a bolt gun a couple years ago. Bartlien 7 twist in a Bighorn/Zermatt TL-3.

Vhit N-110 gave the best ES/SD of any of the powders I tried initially. Once I found a promising load I stared playing with seating depth.

At 100 yards a so-so 3/4 inch group went below a 1/2 inch group with a little seating depth playing.

I sold the rifle and ended up with a DTA bullpup with a Brux barrel and repeated it. Same reamer. Same end result.

With the 300 it worked for me.

Next project is my 510 Whisper. I just need the time and energy to work up a load. Ironically, Vhit N-110 also showed the most promise so far.

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Id like to see the case for that 510 whisper. Is it an actual JD Jones design or are you just using his name on it?

 

[Hoser]

Id like to see the case for that 510 whisper. Is it an actual JD Jones design or are you just using his name on it?

It is an actual 510 Whisper.

 

[Dusty Stevens]

It is an actual 510 Whisper.

Wow man thats awesome!

 

[nakneker]

Wow man thats awesome!

Agreed!

 

[Spook]

Is the n110 better than trailboss? Just curious if you have tried soft seating and getting the bullet up against the lands? Not sure how to get the best combustion with the light charges.

if you can get your hands on some old 4756 or Ramshot Enforcer.... Alliant e3 is showing promise as well. TB is crap.

 

[mgunderson]

Thanks for the information. Are you exploring powders with quickload or another resource? What are the benefits of the other powders and what is bad about TB? Assumng short barrels are better than long. How short s too short?

 

[dellet]

Knowing your cartridge would be very helpful.

In general a bullet deeply seated helps with ES/SD spreads which are critical shooting past 100 yards. That can lead to jumps measured in .100's instead of .001's. A jump tolerant bullet is very helpful.

If your shooting suppressed, the faster the powder, the quieter the shot will be. But if that's in an AR too fast will not cycle the action.

Powders like Trail Boss and Tin Star can work very well, but Trail Boss is one of the fastest powders out there. Depending on bullet weight you can easily start getting into loads that are pushing max pressure before you get max velocity say, 1050 fps. Case capacity comes into play. Trail Boss for example comes in the basic #1 jug and weighs 9 oz. That gives an idea of it's bulk density, or lack of.

Slower powders tend to run very dirty. Using 300 BO as an example and a 190 grain bullet, 10.5 grains of 1680 with 75% burned in a 16" barrel vs N105 or AA#9 using 7 grains and burning 100% in an 8" barrel.

Something hand fed you might have a couple dozen powders that work well and unlimited bullet choice. With an auto loader it might come down to a few powder and bullet combinations depending on caliber.

You can have loads that rival any air gun for noise level or,
something that sounds more like a 22 pistol without a suppressor.

Subsonic shooting can get quite complex if you want to go down that road, or just find a reliable load and plink, it's just a choice.

 

[mgunderson]

I am working with a 308 at the moment with a 16.5 inch barrel. Shooting sierra 175 matchkings. My 10 twist barrel and a few other things are complicating the fun.

 

[Oso]

I am working with a 308 at the moment with a 16.5 inch barrel. Shooting sierra 175 matchkings. My 10 twist barrel and a few other things are complicating the fun.

I assume we are talking about a bolt rifle and no an AR10. Your biggest challenge is case fill. The deeper seating depth of heavy subsonic coupled with the reduced charge weight makes the 300BO case optimal for this reduced case fill. With a 308 you have 3x the case capacity of 300BO. There is a lot of empty space, which typically translates into higher ES/SD as the ignition of the powder varies from shot to shot. The powder may be dispersed within the case completely different for each shot (all piled near the primer or near the bullet or evenly dispersed across the entire bottom of case) that may result in irregular ignitions.... flashover, etc.

The larger volume powders, such as Trail Boss, will provide a more consistent performance from shot to shot by simply reducing the amount of excess space in a 308 case. This should lead to more consistent powder ignitions.

 

[dellet]

Depending on the bullet, a 1/10 will stabilize a lot, maybe most, non VLD bullets up to about 210 grains out to 250-300 yards. A 208 Amax is a favorite because it is very jump tolerant.

Hodgdon has subsonic data but is limited. They used to provide data for Tite-group and Clays by email for the 308. Might be worth a try. I have notes of 8.2 grains of either for a 178 Amax, but that info is probably 10 years old for a 307 W. Basically a rimmed 308. Since your barrel is shorter, you would likely need to up the charge weight.

Accurate used to have a formula for reduced loads with 5744 that is similar to IMR and 4895. 60% of minimum load. With your shorter barrel that would be borderline velocity. But it’s at least published data and method.

Hope that helps get you started. Silencertalk.com is a good resource for all things subsonic.

Hope that helps a bit.

https://hodgdon.com/wp-content/uploads/2020/07/subsonic-data-final.pdf

 

[mgunderson]

Been able to shoot some 6 inch groups @ 300 yards and roughly 1.5 inch groups at 100. Going to have to switch scopes to be able to see anything closer.
Not sure if the seating depth is doing anything or just getting lost in the noise of other issues.

It is a valid point about the bo vs the 308. Less case volume would be helpful in this situation.